Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Plasma Physics
Volume 64, Number 11
Monday–Friday, October 21–25, 2019; Fort Lauderdale, Florida
Session GP10: Poster Session III: Magnetic Fusion: MAST-U, NSTX-U, Pegasus, Spherical Tori, Particle and Power Handling, Divertor Physics and Plasma-Material Interactions, Edge & Pedestal Physics, Measurement, Diagnostic, & Control Techniques, Conventional Tokamaks, HBT-EP. Basic Plasmas: Non-Neutral Plasma, Antimatter Plasmas, Plasma Sheath (9:30am-12:30pm) |
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Room: Exhibit Hall A |
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GP10.00001: Thermal equilibration from the long-range collisions in $e^{\mathrm{-\thinspace }}$/ $H^{\mathrm{-}}$ magnetized NNP. Andrey Kabantsev, C. Fred Driscoll We measure the particle slowing-down rate in a strongly magnetized ($r_{ce} ,r_{cH} \ll \lambda_{D} )$ weakly coupled ${e^{-}} \mathord{\left/ {\vphantom {{e^{-}} {H^{-}}}} \right. \kern-\nulldelimiterspace} {H^{-}}$nonneutral plasma. The plasma consists of near-room-temperature electrons ($T_{e} \approx 0.03-0.1eV)$ with an admixture (up to 20{\%}) of negative hydrogen ion, $H^{-}$. Here, $n_{e} \sim 10^{7}/cm^{3},\;n_{H^{-}} \sim 10^{6}/cm^{3}$. It was suggested recently [1] that for repulsive Coulomb interactions in the strongly magnetized regime, the slowing-down rate can be greatly enhanced by collisions with impact parameter $\rho $ in the range $r_{cH} <\rho <\lambda_{D} $. We measure the thermal equilibration rate $\nu^{H/e}$ between the cold electrons and the bounce-dynamically heated negative ions, and have found the typical rate $\nu^{H/e}\approx 1/\sec $ which is a factor of 5 smaller than the enhanced rate of [1] estimated for the given above plasma parameters. Possible reasons for the observed discrepancy will be analyzed, including hidden effects like a radial mass-separation, non-uniform $T_{e} (r)$ \begin{figure}[htbp] \centerline{\includegraphics[width=0.38in,height=0.19in]{020720191.eps}} \label{fig1} \end{figure} profile, etc. This enhanced particle slowing-down rate contributes also to the collisional damping of plasma waves, and affects the cyclotron cooling rate for electrons. [1] D.H.E. Dubin, Phys. Plasmas 21, 052108 (2014). [Preview Abstract] |
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GP10.00002: TG~waves~in~$\theta $-asymmetric magnetized plasma column. Nicola Panzeri, Kurt Thompson, Andrey Kabantsev, C. Fred Driscoll An unusual~$m_{\theta } =0$ TG wave~amplitude modulation due to the interaction with a~$m_{\theta } =2$ diocotron wave~is observed for the first time~in a~pure electron plasma column.~Trivelpiece-Gould (TG) modes describe longitudinal electrostatic plasma oscillation, modified by the cylindrical boundary, on an axially magnetized plasma column~[1]. Here we perform the first experiments on TG waves propagating in an azimuthally perturbed density (and potential) distribution~in~a pure electron plasma confined in a Penning-Malmberg trap. We~apply~a small $m_{\theta } =2$ diocotron~mode~to an~originally~axisymmetric equilibrium~density distribution $n_{0} (r)$, and~we~measure~the~amplitude modulation~of regular $m_{\theta } =0,\;k_{z} =1,\;m_{r} =1,2,3...$ TG modes:~in the Fourier space, this appears~as~a~wave triplets~consisting of a~central branch (continuation of the original mode) as well as of the upper~and~lower~sideband~shifted in frequency~by~$\Delta f_{SB} $.~The~splitting $\Delta f_{SB} /f_{TG} $ depends on the asymmetry strength $q_{2} $~(the quadrupole moment of the $n(r,\theta ))$ approximately as~$\sqrt {q_{2} } $.~Diagnostics also show two~triplets signals near the~TG wave, with frequency$_{\mathrm{~}}f_{TG} \pm f_{E\times B} $,$_{\mathrm{~}}$each with~$\Delta f_{SB}_{\mathrm{~}}$also related to~$\sqrt {q_{2} } $.~These features in the interaction of two orthogonal waves (diocotron and TG)~are not~yet~explained by nonlinear waves coupling theory. [Preview Abstract] |
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GP10.00003: e-/H- Plasmas: Exceptional inwards transport from a Rotating Wall drive and rapid outwards convection afterwards. K. A. Thompson, A. A. Kabantsev, C. F. Driscoll, N. Panzeri In nonneutral plasmas consisting primarily of electrons with a small fraction of H- ions, we observe exceptional radial compression of the ions by externally-applied rotating electric fields (Rotating Wall drive). In a Penning-Malmberg trap designed to confine electron plasmas, we can accumulate H- ions until they comprise up to 20{\%} of the total charge in the trap. Initially, the ions reside at the outer radii of the rotating plasma column, due to mass-dependent centrifugal forces. We find that the Rotating Wall drive causes preferential inward transport of the ions, with the ions ultimately forming 30{\%} of the total charge density at r$=$0. Nascent theory suggests the ion transport is due to a resonant interaction between the ion axial bounce motion and the applied fields. When the Rotating Wall drive is turned off, the ions convect to the outer edge of the column, apparently as a coherent clump, on a time scale much faster than predicted for diffusive centrifugal separation. Detailed measurements of the temporal evolution of the electron density profile are presented for a variety of Rotating Wall drive configurations. [Preview Abstract] |
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GP10.00004: Parametric Decay of Nonlinear Waves Driven by Weakly-Trapped Particles Daniel Dubin Parametric instabilities, in which longer-wavelength waves grow on a shorter-wavelength nonlinear wave, are endemic in plasmas. This poster discusses a new instability mechanism\textsuperscript{2} caused by a distribution of particles that is weakly-trapped in the potential wells of the nonlinear wave, with energies just below the potential maxima (appearing as phase space rings or holes). Such distributions are common in nonlinear plasma waves. The new theory predicts detrapping and retrapping of such particles that leads to a negative adiabatic compressibility, destabilizing the wavetrain with respect to relative motion of the potential peaks. Simulations of electrostatic Trivelpiece-Gould [TG] traveling waves observe this effect with growth rates in agreement with the theory. Experiments on large amplitude TG standing waves also observe parametric decay, with temperature dependence of the decay rate pointing to the trapped particle effect. New r-z simulations in realistic geometry yield similar parametric decay rates compared to experiments. When trapped particles are removed in the simulations, the instability is suppressed.\newline \ \textsuperscript{2}D. Dubin, Phys. Rev. Lett. {\bf 121}, 015001 (2018); M. Affolter et al., Phys. Rev. Lett. {\bf 121}, 235004 (2018). [Preview Abstract] |
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GP10.00005: Test Particle Diffusion In Correlated Plasmas Francois Anderegg, Jacob Saret, C. Fred Driscoll, Daniel. H. Dubin Inter-particle correlations have several effects on the dynamical properties of plasmas. The correlation parameter $\Gamma \equiv $ e$^{\mathrm{2}}$/aT represents the ratio of the nearest neighbor electrostatic energy to the thermal energy. We cool un-neutralized magnesium ion plasmas in a Penning-Malmberg trap, with density n $\cong $10$^{\mathrm{7}}$ cm$^{\mathrm{-3}}$, the temperature T is controlled over a wide range of 5eV \textgreater T \textgreater 5x10$^{\mathrm{-6}}$eV, resulting in correlation parameters 10$^{\mathrm{-5}}$ \textless $\Gamma $ \textless 10. We have measured the perpendicular to parallel collision rate $\nu _{\mathrm{\bot //\thinspace \thinspace }}$from the non-correlated to strongly correlated regime, and observe the (Salpeter) enhancement of $\nu _{\mathrm{\bot //\thinspace }}$ up to 10$^{\mathrm{9}}$ due to correlations [1]. We have also measured test particle diffusion and heat transport in these plasmas in the absence of correlation ($\Gamma $\textless \textless 1). Here, we propose to extend these measurements into the moderately correlated regime. In the range of 10$^{\mathrm{-3\thinspace }}$\textless $\Gamma $ \textless 1, where no experiment or numerical simulation have been done, we will test theory [2] predicting decreasing diffusion coefficient D as T decreases, with transition from D$\propto $T$^{\mathrm{-1/2}}$ to D$\propto $T and a transition from D$\propto $ B$^{\mathrm{-2}}$ to D$\propto $B$^{\mathrm{-1}}$. [1] F. Anderegg et al., PRL, 102, 185001 (2009). [2] T. Ott and M. Bonitz, PRL, 107, 135003 (2011); S.D. Baalrud and J. Daligaut, Phys. Rev. E, 96, 043202 (2017). [Preview Abstract] |
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GP10.00006: $E \times B$ shear instability of two-dimensional electron filaments subject to applied strain flows Noah Hurst, James Danielson, Daniel Dubin, Clifford Surko Recent work has demonstrated the ability to subject pure electron plasmas to external $E \times B$ flows in a Penning-Malmberg trap in a way that preserves the two-dimensional (2D) drift dynamics [1,2]. The behavior of the electrons in this regime is directly analogous to that of the vorticity in a 2D inviscid, incompressible fluid [3], and so this technique allows one to study driven vortex dynamics using electron plasmas in the laboratory. Presented here are experiments in which thin electron filaments (i.e., $E \times B$ shear layers) are created using a strong external strain flow. Following this, the Kelvin-Helmholtz/Rayleigh instability of these filaments is studied both in the absence and in the presence of external strain flows. The data agree with a simple theoretical model in the linear regime [4]. Nonlinear behavior of the instability is characterized by a reduction of transport perpendicular to the filament as the external strain rate is increased. [1] N. C. Hurst, et. al., Phys. Rev. Lett. 117, 235001 (2016) [2] N. C. Hurst, et. al., J. Fluid Mech. 848, 256 (2018) [3] C. F. Driscoll and K. S. Fine, Phys. Fluids B 2, 1359 (1990) [4] D. G. Dritschel, et. al., J. Fluid Mech. 230, 647 (1991) [Preview Abstract] |
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GP10.00007: Theory of Collisional Relaxation of Temperature Anisotropy in a Strongly Magnetized One Component Plasma Louis Jose, Scott Baalrud Plasma kinetic theory typically assumes that the gyroradius is much greater than the Debye length. As a consequence, particles are unmagnetized within the microscopic collision volume. Since binary interactions depend only on the distance between particles, the interaction volume is spherically symmetric. Conversely, O'Neil et al have developed a kinetic theory for very strongly magnetized plasmas in which the gyroradius is less than the distance of closest approach. Here the particles are strongly magnetized within the collision volume. In this limit, the gyroradius is so small that the collision volume effectively maps to disk surfaces of a cylinder. Here, we consider an intermediate regime where the gyroradius is less than the Debye length but greater than the distance of closest approach. In this regime, the particles gyrate within the collision volume, but the mapping to the disk surface is not possible. We take the collision volume as a cylinder and scattering through both the cylindrical and circular surfaces are included. Screening is included by modelling interactions using the Debye Huckel potential. In this way, the theory transitions from the unmagnetized limit to the strongly magnetized limit. The theory is tested by comparing with molecular dynamics simulations of the temperature anisotropy relaxation of one component plasma. These results are relevant to ongoing experiments in ultracold and non-neutral plasmas. [Preview Abstract] |
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GP10.00008: First principle simulation of ultra-cold ion crystals in a Penning trap with Doppler cooling and a rotating wall potential Chen Tang, Dominic Meiser, John Bollinger, Scott Parker A direct numerical simulation of many interacting ions in a Penning trap with a rotating wall is presented. The ion dynamics is modelled classically. Both axial and planar Doppler laser cooling are modeled using stochastic momentum impulses based on two-level atomic scattering rates. The plasmas being modeled are ultra-cold two-dimensional crystals made up of 100’s of ions. We com- pare Doppler cooled results directly to a previous linear eigenmodes analysis. Agreement in both frequency and mode structure are obtained. Additionally, when Doppler laser cooling is applied, the laser cooled steady state plasma axial temperature agrees with the Doppler cooling limit. Nu- merical simulations using the approach described and benchmarked here will provide insights into the dynamics of large trapped-ion crystals, improving their performance as a platform for quantum simulation and sensing. [Preview Abstract] |
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GP10.00009: Long-time Confinement of Toroidal Electron plasma in SMARTEX-C Lavkesh Lachhvani, Sambaran Pahari, Rajiv Goswami, Prabal Kumar Chattopadhyay Upgradation of vacuum, steady state magnetic field followed by mitigation of instabilities have led to unprecedented confinement of toroidal electron plasmas with purely toroidal B field in a small aspect ratio partial toroidal trap (SMARTEX-C). The confinement time which extends into 10's of second betters the previous record with similar B field topologies by at least an order of magnitude and may improve further with B field and pressure. It may be noted that in such traps existing transport theory mainly due to magnetic pumping phenomena is supposed to have limited the confinement to much lower times. The experiments therefore appear to challenge the limits of confinement proposed theoretically. Detail scaling of confinement time, accompanying analysis and comparison with theoretical predictions along with possible reasons for discrepancies with theory will be discussed in this paper. [Preview Abstract] |
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GP10.00010: Experiments on exploring two-fluid plasma state produced by pure lithium ion and electron plasmas in the BX-U linear trap Haruhiko Himura, Shinji Sowa, Katsushiro Okada, Toshikazu Okada, Akio Sanpei, Meghraj Sengupta Since non-neutral plasmas can be relaxed into rotational thermal equilibria, we use the characteristic to explore two-fluid plasmas in laboratory plasmas. In the BX-U linear trap, a pure electron plasma has been produced in a negative potential well via a relaxation process of at most four independent electron filaments, while a pure lithium ion plasma is produced in a positive potential well by use of a beta-eucryptite. Those plasmas electrically non-neutral so that they inherently rotate in opposite directions perpendicular to B-field each other, owing to their different charge polarity. After they relax into each rotational thermal equilibrium, the ion plasma is translocated into the adjacent nested trap where the electron plasma has been trapped. At this moment, a two-fluid plasma state is thus created. Since the duration of the state can be varied, time evolutions of shapes of both ion and electron plasmas via the state can be observed in their images taken by an ICCD camera. Currently, we have accumulated data for cases where $n_{i} \quad = \quad n_{e}$/10. Actually, there seems to be some equilibria of both the lithium ion and electron plasmas even in the two-fluid plasma state, which may be related with the differential rotational equilibrium. In the meeting, we will show our latest data. [Preview Abstract] |
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GP10.00011: Simulation of solitons and mode coupling in Trivelpiece-Gould modes in a finite-length non-neutral plasma Grant Hart, Mitchell Clingo We use a 2-D (r-z) PIC code to model axisymmetric Trivelpiece-Gould oscillations in a finite-length pure-ion plasma. At high amplitude these modes undergo mode-conversion between low-lying modes. They also can produce nonlinear states that are similar in appearance to solitons propagating on top of the linear normal modes. When the underlying normal modes undergo conversion from one to another this affects the solition part of the state. This interaction can make one of the solitons disappear. The mechanism and parameters of this effect will be discussed. [Preview Abstract] |
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GP10.00012: Overview of recent results and next steps toward confined e+/e- pair plasma experiments E. V. Stenson, J. Horn-Stanja, M. R. Stoneking, U. Hergenhahn, S. Ni\ss l, A. H. Card, N. Belmore, M. Singer, T. Sunn Pedersen, M. Singer, M. Dickmann, C. Hugenschmidt, S. K\"onig, L. Schweikhard, J. R. Danielson, C. M. Surko, H. Saitoh The trapping of small-$\lambda_D$ electron-positron plasmas would enable novel laboratory studies of "pair plasmas", predicted to exhibit significant differences from electron-ion plasmas. To enable experimental tests of some of these predictions, the APEX (A Positron Electron eXperiment) collaboration is working toward the simultaneous confinement of electrons and positrons at plasma densities in toroidal magnetic traps. Experiments within the collaboration encompass a number of parallel efforts. These include development of new settings of the intense, reactor-based cold positron source NEPOMUC; construction and installation of several Penning-Malmberg traps, including a buffer gas trap system and a multi-cell trap; e+ and e- experiments in a prototype dipole trap based on a supported permanent magnet; and the design of two tabletop-sized, superconducting devices for confining the pair plasma: a levitated dipole and an optimized stellarator. This overview will report on the latest results and upcoming developments from the collaboration. [Preview Abstract] |
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GP10.00013: A Buffer-Gas Positron Trap for the NEPOMUC High-Flux Positron Facility. J. R. Danielson, E. V. Stenson, S. Ni{\ss}l}, C. L. Manson, S. Ghosh, C. Hugenschmidt, T. Sunn Pedersen, C. M. Surko The goal of the {\emph APEX} collaboration is to study electron-positron pair plasma phenomena in a superconducting levitated magnetic dipole. This is made possible by exploiting the NEPOMUC high-intensity positron beam located at the FRM-II high-flux research reactor at the Technical University of Munich, which can provide a positron current up to $10^9 {\rm s}^{-1}$. The pair plasma experiment, however, requires pulses of positrons with $N > 1 \times 10^{10}$. This requires conversion of the DC NEPOMUC beam into a pulsed beam and necessitates the use of a positron trap and accumulator. Here we describe plans to reconfigure and upgrade an existing buffer-gas trap (BGT) for installation in the NEPOMUC beamline to provide the large positron pulses required for the pair plasma experiment and useful for other experiments at NEPOMUC as well. We present a mechanical design that accommodates the severe space requirements of the beamline. The traps will be inserted close to large amounts of iron radiation shielding. Magnetic modelling and a plan for correction coils to maintain (the required) uniform fields in the positron traps will be discussed. The results of using electron plasmas to test the BGT before the upgrade will also be presented. [Preview Abstract] |
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GP10.00014: Numerical Studies of Accumulation and Improved Confinement of Positrons in APEX S. Nissl, E.V. Stenson, J. Horn-Stanja, U. Hergenhahn, T. Sunn Pedersen, H. Saitoh, C. Hugenschmidt, M. Singer, M. Stoneking, J.R. Danielson The APEX (A Positron-Electron Experiment) collaboration has the goal to create a magnetized low temperature electron-positron plasma in a magnetic dipole trap. Such a plasma, also called pair plasma, is predicted to have unique characteristics and excellent stability properties due to the equal masses of the participating species. Because the biggest obstacle to producing a low-Debye-length plasma is the number of available positrons, a prerequisite is an efficient positron injection scheme. After multiple iterations of the electrode structure of the experiment and optimization of the applied biases, is has been shown that up to 100\% of the NEPOMUC positron beam can indeed be injected. Simulations have shown that the longest measured confinement times (\textgreater 1s) are limited by positrons hitting the plates that are used to drift-inject the positrons into the dipole field. Further simulations have not only shown that these plates can be shortened without sacrificing the injection efficiency but also suggest that stacking of multiple positron pulses is feasible. [Preview Abstract] |
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GP10.00015: Patch Potential Induced Particle Loss in an Antimatter Penning-Malmberg Trap Andrew Christensen, Joel Fajans, Jonathan Wurtele Even clean, gold plated, grounded metal surfaces inevitably generate anomalous electric fields. These fields are due to ``patch potentials,'' thought to be grain boundaries and charged oxide layers on the metal surfaces. These fields break the cylindrical symmetry of a Penning-Malmberg trap, and are investigated as the possible cause of observed anomalous antiproton escape, particularly in shallow traps with few antiprotons. An in-situ technique was developed for measuring the magnitude of these electric fields which involves using the patch potentials to move electron plasmas off-axis, and observing their subsequent orbit. We find fields on the order of tens of millivolts/cm. We present evidence that ultraviolet lasers used to probe the properties of antihydrogen atoms may strengthen these asymmetric fields. [Preview Abstract] |
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GP10.00016: Recent Progress on Antihydrogen Measurements From Advances in Plasma Physics Joel Fajans, Jonathan Wurtele The antihydrogen trapping rate in CERN's ALPHA experiment has gone from 0.1 antiatoms per hour to 300 antiatoms per hour in the last decade. The recent improvements in the trapping rate have come from improvements in the stabilization and manipulation of the pure-positron and pure-antiproton plasmas from which the antihydrogen is synthesized. In addition, ECR based magnetometry has reached the 1ppm level, increasing the accuracy of measurements on the antiatoms. Taken together, these advances have dramatically improved the speed at which certain measurements can be made; for example entire 1S-2S spectra can be taken in a day, a measurement that used to take weeks. The recent ability to have over 1000 antiatoms trapped simultaneously enables some measurements that previously could not be made at all; for instance, hyperfine measurements that would be too subject to magnet drifts if not taken over a few minutes. Further, laser cooling of antiatoms takes many hours; it would be impractical to cool just a few antiatoms at a time. This poster will review the recent plasma physics advances and some of the measurements they enable. The poster will also discuss nonlinear dynamics/plasma physics concepts that are central to the design of new experiments designed to measure the gravitational attraction of antiatoms to Earth. [Preview Abstract] |
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GP10.00017: Waveguide Mode Analysis of Cyclotron Resonance Heating and Precision Magnetometry Inside Penning-Malmberg Trap Jonathan Wurtele, Eugene Kur, Andrew Christensen, Joel Fajans, Eric Hunter Recent experiments at UC Berkeley [1] have demonstrated precise magnetometry inside a Penning-Malmberg trap by using the structure of resonant heating peaks of a plasma exposed to microwave waveguide radiation. The collection of heating peaks shows electron cyclotron resonance along with sidebands coming from plasma rotation and axial bounce motion. By carefully modeling the peak structure, we identify the cyclotron frequency, providing a measurement of the magnetic field to a precision of 30 parts per billion. In the model, we consider a single electron in the presence of trapping fields, DC self-fields, and electromagnetic modes of a cylindrical waveguide. Evaluating the Lorentz force law to leading order in the Larmor radius produces resonances at the mode frequencies seen in the experiments. [1] Christensen, A., et. al. 60th Annual Meeting of the APS Division of Plasma Physics. American Physical Society. Oregon Convention Center, Portland, Oregon. 8 Nov. 2018. Poster Presentation. [Preview Abstract] |
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GP10.00018: Simulation of ion-acoustic waves and their effect on collisions in an ion presheath. Lucas Beving, Scott Baalrud, Matthew Hopkins, Brett Scheiner It has been predicted that the ion flow in an ion presheath can excite ion-acoustic waves [1]. However, no direct measurement or simulation has yet been made. If these waves are excited, then previous work predicts that wave-particle interactions would significantly enhance the effective coulomb collision rate [1]. Increased collisionality could explain why ion and electron velocity distribution functions are measured with varying degrees of thermalization near the presheath-sheath boundary [2]. PIC simulations will be used to calculate the power spectrum of density fluctuations to determine if the ion-acoustic waves are driven unstable. The simulations also allow for the quantification of wave-particle effects by calculating time correlations between the distribution fluctuations and the electric-field fluctuations \textless $\delta f \delta $E \textgreater . Additionally, the neutral pressure will be varied to turn the instability on or off due to ion-neutral damping. [1] Baalrud S D and Hegna C C 2011 \textit{Plasma Sources Sci. Technol.} \textbf{20} 025013 [2] Yip C, Hershkowitz N and Severn G \textit{2015 Plasma Sources Sci. Technol.} \textbf{24} 015018 [Preview Abstract] |
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GP10.00019: Continuum kinetic simulations of the Weibel instability in plasma sheaths Kolter Bradshaw, Bhuvana Srinivasan When a temperature anisotropy develops across a plasma sheath, growth of the Weibel instability can cause a large magnetic field to develop exponentially from no initial field. Here, simulations of plasma sheaths are performed by directly evolving the ion and electron particle distribution functions with a continuum kinetic code. A steady-state sheath needs to exist for long enough time scales to allow sufficient nonlinear development of the Weibel instability and any associated particle trapping. Creating a particle balance towards a steady-state sheath profile is more straightforward in a particle-in-cell code than a continuum kinetic code. Here, studies of particle balance techniques in a continuum kinetic code will be presented. Furthermore, the effects of a collisional presheath on the Weibel instability will be studied. Using these simulations, the Weibel instability is examined in plasmas sheaths spanning different regimes relevant to nuclear fusion and spacecraft propulsion applications. [Preview Abstract] |
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GP10.00020: Plasma sheath and presheath in the presence of a finite plasma current into the wall Jun Li, Xianzhu Tang During tokamak disruptions, a current-carrying magnetized plasma will inevitably scrape off the first wall. A well-known example is the vertical displacement event (VDE). During such process a large plasma current can enter the wall, resulting in a halo current inside the vessel wall. More generally, in a broad range of low-temperature plasma applications, a plasma current can be driven by electrical biasing using end plates. Here we perform particle-in-cell kinetic simulations using the VPIC code to understand the detailed sheath and presheath physics when the wall-bound plasma current is substantial. The impact of this current on plasma profile (including the plasma potential), Bohm criterion and Bohm speed, sheath energy transmission, etc, is analyzed and elucidated from the simulation data. Of particular interest is the cathode region, in which the plasma sheath can greatly expand in size and the electron distribution are strongly non-Maxwellian. We will go over both steady-state cases and the dynamical evolution of a rapidly cooling plasma. This work was supported by the Office of Fusion Energy Sciences and Office of Advanced Scientific Computing Research through the base theory and SciDAC program. [Preview Abstract] |
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GP10.00021: Influence of collisions on sheath profile Yuzhi Li, Bhuvana Srinivasan, Xianzhu Tang, Jun Li When a plasma is in contact with solid boundaries, due to the greater mobility of electrons, a sheath forms in front of the wall. In the classical sheath model, many assumptions are made in order to obtain a simplified analytical model. The ion exit flow speed is characterized by the Bohm criterion, which is used as a boundary condition for simulations that do not resolve the sheath region. Generally, the Bohm speed equals the sound speed when the plasma is collisionless in the presheath region. However, at the tokamak edge, where sharp temperature gradients, strong inelastic collisions, and high recycling regime exists, the sheath profile may have some collisional dependence. Thus, a kinetic study of divertor sheaths will be helpful to understand the physics. In the present work, we developed a Monte Carlo Collision (MCC) package for vector particle in cell (VPIC) code, where elastic and inelastic interactions between charged particles and neutrals are included. Simulation results of plasma distribution profile and sheath parameters will be presented. [Preview Abstract] |
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GP10.00022: On the difference of plasma potentials measured by Langmuir probes and emissive probes in the presheath Peixuang Li, Noah Hershkowitz, Eugene Wackerbarth, AND Greg Severn It is conventional wisdom that Langmuir probes (LPs) do not work in sheaths, but are supposed to yield accurate plasma potentials ($\phi_{pl}$) in quasineutral plasma. Experiments that compared $\phi_{pl}$ measurements made by partially coated and uncoated LPs and cylindrical LPs with measurements made by emissive probes, were performed in low pressure unmagnetized argon discharges ($ 0.1\leq P_n \leq 1mTorr$), with $T_e$ and $n_e$ between 1 and 5 $eV$, and $\tento{1}{9}$ and $\tento{1}{10} cm^{-3}$, respectively. Presheaths were set up in the plasma using negatively biased electrodes. We used both grids and plates for the biased electrode. Results indicate that the emissive probe $\phi_{pl}$ measurements (in the limit of zero emission) were more negatve than LP measurements in the presheath, in the bulk, more positive. In the bulk, the difference was not proportional to $T_e$ as previously thought. In the sheath, most LP measurements did not go negative but rather became increasingly positive. Only the emissive probe measurements worked in the sheath. These differences are thought to be caused by the ion flow in the presheath toward the negatively biased electrode, characteristic of sheath formation. [Preview Abstract] |
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GP10.00023: Characterization of a metal vapor arc discharge Matthew Bledsoe, Jacob Simmonds, Yevgeny Raitses Arc discharges are used in many applications including welding, lighting, and the deposition of metal films. In this work, we characterize the initiation and operation of an arc discharge between a hollow cathode and an anode plate in a metal vapor environment and compare it to an arc discharge with the same setup in an argon environment. The discharges are operated at sub-Torr pressure. The hollow cathode contains an independently heated tungsten filament that acts as an initial source of electrons to aid in discharge initiation without requiring initial physical contact between the electrodes. The interaction between the arc plasma and the molten metal cathode is analyzed and discussed in this work. [Preview Abstract] |
(Author Not Attending)
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GP10.00024: Angular dependence measurements of Magnum-PSI plasmas using MAST-U flush mounted Langmuir probes Jack Leland, Sarah Elmore, Andrew Kirk, Hennie Van Der Meiden, James Bradley Langmuir probe measurements in tokamaks are difficult to interpret when operating at grazing angles of magnetic field incidence due to the effects of sheath expansion on the probe collection area. A probe array from MAST-U, with a novel tip design, was taken to Magnum-PSI to investigate whether temperature ($T_e$) and density ($n_e$) measurements could be performed at the low angles of incidence (0-10$^{\circ}$) possible in the MAST-U divertor. Incidence angle scans were made at a range of plasma parameters, with temperatures and densities measured using the Bergmann 4-parameter model and compared to the Thomson scattering system on Magnum-PSI. Comparisons are also made to simulations of the probe tip using particle-in-cell code SPICE. The measured plasma parameters suggest that the tip design is successfully mitigating the effects of sheath expansion at low angles of magnetic field incidence. [Preview Abstract] |
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GP10.00025: Self-Driven Liquid Lithium Target for High Heat Flux Handling Steven Stemmley, Matt Szott, David Ruzic Liquid lithium has gained interest as a plasma facing material because of its ability to handle large heat and particle fluxes, reduce edge recycling, and increase plasma performance. In the past, the Liquid Metal Infused Trench (LiMIT) concept, developed at the University of Illinois, has been shown to work well under fusion relevant conditions. Recently, this concept has been extended to create compact, self-flowing liquid lithium targets for beam-target fusion neutron generators, which can produce heat fluxes on the order of 10's to 100's of MW/m$^{\mathrm{2}}$. The liquid lithium surface acts as a self-healing plasma facing material and allows for the production of fusion relevant neutron spectra without tritium for materials testing by utilizing the Li-7(d,n) and D(d,n) reactions. Initial experiments, where a temperature gradient was imposed only via cooling, peak velocities of 16 \textpm 4 cm/s were observed. For heat fluxes greater than 10 MW/m$^{\mathrm{2}}$, COMSOL models have shown that sufficient velocities (\textasciitilde 70 cm/s) are attainable to prevent significant lithium evaporation. Expected yields of this system would be 10$^{\mathrm{7}}$ n/s for 13.5 MeV neutrons and 10$^{\mathrm{8}}$ n/s for 2.45 MeV neutrons. Future work will be aimed at experimentally demonstrating the viability of these targets under large heat loads and determining the neutron output of the system. The preliminary results and discussion will be presented. [Preview Abstract] |
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GP10.00026: Improving Heat Flux Handling of TEMHD-Driven Liquid Lithium PFCs Matthew Szott, Steven Stemmley, David Neil Ruzic Liquid lithium displays increasing promise as an alternative to solid plasma facing components (PFC) in fusion device applications. Liquid lithium PFCs reduce erosion and thermal stress damage, prolonging device lifetime, and have been shown to decrease edge recycling, reduce impurities, and enhance plasma performance. The Liquid Metal Infused Trench (LiMIT) concept developed at UIUC successfully demonstrates horizontal and vertical thermoelectric magnetohydrodynamic (TEMHD) flow of liquid lithium through metal trenches for use as a PFC. The LiMIT device has been successfully tested at UIUC and in devices around the world, including the HT-7 tokamak and the Magnum PSI linear plasma device, at heat fluxes up to 3 MW/m$^{\mathrm{2}}$. As peak heat flux increases, lithium dryout is possible due to strong thermal gradients. Maintaining a steady flowing liquid surface in the face of extreme heat fluxes is imperative for continued application of flowing liquid lithium PFCs. To that end, novel geometries are being developed that maintain the propensity for TEMHD flow while eliminating the risk of dryout. Results of computational modeling and experimental testing will be presented. [Preview Abstract] |
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GP10.00027: Distillation Column for Fuel Recovery From Liquid Lithium PFCs Cody Moynihan, David Ruzic Use of liquid lithium as a plasma-facing material may present benefits over the use of high-Z refractory metals. Lithium's high affinity for ionized species is one of the major concerns surrounding the use of lithium plasma-facing components. Recovery of the fuel species, specifically tritium, is required for the technological readiness of flowing lithium loops and lithium PFCs. A distillation column has been designed for the recovery of hydrogen isotopes from lithium melts at the Center for Plasma-Material Interactions at the University of Illinois. This prototype column utilizes thermal treatment of lithium and lithium-hydride melts to desorb hydrogen isotopes, along with condensation stages to collect the clean lithium. Previous work has shown that the column can recover hydrogen to balance the fuel wall losses in an ignited ITER scenario. Moving forward, the column needs to be integrated into a flowing lithium loop to demonstrate steady-state operation. Before designing a second generation column, the parameters that effect hydrogen desorption must be understood. This work gives insight into the effect of sample geometry and hydrogen concentration on hydrogen evolution from bulk lithium mixtures. These results guide the design of the next-generation column. [Preview Abstract] |
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GP10.00028: Boundary conditions for flowing magnetized plasmas at material surfaces. Adil Hassam, Ian Abel, Yi-Min Huang At the edge of confined magnetized plasmas, field lines intersect material surfaces. In tokamaks, field lines in the SOL strike diverter plates;~in smaller experiments (LAPD, MCX), field lines intersect conducting vacuum vessels or insulators.~ In these cases, ExB flows and MHD stability can be strongly affected depending on the material. We study flows in the vicinity of both conductors and insulators. The general boundary condition for all materials is that the tangential electric field is continuous across the surface. For perfect conductors, this results in ``line-tied'' boundary conditions on the magnetic field. Thus, the field line is effectively anchored to the immobile conductor. Effects that break frozen-in, such as resistivity, result only in very small flows. In the case of insulators, field lines are not frozen into the insulator, thus allowing freer flow. However, no-slip boundary conditions on the tangential mass flow require relative motion between ions and the ExB flow. This difference can be actualized via resistivity or by Hall physics. We will show that the latter gives more slippage. Finally, in the case of centrifugal plasmas, the exponential drop in density may effectively decouple the core and the material boundary, allowing freer flow. [Preview Abstract] |
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GP10.00029: Recent upgrades on BOUT++ framework Ben Zhu, Xueqiao Xu We report a few recent upgrades on BOUT++'s drift-reduced six-field turbulence model, in particular the flux-driven source option and the new Laplacian inversion solver which is able to capture $n=0$ mode evolution. With flux driven particle and energy sources from the core side, BOUT++ global simulations self-consistently evolve plasma backgrounds in transport time-scale runs. Moreover, in the previous simulations, due to numerical inaccuracy $n=0$ zonal mode is usually omitted when evaluating the electrostatic potential $\phi$ by inverting vorticity. Now a new Laplacian inversion solver that captures $n=0$ mode, first developed in Hermes-1 [1] -- a BOUT++ based five field model; and more recently in a four field shift circular model [2] has been extended and implemented in the six-field turbulence model, in both limited and diverted configurations. These improvements on BOUT++ is essential towards a fully self-consistent edge turbulence model capable of both transient (e.g., ELM, disruption) and transport time-scale simulations. [1] B. Dudson and J. Leddy, Plasma Phys. Control. Fusion \textbf{59} 054010 (2017) [2] H. Seto et.al, Phys. Plasma \textbf{26} 052507 (2019) [Preview Abstract] |
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GP10.00030: Transport and turbulence dynamics of ITER SSO plasmas in a grassy-ELM regime Xueyun Wang, Xueqiao Xu, Zeyu Li BOUT$++$ six-field two-fluid transport and turbulence codes are used to investigate the pedestal and SOL turbulent transport dynamics of ITER SSO plasmas. Starting from the initial SSO (Steady-State Operation) scenario profiles with q$_{\mathrm{95}}=$5.12, beta\textunderscore p$=$1.57, triangularity $=$0.48, we use BOUT$++$ transport code to evolve plasma parameters and radial electric field to steady state. These steady-state plasma profiles are used as initial input profiles to BOUT$++$ turbulent code, which are further evolved into steady-state turbulence. Simulation results show that under ITER SSO scenario, the most unstable toroidal mode numbers are at intermediate range n$=$15-20 in linear stage, and the peeling-ballooning mode is the most likely dominated instability. In non-linear stage, the instabilities evolve into grassy ELMs. These results are dramatically different from the ITER 15MA baseline scenario [1], in which ballooning modes mixed with drift-Alfven instability with high toroidal mode numbers dominate in the linear stage, which leads to a large Type-I ELM crash in the non-linear stage. Compared with ITER 15MA baseline scenario, the ITER SSO scenario has approximately one-order-amplitude reduction of divertor heat flux. Parameters that may influence the divertor heat flux width are under investigation. [1] Ze-Yu Li \textit{et al }2019 \textit{Nucl. Fusion }\textbf{59 }04601 [Preview Abstract] |
(Author Not Attending)
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GP10.00031: Impact of density on ELM behaviors in the EAST tokamak Fubin Zhong, Tao Zhang, Xueqiao Xu, Yumin Wang, Fei Wen, Mingfu Wu, Kaixuan Ye, Jia Huang, Gongshun Li, Haoming Xiang, Kangning Geng, Xiang Gao The ELM behaviors at different densities have been researched on EAST while other parameters remain the same. The ELM frequency increases gradually with density at constant heating power. And the amplitude of ELM declines along with the rise of the frequency. The relative decrease in stored energy at high density plasma is about 25{\%}, in comparison with the plasma at lowest density. And the confinement enhancement factor H$_{\mathrm{98}}$ decreases (about 40{\%}) with density. The evolutions of pedestal density profile relate to ELM bursts have been investigated in detail. It shows that the effect of ELM bursts on density profile is slight and just locates in a very narrow region near the pedestal top at high density plasma. Also the analysis of the MHD signature and power deposition characteristics associated with ELMs have been made. The ELM characteristics at different densities will be simulated using BOUT$++$ code and the simulation results will be compared with experimental measurements. [Preview Abstract] |
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GP10.00032: Simulations of ELM sizes and heat fluxes in an ELMy H-mode discharge on HL-2A Xiaoxue He, Tianyang Xia, Yue Liu, Xueqiao Xu As we know that the equilibrium profiles have directly impact on ELM size and heat fluxes. In this work we take seven profiles trying to find out the influence of the pedestal profiles on the ELM size and electron heat flux to the outer target. The equilibrium profiles from the ELMy H-mode HL-2A discharge {\#} 24953 are adopted as the initial condition in the original case, and six more equilibriums are constructed upon the original case to scan the pedestal height and position. The BOUT$++$ six-field two-fluid model is used to reproduce the peak of the electron heat flux to the outer divertor target during the ELM bursts. Results indicate that the ELM size increases with the higher pedestal for the larger pressure gradient, and it decreases because the enhancement of the local magnetic shear suppresses the curvature driving term when the pedestal is closer to the last closed flux surface. And the heat flux increases in both processes. Furthermore, theoretical analysis and the simulation results consistently present the heat flux $q_{\parallel e} $ is proportional to $n_{e0} T_{e0}^{\frac{3}{2}} $, which means that the heat flux is a fixed value as long as the term $n_{e0} T_{e0}^{\frac{3}{2}} $ remain unchanged. [Preview Abstract] |
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GP10.00033: Linear and nonlinear Analyses of Kinetic Ballooning Modes (KBM) in High-beta Pedestal Plasma . Li Pengfei, Xu Xueqiao We present the 3D simulations of edge plasma instabilities using a set of shifted circular geometry equilibria based on the gyro-Landau-fluid (GLF) model in the BOUT$++$ framework. The initial realistic equilibria are generated by a global equilibrium solver CORSICA, in which the Shafranov shift, elongation effects and bootstrap current are scanned. The linear growth rate spectrum shows that with the consideration of bootstrap current the growth rates of instabilities are smaller. The unstable region decreases compared to the region neglecting the bootstrap current, and it shifts to the direction of low toroidal mode number because of the kink drive of the edge bootstrap current. Considering the three kinetic effects, the FLR effect is the main stabilizing effect, the Toroidal Resonance has weak impact on KBM and the Landau Damping broadens the growth rate spectrum. In the nonlinear simulation, the saturated state is observed. The heat and particle transport are calculated under different $\beta $ and different fractions of bootstrap current including all the three kinds of kinetic effects. The threshold of the instability is also checked. When the system comes into the nonlinear phase there are big energy transport events. Then the system enter turbulence stage, in which the prturbation level decreases. This part of work is in progress. More results will be shown soon. [Preview Abstract] |
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GP10.00034: Implementation of plasma turbulence model for tokamak scrape-off layer and divertor M.V. Umansky, B.I. Cohen, J.R. Myra Tokamak edge turbulence model SOLT3D for scrape-off layer and divertor is motivated by the older model SOLT [1], which in spite of its relative simplicity has produced a number of encouraging results. The original SOLT model solves for time-evolution of plasma density, temperature, and electrostatic potential in a 2D plane perpendicular to the magnetic field B in the outboard mid-plane region of the tokamak; the dynamics along the magnetic field is approximated by a parallel scale length parameter. The new SOLT3D model, being developed in the BOUT$++$ framework [2], roughly follows the original SOLT design but includes the dimension along the magnetic field line and solves for parallel variations of plasma fields and for electron dynamics along the magnetic field line. The model supports linear instabilities relevant to SOL turbulence: drift-resistive-ballooning mode instability driven by the magnetic curvature and the radial gradient of plasma pressure, and the conducting-wall mode instability driven by the end-plate sheath boundary conditions and radial gradient of plasma temperature. Testing of the model includes verification of linear dispersion relations and some known nonlinear solutions. [1] Russell et al., Phys. Plas. \textbf{22}, 092311 (2015); [2] Dudson et al. Comp. Phys. Comm. \textbf{180}, 1467--1480 (2009). [Preview Abstract] |
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GP10.00035: The path toward self-consistent, kinetic simulation of detached divertors George Wilkie, Daren Stotler, Michael Churchill, Robert Hager, Julien Dominksi, Seung-Hoe Ku, Choong-Seock Chang Achieving predictive theoretical capability for divertor detachment is complicated by several outstanding challenges. Among these include: the plasma turbulence and transport across the separatrix, in the scrape off layer, and in the private flux region; the role of impurities; and robust kinetic predictions of neutral atoms and molecules. We simulate the behavior of neutrals with the Monte Carlo particle code DEGAS2, coupled to the XGC edge gyrokinetic code for the plasma dynamics. Ensuring energy conservation in the coupling between these computational models will be critical. The lower electron temperatures of the detached divertor mean that volume recombination must be included as a source of neutrals and sink of plasma ions and electrons. The low temperatures also result in longer lifetimes for molecular hydrogen. The associated higher neutral densities increase the importance of neutral-neutral collisions and radiation trapping. In this poster, we will present a roadmap outlining how these key challenges are to be addressed and the solutions consolidated into a unified theoretical framework for predicting divertor performance. [Preview Abstract] |
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GP10.00036: Simulation of Edge Plasma Transport with Transient Impurity Sources due to Dust Influx Roman Smirnov, Sergei Krasheninnikov Accidental dust injection into edge plasmas of magnetic fusion devices can occur due to damage of plasma facing components by high transient heat loads or due to mobilization of previously accumulated dust from plasma exposed surfaces. Such events can produce large transient impurity sources in fusion edge plasmas caused by ablation of the dust grains. As the dust itself is mobile due to various forces acting on the grains in the plasma, impact of the dust-produced impurities on edge plasma depends on the dust transport. To simulate the dust, impurity, and plasma transport in tokamak edge self-consistently we use coupled UEDGE-DUSTT code in time-dependent mode. In this work we investigate magnitudes of perturbations induced by tungsten dust injection in ITER-like H-mode edge plasma and the following plasma recovery dynamics for various amounts of dust injected. The impurity fluxes to the plasma core and associated reduction of the pedestal temperatures are simulated, as well as characteristic times of the pedestal recovery following dust injection. We also investigate impact of transient dust influx on divertor operation. The simulations demonstrate that injection of dust in divertor plasma can trigger asymmetric detachment/re-attachment of inner-outer divertor plasmas, which may not spontaneously recover after the dust injection event. [Preview Abstract] |
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GP10.00037: SOLPS-ITER simulations of ITER divertor operation with high flux expansion J.M. Canik, X. Bonnin, R.A. Pitts, J.D. Lore, Y. Gribov, A.A. Kavin, V.E. Lukash, R.R. Khayrutdinov The ITER divertor has been designed based on a large set of 2D fluid plasma/kinetic neutral transport simulations using SOLPS [1], identifying a window in divertor neutral pressure with heat fluxes reduced to acceptable levels. Recently a new magnetic equilibrium has been calculated with higher divertor flux expansion, with the magnetic field angle at the outer strike point reduced from 2.7$^{\mathrm{o}}$ in the standard configuration to 1.5$^{\mathrm{o}}$ (not accounting for toroidal shaping of the divertor tiles). SOLPS-ITER [2] simulations have been performed in this new geometry to evaluate the impact on divertor operations. The calculations indicate a reduction in the peak heat flux to the outer target by \textasciitilde 40{\%}, consistent with the increase in flux expansion. The simulations also indicate that heat flux mitigation can be achieved at reduced divertor neutral pressure compared to the standard configuration. The potential benefits of this configuration for power handling, including the prospects for reducing heat flux with narrow scrape-off-layer widths, will be presented.[1] Kukushkin \textit{et al.}, Nucl. Fusion \textbf{43} (2003) 716. [2] Wiesen \textit{et al.}, J. Nucl. Mat. \textbf{463} (2015) 480. [Preview Abstract] |
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GP10.00038: Investigation of Divertor Heat Flux Width with Different Upstream Pedestal for CFETR. Zeyu Li, Xueqiao Xu, Vincent S. Chan, Yiren Zhu, Jiale Chen, Xueyun Wang Investigation on the turbulent transport dynamics in Scrape-off-layer (SOL) and divertor heat flux width prediction is performed in BOUT$++$ simulation [1]. Further study on the impact of different pedestal structure on China Fusion Experimental Test Reactor (CFETR) divertor heat flux width is performed using BOUT$++$ six-field turbulence and transport module. Different scenarios of CFETR, such as steady state operation and hybrid scenario, are investigated separately to identify relation between upstream pedestal condition and downstream divertor heat flux width. Furthermore, the study of CFETR R7.2m 1GW hybrid operation is carried out by changing the pedestal height and width meanwhile keeping the core plasma profile unchanged. Higher pedestal is typically more unstable, which leads to larger turbulent flux transported crossing the separatrix into the SOL. This leads to the broadening of the SOL perpendicular heat decay width. Pedestal structure is important in determining the divertor heat flux width. It is found in CFETR that grassy ELM operation might be favorable for broadening the divertor heat flux width. Further study needs to be made to optimize the divertor heat load and confinement. [1] Zeyu Li et al. 2019 Nucl. Fusion 59 046014. [Preview Abstract] |
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GP10.00039: Influence of the inverse sheath on divertor plasma performance in tokamak edge plasma simulations Rebecca Masline, Roman Smirnov, Sergei Krasheninnikov The 2-dimensional multifluid tokamak edge plasma code UEDGE is used to assess the effect of the newly proposed "inverse sheath" at the divertor plate on plasma detachment. The inverse sheath is a monotonic positive potential sheath at the plasma edge near the wall that is different than the space-charge-limited sheath, and results from a combination of strong thermionic emission from material surfaces with ion cooling due to charge exchange and other collisions. Plasma dynamics shown in kinetic simulations of the inverse sheath regime indicate extreme cooling at the plasma edge. This has been proposed as a novel mechanism to promote divertor detachment through mitigating plasma-surface interactions via electron cooling precipitated by thermionic emission from tungsten divertor plates. We use a DIII-D-like geometry to model the detachment of a plasma using different wall boundary conditions that model both the "conventional" sheath and the "inverse" sheath regimes. Our simulations indicate that the inverse sheath conditions result in different plasma dynamics that modify detachment transition conditions when compared to the conventional sheath regime. The extension of the inverse sheath regime to practical applications in tokamak divertors is discussed. [Preview Abstract] |
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GP10.00040: Enhanced non-coronal carbon radiation facilitating detachment in DIII-D X-divertor: a SOLPS-ITER study Zhongping Chen, Mike Kotschenreuther, Swadesh Mahajan The X-divertor(XD) in DIII-D has demonstrated, through simulation, abilities to boost carbon radiation and hence to achieve similar detached conditions (target temperature < 2eV within the first heat flux scrape-off-layer width) with less than half the upstream density of the standard divertor(SD). Simulations suggest that the XD geometry vastly expanded the carbon line radiation volume near the target through multiple mechanisms that enhance the non-coronal effect which strongly increases the carbon emissivity in 10-30 eV range. We will provide a few physics pictures supported by simulation data to demonstrate the critical role of the ultra shallow field angle in the XD in relevance to carbon radiation. [Preview Abstract] |
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GP10.00041: Investigation of detachment in Double-Null configurations in the TCV tokamak. Olivier Fevrier, Stefano Coda, Christian Theiler, Hugo de Oliveira, Basil P. Duval, Benoit Labit, Roberto Maurizio, Holger Reimerdes, Andrew Thornton Safe power exhaust in future fusion reactors will require, at the least, partially detached divertor operation. Alternative divertor configurations could facilitate access to such a regime. In particular, Double-Null (DN) configurations allow splitting most of the exhaust power between two outer legs and potentially reaching higher level of radiation thanks to the presence of two X-Points. In this work, we investigate the detachment on TCV in DN geometries with different outer leg positions, including a double Super-X configuration. Preliminary results show that detachment onset, as measured by the movement of the CIII front away from the targets, happens at lower density than in equivalent Lower Single-Null (LSN), while, for the same line-averaged density \textless ne\textgreater , a higher fraction (between 10{\%} and 50{\%}, depending on shape and \textless ne\textgreater ) of the input power is radiated. However, this enhanced accessibility of the detached regime appears to come at the price of a reduced detachment window. The double-null configurations disrupt at lower (between 10{\%} - 20{\%}) line-averaged densities than the equivalent LSN, after a quick movement of the CIII front towards the X-Point. [Preview Abstract] |
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GP10.00042: Two-Photon Absorption Laser Induced Fluorescence on Proto-MPEX Thomas Steinberger, Jacob McLaughlin, Theodore Biewer, Earl Scime Two-photon Absorption Laser Induced Fluorescence (TALIF) is a non-perturbative spectroscopic technique that provides direct measurements of the temperature, bulk flow, and absolute density of neutral hydrogen in fusion-class plasmas. Recently, TALIF has been added to the suite of diagnostics on the Prototype Material Plasma Exposure eXperiment (Proto-MPEX) at Oak Ridge National Laboratory. Since TALIF is typically used to interrogate energetic ground state transitions, high intensity ultra-violet (UV) light is required. Here we generate 4 mJ, 8 ns pulses of 205 nm light with a Sirah Cobra-Stretch dye- laser. Laser light is injected in Proto-MPEX through high UV transmission sapphire vacuum windows. Implementation of TALIF on Proto-MPEX necessitates an injection beam path length of $\sim $ 20 meters. We present measurements in Proto-MPEX using both free space injection and fiber coupled injection. For both beamline options, measurements in krypton and xenon calibration gasses as well as the targeted neutral deuterium atoms were obtained confocally. Neutral deuterium measurements were made upstream of the helicon source region of Proto-MPEX for a range of D2 fuel gas pressures. [Preview Abstract] |
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GP10.00043: PHAse Space MApping (PHASMA) Experiment Earl Scime, Cuyler Beatty, David Caron, Regis John, Matthew Lazo, Olivia Lehki, Jacob McLaughlin, Michael Moran, Mitchell Paul, Ethan Scime, Thomas Steinberger, Derek Thompson A new experiment, called the PHAse Space MeAsurements (PHASMA), features laser induced fluorescence diagnostics for ion measurements, Thomson scattering diagnostics for electron velocity distribution function measurements, and a microwave scattering system for turbulence measurements. PHASMA is designed to enable the direct measurement of ion and electron vdfs in space-relevant plasma phenomena including reconnection, shocks, and turbulence. To create the conditions necessary for different experimental regimes, PHASMA will employ a 5 kW, steady-state helicon source capable of generating variable-density background hydrogen, helium, and argon plasmas with controllable plasma pressure (relative to the magnetic pressure), collisionality, and azimuthal flow shear. Reconnecting flux ropes will be created through the merging of discharges from two pulsed plasma guns. Measurement objectives include fully 3D ivdf measurements in a 3D volume with spatial resolution less than 0.2 cm (the expected electron skin depth in PHASMA) and similarly resolved evdf measurements, in all three laboratory coordinate axes. We present initial ion velocity distribution function measurements, Langmuir probe measurements, and electromagnetic fluctuation measurements in PHASMA. [Preview Abstract] |
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GP10.00044: HELIOS Diagnostic for the Prototype Material Plasma Exposure eXperiment Holly Ray, Theodore Biewer, Juan Caneses, Nischal Kafle, Jorge M. Muñoz Burgos, Ezekial Unterberg, Oliver Schmitz A helium line-ratio spectral monitoring (HELIOS) diagnostic has been implemented on Oak Ridge National Laboratory's (ORNL) Prototype Material Plasma Exposure eXperiment (Proto-MPEX). A Filterscope and a 1 meter McPherson spectrometer are used to measure the intensity of three separate helium lines: 667.9 nm, 706.53 nm, and 728.0 nm. The open magnetic geometry of Proto-MPEX allows for direct comparison between the derived n$_{\mathrm{e}}$ and T$_{\mathrm{e\thinspace }}$values to nearby double Langmuir probes (DLP) and Thomson scattering (TS) measurements. Preliminary HELIOS measurements give T$_{\mathrm{e}}$ values of 5 - 8 eV and density values of 2.71e18 m$^{\mathrm{-3}}$ -- 9.34e18 m$^{\mathrm{-3}}$, which are consistent with edge Thomson Scattering T$_{\mathrm{e}}$ and n$_{\mathrm{e}}$. The ability to measure the core density (\textgreater 2.00e19 m$^{\mathrm{-3}})$ and low temperature (\textless 5 eV) regime of Proto-MPEX with HELIOS is of great interest; however, the gas puff penetration into the plasma column is limited by the n$_{\mathrm{e}}$ of the core. The gas puff was increased until signs of core gas penetration were observed. The data suggest that the high neutral helium density introduced to the chamber during the gas puff is causing radiation trapping of the ground state singlet transitions: 2$^{\mathrm{1}}$P$\to $3$^{\mathrm{1}}$D (667.9 nm) and 2$^{\mathrm{1}}$P$\to $3$^{\mathrm{1}}$S (728.0 nm). Correcting for this brings the HELIOS measured n$_{\mathrm{e}}$ and T$_{\mathrm{e}}$ in the core of the discharge into agreement with DLP measurements. [Preview Abstract] |
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GP10.00045: Assessing Digital Holography for potential deployment on Proto-MPEX and DIII-D E.G. Lindquist, T.M. Biewer, C.D. Smith, C.E. Thomas A 3D IR digital holography diagnostic for \textit{in situ}, real-time plasma facing component (PFC) measurements in development at Oak Ridge National Laboratory. Erosion measurements are typically \textit{post mortem} because \textit{in situ} diagnostics are still being developed. With ITER divertor cooling channels \textasciitilde 8 mm below the surface, the \textit{in situ} capabilities of digital holography (DH) can provide measurements during and after discharges to inform operations when divertor erosion may breach the coolant. The benchtop system at ORNL could be put on a linear plasma device at ORNL for proof-of-principle measurements on C and W targets to quantify Proto-MPEX erosion rates and to determine DH limits of detection. Then DH could be deployed on DIII-D for a first-of-kind \textit{in situ} measurements to demonstrate the DH technique in a fusion plasma environment. Implementation of DH would provide first \textit{in situ} measurements and advance DIII-D PMI goals. The DIII-D divertor strike point width, $\lambda q_{\vert \vert }$ is calculated to be 1-2 mm and DH can monitor \textasciitilde 1 cm$^{\mathrm{2\thinspace }}$region. Hence, DH will be able to measure the net erosion profile at and around the strike point. This poster will present an assessment of the requirements for staging the DH diagnostic on the Proto-MPEX linear device and subsequently on the DIII-D tokamak. [Preview Abstract] |
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GP10.00046: Effects of Vibration on Dual-Laser Digital Holography for In-Situ PFC Surface Characterization C. E. Thomas, C.D. Smith, T.E. Gebhart, A.D. Greenhalgh, E.G. Lindquist, X. Ren, T.M. Biewer The ability to make real-time in-situ measurements of surface changes of Plasma Facing Components (PFCs) in magnetic fusion devices would be a considerable advantage to fusion researchers. A digital holography device is under development at Oak Ridge National Laboratory for in-situ surface characterization of PFCs. A brief review of single laser and dual laser techniques, along with measurements of targets with known surface variations, will be presented. Additionally, ex situ measurements of targets exposed in the Proto-MPEX linear plasma device, and also an initial target exposed to a high power Electro-Thermal arc plasma device, will be shown. The relative vibration of the diagnostic table and the plasma target plays a large role in determining the measurement resolution of the technique, as the diagnostic system transitions from ex situ to in situ characterization of PFCs during plasma exposure. [Preview Abstract] |
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GP10.00047: Characterizing Nanomaterial Response for sub-100 ps X-ray Scintillation Michael Sherburne, Tod Laurvick, Larry Burggraf, Ian Bean, Patrick Crandall, Minzhen Du, Colin Adams, Eric Burke, Marek Osinski, Arjun Senthil, Dominic Bosomtwi, Shruti Gharde, Gema Alas, Sergei Ivanov, Victor Klimov, Igor Fedin, Thomas Weber Measuring sub 100 ps quantum dynamics of x-rays in the 1 -- 100 keV range is a need shared by many experimental programs and is beyond the capability of current techniques. We are investigating the feasibility of using colloidal quantum dots (CQDs) as a fast x-ray scintillator by functionalizing then experimentally characterizing the x-ray excitation and decay response of 13 varieties of CQDs. All variants will be evaluated for their stability, temporal decay characteristics. Functionalization will be using two fabrication methods; loading the nanomaterials into a polymer and drawing them into a microstructured photonic crystal fiber. Radiation response will be characterized at typical laboratory conditions (e.g, 23$^{\mathrm{o}}$C). Based on previous studies we expect that CdSe nanoplatelets and CsPbCl$_{\mathrm{3}}$ perovskite nanoplatelets will exhibit temporal resolutions under 100 ps. Photoluminescence decay will also be determined for the additional 11 nanomaterials when excited by a pulsed x-ray source. [Preview Abstract] |
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GP10.00048: Ion Beam Analysis of Deuterium and Helium Retention in Li-coated Plasma-Facing Components F. Bedoya, K.B. Woller, D.G. Whyte Plasma-Facing Components (PFCs) influence plasma behavior in magnetic confinement devices, and conditioning of the plasma-facing surfaces is highly relevant to optimize their performance. The deposition of low-Z thin films on PFCs has been successful in decreasing contamination and radiation losses from the plasma. In particular, the use of Li coatings on PFCs has shown encouraging results in several tokamaks. The outstanding capability of Li to bind D and O allows low recycling walls and flatter temperature profiles, reducing edge instabilities. To elucidate the complex relationship between plasma, coating and substrate, we studied thin Li coatings ($<$100$\mu$m), vapor-deposited in-situ on graphite and TZM using Ion Beam Analysis (IBA). The samples were irradiated with plasma fluences of up to 10$^{24}$ m$^{-2}$ of either D or helium (He). The exposures were performed with relatively low ion incident energy (~15 eV) and fluxes in the range of 10$^{21}$-10$^{22}$m$^{-2}$s$^{-1}$. Lithium-coated graphite and TZM had increased deuterium retention, with D/Li greater than 1.0 on the graphite, in contrast with the maximum D/C=0.1 on bare graphite and near zero retention observed with bare TZM. These results support the idea that Li PFC are a promissory solution to the PMI issue. [Preview Abstract] |
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GP10.00049: Surface chemistry of conditioned plasma facing components in the National Spherical Torus Experiment Upgrade (NSTX-U) Heather Sandefur, Hanna Schamis, Jean Paul Allain The National Spherical Torus Experiment Upgrade (NSTX-U) has been used to investigate the effect of wall tile surface conditioning on plasma performance during operation. Previous campaigns have demonstrated the enhanced suppression of edge-localized modes (ELMs) and reduced divertor recycling when reactor walls were conditioned with lithium. Performance was also improved when wall tile conditioning via boronization was performed, and high confinement (H-mode) operating conditions were routinely achieved during operation after conditioning. In order to better understand the impact of surface conditioning and subsequent plasma exposure on wall materials, cored sample of the exposed NSTX-U wall tiles were obtained and their surface chemistry was analyzed. The Ion-Gas-Neutral Interactions with Surfaces (IGNIS) system was used to analyze boronized NSTX-U samples using x-ray photoelectron spectroscopy (XPS) and low energy ion scattering spectroscopy (LEISS). The near-surface chemical composition of the samples was determined using time of flight secondary ion mass spectrometry (TOF-SIMS) and Rutherford backscattering spectrometry (RBS). In addition, variations in surface morphology in each tile were observed using atomic force microscopy and Keyence profilometry. [Preview Abstract] |
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GP10.00050: Deuterium and helium behaviour and morphology effects in porous tungsten/liquid lithium plasma facing component system Aveek Kapat, Felipe Bedoya, Kevin Woller, Jean Paul Allain The focus of this work is to develop a material system that has the favourable bulk properties of W while changing the interface material between the impinging plasma and the structural tungsten. A porous W-liquid metal hybrid system is manufactured and tested for bulk thermomechanical properties as well as plasma surface interactions such as He-induced morphology, vapor shielding and H isotope inventory. Porous W-substrates have been fabricated via SPS and subjected to He and D plasma exposures in both Magnum-PSI and DIONISOS. D inventory in porous W substrates with 1$\mu $m Li deposited and melted is quantified with \textit{in-operando }NRA during 60eV$^{\mathrm{\thinspace }}$D$^{\mathrm{+}}$ plasma exposure to a fluence of 2E24 m$^{\mathrm{-2}}$ with the retention behaviour relative to Li percolation quantified with \textit{in-operando} He ERD as well as \textit{post-mortem }SIMS. Morphology resistance is tested by SEM examination of porous W samples exposed in Magnum-PSI to 1E26m$^{\mathrm{-2}}_{\mathrm{\thinspace }}$D, 5E26m$^{\mathrm{-2\thinspace }}$He, and 5E26m$^{\mathrm{-2}}$ He with 1ms pulses between 0.1-0.5 GW m$^{\mathrm{-2}}$ @ 0.1 Hz, all biased to 33eV and heated to 1000\textdegree C. The effects of porous structure on the implantation and retention of He/D are studied on Magnum-PSI exposed samples with \textit{post-mortem }SIMS. [Preview Abstract] |
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GP10.00051: \textbf{Producing a Coherent Liquid Lithium Droplet Injector and Developing Future Liquid Metal Injectors} Daniel O'Dea, Daniel Andruczyk, Andrew Shone Injection of granular lithium pellets, with a diameter under 1mm, into NSTX-U has been shown to effectively pace ELMs by simulating higher frequency lower power ELMs. However, this current granular injector produces randomly spaced and sized droplets which can cause several miss-hits. To this end researchers at University of Illinois (UIUC) are developing a Liquid Lithium Droplet Injector (LLDI,) using a chamber that is on loan from Princeton Plasma Physics Lab, with the aim of producing a coherent stream of liquid metal droplets for injection into fusion devices. Various methods for stimulating coherent breakup have been investigated. These include: using a vibrating rod to stimulate an instability in the capillary jet to stimulate breakup at a set frequency, charging droplets to ensure they do not coalesce and finally looking at JxB injection (instead of gas back pressure) to stimulate droplet break-up. Concurrently to this new injector designs are being explored, with the designs being influenced by the results from the LLDI. These new designs will be tailored for purpose, i.e. different drop sizes, spacing and flow rates, and should offer a safe and reliable solution for liquid metal injection. [Preview Abstract] |
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GP10.00052: Atomistic Simulation of Radiation Resistivity of Tungsten-based High Entropy Alloys Proposed as Plasma-facing Materials Muhammad Abdelghany, Ashley Zhao, Jean Paul Allain High Entropy Alloys (HEAs) are proposed as potential plasma-facing materials for fusion reactors for their high structure stability and radiation resistivity. An atomistic simulation study of surface changes due to plasma irradiation of W-based HEA, W-Ta-Cr-V, has been performed. The main challenge is to develop a proper surface potential of this alloy, that can capture the properties of the system especially at the surface in such far from equilibrium condition. A genetic-algorithm is used to find an optimized Embedded Atom Method (EAM) potential. This algorithm combines the single-element potentials to generate binary potentials, which are combined to form multi-component alloy potentials. The main idea is to optimize the binary potential of each pair such that the Molecular Dynamics (MD) calculations of selected surface properties obtained using these potentials approach the Density Function Theory (DFT) calculations of the corresponding properties of these pairs. This optimized potential is used to do an MD simulation of the surface changes. DYNAMIX, a binary collision approximation code, is used to understand ion-surface interactions. Surface characterization is being done using IGNIS, in-situ facility, to experimentally validate this computational study. [Preview Abstract] |
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GP10.00053: Studies of Statistical Time Lag for High-Voltage DC Flashover and Punch-Through in Pressurized Air and Oil for Pulsed Power Applications I.A. Bean, C.S. Adams, T.E. Weber Surface flashover and punch-through of dielectric materials are primary limiting factors for high power, low-inductance pulsed-power-driven experiments. We are investigating the formation of these discharges for varying dielectric materials and geometries common in pulsed power systems. The onset of these discharges in high-voltage DC environments have a strong dependence on the statistical time lag to formation which has not been thoroughly investigated in available literature. The results presented here detail the time dependence of these failure modes under a variety of conditions, allowing for a more accurate prediction of their inception. [Preview Abstract] |
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GP10.00054: Tokamak edge modeling with the 5D full-F continuum gyrokinetic code COGENT Mikhail Dorf, Milo Dorr COGENT is a full-F continuum gyrokinetic code being developed by the Edge Simulation Laboratory for edge plasma modeling. The code is distinguished by the use of a high-order finite-volume (conservative) discretization combined with arbitrary mapped multiblock grid technology. Our recent work is focused on development of a 5D version of the COGENT code to model edge plasma turbulence. A numerical algorithm utilizing locally field-aligned multiblock coordinate system has been developed to facilitate simulations of highly-anisotropic microturbulence in the presence of a strong magnetic shear. In this approach, the toroidal direction is divided into blocks, such that within each block the control cells are field-aligned and a non-matching (non-conformal) grid interface is allowed at block boundaries. The algorithm has been implemented and tested for various magnetic geometry configurations including a single-null geometry. Special emphasize is placed on development of reduced fluid electron models and the corresponding models for self-consistent variations of electrostatic potential spanning both open and closed field lines. [Preview Abstract] |
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GP10.00055: Higher Order Fluid Moments for the Gyrokinetic MHD Equations including the Finite-Larmor-Radius Effects W. W. Lee W. W. Lee, Princeton Plasma Physics Laboratory, Princeton, NJ 08540 — As shown earlier, gyrokinetic MHD equations including the Finite-Larmor-Radius effects [1] can produce some interesting physics in the steady state of the tokamak/stellarator experimrnts such as island formations [2] and force-free configurations [3]. Here, we will present the extension of these equations by including all the relevant fluid quantities such as vorticity, parallel current, pressure and heat flux. The numerical as well as the conservation properties of these equation will also be discussed. *This work is partially supported by DoE grant to PPPL and partially supported by Social Security and TIAA funds. [1] W. W. Lee, Phys. Plasmas {\bf 23}, 070705 (2016); [2] W. W. Lee, S. R. Hudson and C. H. Ma, Phys. Plasmas {\bf 24}, 124508 (2017); [3] W. W. Lee and R. B. White, Phys. Plasmas {\bf 24}, 081204 (2017); Phys. Plasmas {\bf 25}, 054702 (2018); [4] W. W. Lee and R. B. White, Phys. Plasmas {\bf 26}, 040701 (2019) [Preview Abstract] |
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GP10.00056: Total-f gyrokinetic edge physics from XGC in realistic diverted geometry C.S. Chang Tokamak edge plasma is in a non-Maxwellian state, dominated by multiscale multi-physics kinetic dynamics. Pedestal has steep gradient with its width comparable to the ion orbit width. Neoclassical, micro-turbulence, MHD/fluid type long wavelength modes, and neutral particle physics are all coupled together in complicated geometry that includes divertor and magnetic X-point. The collisionality changes from banana regime at the pedestal top to highly collisional regime in scrape-off layer. Impurity particles are also important players. We will present the current status of the edge gyrokinetic solutions from XGC in the present tokamaks and the future ITER plasmas: including the L-H bifurcation dynamics, divertor heat-flux width, neutral particle effect on edge turbulence, RMP physics, electromagnetic effect, importance of the X-point orbit loss physics, pedestal shape, toroidal rotation source at edge, blob physics, etc. We will also present the future plans. [Preview Abstract] |
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GP10.00057: Deep neural-network enabled study of XGC-generated blob dynamics Ralph Kube, Randy Michael Churchill, Seung-Hoe Ku, CS Chang Filamentary coherent turbulence structures, commonly called blobs, may have an important implication on the edge physics: edge plasma transport, nonlocal L-H transition, core-edge-SOL physics coupling, divertor heat-load width, etc. Recent analysis of X-point view GPI data suggests that blobs are also present in this high-shear region, with their dynamical properties to appear different from the blobs near the midplane. Blobs may also exists in the divertor chamber below the X-point, with their dynamical property and the correlation with the upstream blobs again different. Here we analyze the dynamical properties of blobs in a high-Ip Alcator C-Mod discharge, simulated using the XGC1 code. Pressure contour images taken in these three regions, viewed in a radial-poloidal plane are analyzed. Blobs are identified using a deep neural network that performs semantic segmentation of the image data. Semantic segmentation learns the structure of plasma blobs from a set of training data instead of relying on a thresholding method. The distribution of blob cross-field size, velocity, and amplitude distributions are compared for the three regions. [Preview Abstract] |
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GP10.00058: Physics of the Er-well structure development after L-H bifurcation from the edge gyrokinetic code XGC Seung-Hoe Ku, Choong-Seock Chang, R.M. Churchil, Robert Hager Previous gyrokinetic simulation of the L-H bifurcation [1,2] ended, due to lack of computing time, right after the turbulence suppression. The turbulent Reynolds-stress driven ExB-shearing started the turbulence-suppression process and the X-point orbit-loss driven ExB-shearing finished the process. Even though the ExB shearing rate from the curvature in the edge electrostatic potential was sufficient to give the turbulence suppression, an Er-well was not formed yet right after the bifurcation. This year, with the operation of the world #1 computer Summit, we continue the simulation until the Er-well is formed, pressure gradient steepens, and the neoclassical equilibrium Er becomes dominant. We will give a detailed report on how the Reynolds stress, the X-point orbit loss, the neoclassical force-balance physics, the neutral particles, and edge rotation play their roles in building up the pedestal right after the bifurcation event. Details of the turbulence dynamics change will also be reported. \newline [1] C.S. Chang et al., Phys. Rev. Lett. 118, 175001 (2017) \newline [2] S. Ku et al., Phys. Plasmas, 25, 056107 (2018) [Preview Abstract] |
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GP10.00059: Understanding pedestal transport via combined gyrokinetic and edge modeling David Hatch, M. Kotschenreuther, S. Mahajan, M. Halfmoon, E. Hassan, G. Merlo, C. Michoski, J. Canik, I. Joseph, M. Umansky, W. Guttenfelder, A. Diallo, R. Groebner, A. Nelson, F. Laggner, J. Hughes, S. Mordijck This presentation will report on the FY19 theory performance target (TPT), whose goal is to identify the turbulent transport mechanisms that affect pedestal dynamics. This is achieved via two sets of computational tools: (1) gyrokinetic codes (GENE and CGYRO), which can analyze the instabilities and resulting transport in the pedestal, and (2) edge codes (SOLPS and UEDGE), which, when operated in interpretive mode, can provide the best possible estimate of particle and heat sources--e.g., the ionization density source and the atomic energy loss channels due to ionization, charge exchange, and radiation. Such information, in combination with available fluctuation data and observed inter-ELM profile evolution, provides powerful constraints on the candidate instabilities that may govern pedestal transport. Comparisons will be made with discharges spanning multiple devices, large ranges in dimensionless parameters, multiple modes of operation (e.g., ELMy H-mode, I-mode, QH mode, etc.), wall materials, and fueling levels. [Preview Abstract] |
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GP10.00060: ETG Benchmarking for GENE/CGYRO/GEM Gyrokinetic Codes in the Pedestal Region Ehab Hassan, Walter Guttenfelder, David Hatch, Youjun Hu, Yang Chen, Scott Parker Electron temperature gradient (ETG) driven turbulence is thought to be a major contributor to heat transport in H-mode pedestals. Despite its importance, no systematic benchmarking exercise has been undertaken to establish the agreement of multiple codes for such turbulence. The pedestal is characterized by steep gradients and strong geometric shaping and produces slab-like instabilities and isotropic fluctuations. All of these elements are in contrast with the more-familiar core scenarios that have been the focus of previous benchmarking studies. We will describe a benchmarking exercise between three gyrokinetic codes: GENE, CGYRO, and GEM at multiple locations in the Pedestal region. Comparisons between growth rates, frequencies, and mode structures will be presented. Initial studies demonstrate limited qualitative agreement. Careful treatments of, for example, geometry and collisions are expected to resolve remaining discrepancies. [Preview Abstract] |
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GP10.00061: $T_{e}$-fluctuation measurements within the pedestal in dominantly electron-heated wide pedestal quiescent H-mode in DIII-D S. Houshmandyar, D. Ernst, A. Ashourvan, M. Austin, X. Chen, G. McKee, T. Rhodes, G. Wang The recently discovered wide pedestal quiescent H-mode (WPQH) in DIII-D is distinguished by its ELM-free nature, high confinement, and broadband turbulent fluctuations which limit the pedestal gradient. Recent experiments have shown that supplementing NBI heating with ECH heating improves the WPQH confinement. For dominantly electron heated WPQH plasmas, the fluctuation diagnostics have measured increased $n_{e}$ and $T_{e}$ fluctuations in the outer core as NBI power is exchanged for ECH power, which also leads to electron internal transport barrier formation [Ernst \textit{et al}, IAEA 2018]. However, our results show an unexpected and significant reduction in the $T_{e}$-fluctuation level in the \textit{pedestal} during the ECH. The $T_{e}$-fluctuation measurements in the pedestal were facilitated by the wide and high pedestal nature of the WPQH plasmas which results in sufficient optical thickness required for ECE measurements. Here we present the $T_{e}$-fluctuation measurements using the YIG ECE channels and the CECE analysis, in which a cluster of the channels were placed in the vicinity of the pedestal (0.8 \textless $\rho $ \textless 0.95) for the intent of increasing the spatial resolution of the $T_{e}$-profile. Transport analysis and the gyrokinetic simulation with the measured $T_{e}$-fluctuation will be presented. [Preview Abstract] |
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GP10.00062: Reduced Kinetic MHD model of an L-mode Rahul Gaur, Ian Abel, William Dorland Knowledge of the $L$-mode edge is crucial to understanding the transport and stability of an $L$-mode plasma and the $L-H$ transition. Experiments have shown that the density fluctuations become large ($O(1)$) and average gradients steepen as we move from the core to the edge of an L-mode plasma. Trying to understand such a system solely using fluid equations will neglect important kinetic effects such as Landau resonances and trapped particle modes. We present a computationally efficient and first-principles model of such a plasma. Our model consists of a closed set of hybrid fluid-kinetic equations. We assume $O(1)$ corrections to the distribution function and electromagnetic fluctuations. These fluctuations have a wavelength comparable to the perpendicular length scale and evolve on a parallel streaming time scale at the speed of sound. The model comprises a kinetic equation for the ions, fluid-like equations for the electron density and temperature, and a vorticity equation for the electrostatic potential. To validate our model, we examine the behaviour of zonal flows and Geodesic Acoustic modes, reproducing known results. To understand the physics of large-scale fluctuations in the edge, we examine the linear stability of ITG modes and Trapped-Particle Modes in our system. [Preview Abstract] |
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GP10.00063: Predicting and Optimizing the Pedestal in Present and Future Tokamaks Philip Snyder, Jerry Hughes, Matthias Knolker, Joseph McClenaghan, Orso Meneghini, Tom Osborne, Samuli Saarelma, Howard Wilson The pressure and temperature at the top of the pedestal play a key role in fusion performance of tokamaks. We review comparisons of pedestal predictions with the EPED model to observations on several tokamaks, focusing on high pedestal regimes such as Super H Mode. The role of both plasma shaping and aspect ratio is studied in detail, noting that even small changes to aspect ratio and shape can have profound implications for access to high pedestal states. Updates to the EPED model are discussed, addressing generalizations of the model and numerical and formulational challenges specific to lower aspect ratio devices. Predictions for DIII-D, JET, and MAST-U are discussed, as well as pedestal optimization for next step devices such as a Sustained High Power Density device and Compact Pilot Plant. [Preview Abstract] |
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GP10.00064: Orbit Modulation and Plasma Boundary Losses T. Stoltzfus-Dueck Ion orbit loss has long been invoked to explain mysterious near-separatrix observations including the L-H transition and edge intrinsic toroidal rotation. The basic idea is sound: Within about one poloidal gyroradius of the last closed flux surface (LCFS), some ions' drift orbits will intersect the divertor plate or vessel wall, causing those ions to be lost. However, orbit-loss models often struggle to maintain self-consistency: In steady state, the outgoing flux of ions on any given loss orbit should be restricted to the rate at which ions are supplied to that loss orbit, by turbulence, collisions, or otherwise. In this work, a general conservative gyrokinetic framework explicitly gives the steady-state orbit-loss boundary fluxes as a function of upstream transport (turbulent and collisional) and upstream sources. The details of the equilibrium orbits contribute only by modulating upstream transport and sources. The explicit reformulation of the orbit-loss terms facilitates their evaluation via numerical diagnostics or reduced models. For example, the orbit-modulation reformulation presents self-consistent avenues to determine the orbit-loss contribution to the edge toroidal rotation and $E_{r}$, and thereby to address questions like the L-H power threshold asymmetry. [Preview Abstract] |
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GP10.00065: Observation of low-n edge harmonics oscillations at high field side in JT-60U QH-mode plasmas Kensaku Kamiya, Naoyuki Oyama, Nobuyuki Aiba, Kimitaka Itoh We found the edge harmonics oscillation (EHO) having dominant toroidal mode number of n $=$ 1 at both Low/High Field Side (LFS/HFS) of the tokamak edge region in JT-60U quiescent H-mode (QH-mode) plasmas with counter-NBI heating under the configuration having an optimized-gaps between the plasma and the wall, regardless of a weak ExB (and/or toroidal) flow shear even at its shearing rate of less than 1 MHz at the pedestal collisionality of \textasciitilde 0.4). On the other hand, the edge localized modes (ELMs) were not completely suppressed when the EHO can be seen only at LFS under the configuration having a narrower inner-gap (wider outer-gap). This observation provides the first direct experimental evidence for its peeling nature, exhibiting necessary condition for keeping the EHO saturated both at LFS and HFS equally everywhere as possible, and represents a significant challenge of the theory. [Preview Abstract] |
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GP10.00066: Modeling RMP Footprint in NIMROD E.C. Howell, J.R. King, S.E. Kruger, D. Orlov, R.A. Moyer, T. Evans NIMROD calculations are used to study RMP induced footprint structures in DIII-D. The RMP fields alter the magnetic field structure near the divertor, splitting the separatrix in a homoclinic tangle of spiral lobes. Large asymmetric heat and particle fluxes may result where these lobes intersect the divertor (the magnetic footprint). Impurity radiation in the divertor may act to smooth out the heat flux reducing the asymmetry. Accurate modeling of the footprint, including the plasma response, is needed to predict and control the asymmetric fluxes to the divertor. Due to the chaotic nature of the magnetic field lines in the homoclinic tangle, the magnetic footprint structure is sensitive to the plasma response to the RMP. Here we use resistive MHD to model the plasma response. The model allows field penetration to drive the formation of islands, and equilibrium flow profiles are included to account for the plasma screening. Modeling shows that coupling to a marginally stable core 1/1 modes amplifies the RMP excited edge modes and that this coupling is sensitive to the core rotation. The effect of this amplification on the edge stochasticity and footprint structure is investigated using field line integration. [Preview Abstract] |
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GP10.00067: Multi-mode Plasma Response to Resonant Magnetic Perturbations and its Application in Controlling Edge Localized Modes Shuai Gu, Youwen Sun, Nan Chu, Yueqiang Liu, Tonghui Shi, Huihui Wang It has been demonstrated in recent years that multi-mode plasma response to resonant magnetic perturbations (RMPs) plays important role in controlling edge localized modes (ELMs). Therefore, in order to understand the mechanism of ELM control, it is necessary to extract the multi-mode plasma response to RMP fields and illustrate its role in ELM control. In this work, a new technique for multi-mode plasma response extraction is presented, as well as a new criterion to identify modes that related to edge localized mode control. This technique extracts modes by separating the full poloidal cross-section structure from the phase difference between upper and lower RMP coils using singular value decomposition. The phase difference dependence of extracted modes are compared to the effect of ELM control using both n $=$ 2 and n $=$ 3 RMP in wide q95 operating regimes. It shows that the mode with highest resonant plasma response near the pedestal top is strongly correlated with ELM mitigation or suppression, even if the mode is not the dominant one in plasma response. [Preview Abstract] |
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GP10.00068: Data-driven study on density limit and radiation collapse in stellarator-heliotron plasmas on LHD Tatsuya Yokoyama, Hiroshi Yamada, Suguru Masuzaki, Junichi Miyazawa, Kiyofumi Mukai, Byron Peterson, Naoki Tamura The boundary between stable-state and radiation collapse in stellarator-heliotron plasmas has been discussed using machine learning methods based on the experiment data in LHD. The density limit in stellarator-heliotron plasmas is defined by radiation collapse and the Sudo scaling provides brief physics picture. It should be noted that there are much more parameters than in the Sudo scaling involved in physics of radiation collapse and density limit. In this study, a linear SVM, which is one of machine learning technique, has been used to divide stable and collapse regions. The key parameters of density limit in LHD have been extracted using a sparse modeling, which exploits the inherent sparseness in all high-dimensional data and extracts the maximum amount of information from the data. Among about 20 physical parameters, several parameters such as line averaged electron density, heating power, and neutral gas pressure have been extracted as key parameters. With these parameters, 95\% of stable-state data and 82\% of radiation collapse data were classified correctly. Density-limit scaling using the extracted key parameters and likelihood of radiation collapse is discussed. [Preview Abstract] |
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GP10.00069: Installation of Lyman Alpha Pinhole Cameras on DIII-D and Initial Measurements of Main Chamber Neutrals Aaron Rosenthal, Alessandro Bortolon, Jerry Hughes, Florian Laggner, Theresa Wilks, Rui Vieira A one dimensional hydrogen Lyman-alpha (Ly-$\alpha$) edge diagnostic was recently installed and commissioned on the DIII-D tokamak. The system consists of two cameras, one viewing the low field and the other the high field side of the tokamak, each providing a toroidal fan of twenty lines of sight covering the scrape off layer and pedestal region below the midplane. Each camera has an aperture, Ly-$\alpha$ reflective mirror, Ly-$\alpha$ transmission filter, and AXUV detector array providing a radial resolution of 8 mm with a total coverage of 214 mm. The system was aligned on the bench and its positioning verified in vessel using a coordinate measuring machine, accurately matching the design parameters without the need of in vessel laser alignment. The Ly-$\alpha$ camera began taking data early in the 2019 DIII-D experiment campaign, providing an improved characterization of neutrals by measuring the Ly-$\alpha$ brightness. Using standard rate coefficients and electron density and temperature profiles, brightness profiles can be inverted to obtain Ly-$\alpha$ emissivity profiles, which can yield atomic deuterium and ionization rate profiles. Installation, alignment, initial data analysis, and preliminary results will be discussed. [Preview Abstract] |
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GP10.00070: SOLPS $+$ DIVIMP studies of impurity accumulation near the outside midplane separatrix J. David Elder, Peter Stangeby, Xavier Bonnin, John Canik, Jeremy Lore, David Moulton, Jacob Nichols, Richard Pitts SOLPS code studies of ITER cases find impurity accumulation near the outside midplane (OMP) separatrix. Since the impurity density, n$_{\mathrm{z}}$, there is the boundary value for the core n$_{\mathrm{z}}$ this effect is of potential significance. The effect also appears to be fairly robust. SOLPS case 2316 is detached at both targets for the flux tubes near the separatrix; case 2408 is attached at the outer and detached at the inner target. Nevertheless, for both cases a n$_{\mathrm{Ne}}^{\mathrm{10+}}$ peak of almost the same magnitude occurs. The neon enrichment there is \textless 1 since the concentration, n$_{\mathrm{Ne}}^{\mathrm{total}}$/n$_{\mathrm{e}}$, at the OMP is greater than the divertor concentration; however, the compression is \textgreater 1 since n$_{\mathrm{Ne}}^{\mathrm{total}}$ is higher in the divertor. As the computational time to reach converged SOLPS solutions is long, it is difficult to explore the controlling physics involved. The DIVIMP code has been used to launch Ne into the SOLPS-calculated `plasma background' for cases 2316, 2408. The value of D$_{\mathrm{\bot }}$ was changed from the SOLPS value of 0.3 m$^{\mathrm{2}}$/s to 1, and also to 0.1, while keeping the number of injected Ne particles fixed. For 0.3 $\to $ 1 m$^{\mathrm{2}}$/s the n$_{\mathrm{Ne}}^{\mathrm{10+}}$ peak value decreased by \textasciitilde 2X, while for 0.3 $\to $ 0.1 m$^{\mathrm{2}}$/s it increased by \textasciitilde 2X. It is shown that these results are broadly in accord with a simple fluid model for impurities. [Preview Abstract] |
(Author Not Attending)
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GP10.00071: Upgrades to the TCV Divertor Spectroscopy System Christopher Dodson, Lorenzo Martinelli, Basil Duval, Yanis Andrebe, Hammam Elaian, David Moret, Mirko Wensing, Kevin Verhaegh The divertor spectroscopy system on the TCV tokamak resolves spectra in visible wavelengths for determining plasma properties such as density, temperature, and the rates of atomic and molecular processes. This diagnostic system is being upgraded to improve coverage, signal-to-noise ratio, spectral range, and operational flexibility for divertor detachment studies. The recent addition of baffles for neutral compression requires modifications to provide coverage of the divertor legs from the separatrix to the strike points. View chords will be added on a lower port to observe the inner divertor leg, and modifications to the horizontal port will enable increased angular coverage of the outer divertor leg. A third spectrometer will also be added to increase spectral coverage, and upgrades to the fiber-spectrometer coupling will enable quick selection of view chords between shots. Modifications of one of the spectrometers will improve the time resolution of a narrow spectral range for measurements of transient events such as ELM's and disruptions. This presentation will illustrate these upgrades and present results of synthetic diagnostic analysis used for the design. [Preview Abstract] |
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GP10.00072: \textbf{Study of Argon impurity line emissions using high resolution spectroscopy diagnostic in Aditya-U tokamak } Kajal Shah, Gaurav Shukla, Malay Chowdhuri, Ranjana Manchanda, Kumarpalsinh Jadeja, Kaushal Patel, Rakesh Tanna, Balamurali Krishna Mayya Kolake, Joydeep Ghosh \textbf{Argon impurity line emissions in the visible range have been observed in Aditya-U tokamak using 1m }\textbf{\textit{f}}\textbf{/8.7 Czerny Turner configuration spectrometer together with a charge coupled device (CCD) detector. The diagnostic allows measurements from core to edge covering the complete minor radius of the plasma using a set of optical fibers giving multiple lines of sight. Temporal behaviour of the lines has been examined for reliable identification. Moreover, experimental observation of the Ar lines was reproduced to obtain emissivity profiles using 1D impurity transport code STRAHL. The emissivity profiles reproduced is then analysed to estimate signal strength of Helium like argon line emissions to be used as a feasibility study for X-ray Imaging Crystal Spectroscopic diagnostic proposed for Aditya-U. In this paper, we present observation and analysis of Ar impurity lines in Aditya-U tokamak.} [Preview Abstract] |
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GP10.00073: Improvements to Plasma Modeling and Hardware Systems for Spectroscopic Analysis on HIDRA Matthew Parsons, Elizabeth Perez, Daniel Andruczyk The HIDRA stellarator/tokamak facility at the University of Illinois at Urbana-Champaign is dedicated to the development of novel materials technologies for fusion reactors. The machine is equipped with a pair of optical spectrometers for the analysis of plasma characteristics, and a pair of infrared cameras for the monitoring and analysis of plasma-limiting components. Extensive work has been done in the past year to improve both the modeling and hardware aspects of these systems. A Collisional-Radiative Model is now employed to reconstruct 1-D profiles of plasma densities and temperatures from line-integrated measurements with the optical spectrometers. The infrared camera data can be compared to recent work on the modeling of HIDRA plasmas for the analysis of heat fluxes to limiting surfaces. On the hardware side, significant modifications to the original infrared optical system allow for the cameras to be placed outside of the magnetic field, and further permits the interchange of lenses to vary the viewable area inside of HIDRA. [Preview Abstract] |
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GP10.00074: \textbf{Extracting information on shard penetration from plasma discharge videos: a new OpenCV based bright object tracker} Cristian Sommariva, Scott Silburn, Jonathan Graves, Cedric Reux, Joan Decker, JET Contributors Disruptions are magnetohydrodynamic instabilities characterized by a sudden loss of plasma confinement and can create dangerous levels of heat loads, electromechanical stresses and runaway electron currents. Therefore, an effective disruption mitigation scheme is required for safe tokamak operations. The Shattered Pellet Injector is the chosen mitigation system for ITER and prototypes have been used in DIII-D while tests in JET will be conducted in the forthcoming months. Their effectiveness depends on the shard penetration length into the plasma, which, in JET, can be measured by analyzing the data taken from fast visible camera looking at the SPI injection point. For doing so, a new particle tracker has been developed for identifying and tracking bright objects visible in plasma discharge videos. In the JET video particle tracker, this is obtained using general-purpose open-source and commercial computer vision technologies. Tracking robustness is improved by blending different methods via a weighting scheme. After an introduction of the new tool, this work reports code performance, and recent tests conducted on videos showing fueling and ELM pacing pellets injected into JET plasmas. If data on JET mitigated disruption via SPI are available, first results will be reported. [Preview Abstract] |
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GP10.00075: New neutron-gamma scintillator diagnostics at the Madison Symmetric Torus Luigi Cordaro, Matteo Zuin, Jay K Anderson, Luca Stevanato, Cristiano Fontana, Isacco Bonesso, Jungha Kim The energy calibration and the first experimental measurements of a diagnostic for the neutron-gamma detection are presented. The system is composed of 6 scintillator cells coupled with flat photomultiplier tubes. One detector cell is based on NaI(Tl) crystal scintillator, the remaining ones use EJ-309 organic liquid scintillators. While the former is used for gamma-ray spectra analysis, the latter are used for fast neutron detection, by means of the Pulse Shape Discrimination (PSD) technique. Energy calibration has been performed with several calibrated radioactive sources, the neutron response function has been analyzed as a function of the neutron time-of-flight, using a $^{\mathrm{252}}$Cf source tagged by an additional fast plastic scintillator. A high neutron flux has been detected in discharges operated with NBI. The overall diagnostic has proved to withstand flows, up to several Mcounts/ms. The diagnostics is able to detect fast transient phenomena, such as those detected during discrete plasma relaxation events. Fast transient peaks have been detected, allowing the study of fast particles induced by magnetic reconnection events in plasma. [Preview Abstract] |
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GP10.00076: Recommissioning the KA2 Fast Ion Loss Detector on JET in Preparation for DT-Campaign P. J. Bonofiglo, M. Podesta, V. Kiptily, P. Beaumont The KA2 Faraday cup fast ion lost detector has been recommissioned on JET in preparation for the upcoming DT-campaign. The detector consists of five particle collector receptacles spaced poloidally along a fixed toroidal location from the midplane to $\sim$0.7 m below. Each receptacle contains three radially displaced apertures which consist of a stack of alternating current collecting foils and insulating material. The detector provides 15\% incident particle energy resolution from $\sim$0.5-7 MeV with limited pitch discrimination, allowing for the measurement of lost energetic ions from ICRH, NBI heating, and alpha particle production. Initial results and measurements are presented from JET’s 2019 deuterium campaign in preparation for recording alpha losses during the upcoming DT-campaign. New cabling and signal filtering have been implemented on the detectors to reduce ambient plasma pickup and electrical noise for an improved signal-to noise ratio. Emphasis has been placed on assessing the systems performance to study Alf\'{e}n mode resonant transport of energetic ions which is particularly detrimental to the confinement of the ion population. [Preview Abstract] |
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GP10.00077: Fast wave interferometer/reflectometer for ion diagnostics T. Akiyama, R.I. Pinsker, D.C. Pace, M.A. Van Zeeland, C.M. Muscatello, R.L. Boivin, W.W. Heidbrink The fast wave, with frequency of several tens of MHz, couples with ions in plasmas. A fast wave interferometer and reflectometer measure line-integrated ion mass density and concentrations of the different species such as D and T. Such fast wave diagnostics are robust even in the harsh environment of burning plasma devices. They typically use small antennas, which are less prone to neutron/gamma issues, and not affected by neutral/impurity deposition like first mirrors in optical systems. This makes the development of fast wave diagnostics especially relevant for future devices. The fast wave diagnostic system, which uses a high power source up to 75 W, is being designed. The system will inject a wave, whose frequency is swept from 10 to 60 MHz to cover an entire plasma region, for the reflectometer. The fixed frequency at above 60 MHz will be used for the interferometer. To obtain sufficient received signal power, fast wave antennas on the low field side, previously used for high-power fast wave current drive, will be used for launching and receiving the probing wave. The phase will be extracted by analog or digital demodulators, whose bandwidths will be MHz range. In case of ITER-like D/T burning plasma, the similar frequency range is available. [Preview Abstract] |
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GP10.00078: Development of a heterodyne Phase Contrast Imaging system for Ion Cyclotron Emission detection on DIII-D Alessandro Marinoni, Charles P. Moeller, Miklos Porkolab, John C. Rost The Phase Contrast Imaging diagnostic on DIII-D has been upgraded with a novel optical heterodyne detection scheme to measure the spatial structure of Ion Cyclotron Emission (ICE), thus extending the purely temporal measurements available so far. The PCI is an absolutely calibrated internal-reference interferometer that creates an image of line-integrated electron density fluctuations. Due to technological limitations in manufacturing arrays of cryogenically cooled detectors, worldwide PCI systems operate with a 2 MHz bandwidth (BW) which, although suitable for turbulence and low-f waves, precludes the study of faster phenomena such as ICE, which occurs at frequencies of tens of MHz. The laser beam power is modulated in such a way that the frequency of the wave of interest is within the PCI detector BW, thus making the imaging method applicable at higher frequencies. A transverse Pockels cell made of a water cooled CdTe birefringent crystal, driven by a matched oscillator that provides a 2 kV pk-pk voltage, is used for the beam modulation. Bench-top tests using a 10 MHz oscillator are in agreement with the expected response. Initial measurements will be presented using an upgraded oscillator at variable frequency, suitable for various values of the confining magnetic field. [Preview Abstract] |
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GP10.00079: Upgrade of radial interferometer-polarimeter for internal magnetic fluctuation measurements on DIII-D Ryota Yoneda, Jie Chen, David Brower, Weixing Ding, Daniel Finkenthal The Faraday-effect radial interferometer-polarimeter diagnostic on DIII-D tokamak, using two probe waves at 650 GHz with right- and left-handed circular polarizations, has been upgraded for high resolution internal magnetic and density fluctuation measurements. A novel correlation polarimetry technique, which measures Faraday rotation using two sets of independent mixers viewing an identical line-of-sight, has been developed for detection of small amplitude internal magnetic fluctuations. Using this approach, the polarimeter phase noise floor has been reduced from $\sim $1 to below 0.1 Gauss*m/sqrt(kHz) under medium plasma density condition (5$\times $10$^{\mathrm{19}}$m$^{\mathrm{-3}})$. A third 650 GHz solid-state microwave source has also been added, enabling simultaneous measurement of internal line-integrated Faraday and density fluctuation along three horizontal chords at Z$=$0 and $\pm $13.5 cm. New optical design, with small incident angle on mesh beam splitters is adopted, allowing smaller distortion of probe beam polarization. Digital phase demodulator has been upgraded to process the data from three-waves and correlation measurement in real-time. Results from bench test and plasma experiments will be presented. [Preview Abstract] |
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GP10.00080: Beam diagnostic innovations to deepen understanding of pedestal and edge phenomena such as the onset and evolution of transport barriers and ELMs P.J. Fimognari, T.P. Crowley, D.R. Demers Deeper understanding of the onset and evolution of transport barriers as well as MHD stability and transport, including edge localized modes (ELMs), is possible with direct measurements throughout the pedestal and edge regions. We are pursuing interrelated diagnostic beam innovations to improve diagnosis of these regions of magnetically confined plasmas. The common thread among the innovations is the non-perturbing determination of spatially localized quantities from measurement of secondary particles created by interaction of a diagnostic beam with the plasma. The (typically alkali metal) beam diagnostic technique uses the conservation of energy and angular momentum to infer the fluctuations in electron density, and the electric and magnetic fields at locations where secondary particles are created. We will discuss several areas of measurement innovation including determination of a component of the magnetic vector potential in a magnetically confined plasma using a singly charged alkali ion beam (enabling inference of the edge current density profile and, correspondingly, evolution of transport barriers and ELMs) and simulations of the ability of neutral alkali beams to probe the edge of large toroidal devices. [Preview Abstract] |
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GP10.00081: Study of toroidal and poloidal rotation on Aditya-U tokamak Gaurav Shukla, Kajal Shah, Malay Chowdhuri, Ranjana Manchanda, Kumarpalsinh Jadeja, Kaushal Patel, Rakesh Tanna, BalamuraliKrishna Mayya Kolake, Joydeep Ghosh Toroidal and poloidal rotation velocity profiles have been measured on Aditya-U tokamak using a high resolution spectroscopic diagnostic. Impurity ion temperature is also measured using the diagnostic. Rotation velocity and ion temperature have been measured using Doppler shift and Doppler broadening of the line emission respectively. Carbon impurity was chosen for the measurements because it remains the main intrinsic impurity in Aditya-U discharges due to the graphite limiters. For the toroidal rotation profile measurement, passive charge exchange (PCX) line of C VI at 529 nm was measured covering the complete plasma minor radius from core to edge towards low field side. Poloidal rotation velocity measurements are performed using C III line emission at 464.74 nm. Matrix inversion technique was applied to obtain localized measurements from the line integrated measurements. [Preview Abstract] |
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GP10.00082: Experimental Program to Study Centrifugal Rotation in Highly Magnetized Hydrogen Plasmas and in Dusty Plasmas C. A. Romero-Talamas, N. J. Eschbach, J. N. Stefancik, K. N. Frost, J. M. Drummond, A. J. Chen, B. Tsao Experimental plans and preliminary diagnostic results for a new high-field electromagnet facility under construction at UMBC are presented. The magnet, called Adjustable Long Pulse High-Field Apparatus (ALPHA), is a Bitter-type electromagnet designed to deliver up to 10-T in a 15-cm bore for more than 10 seconds. ALPHA is being constructed to accommodate two magnetic configurations: i) continuous stacking, which will allow for the highest steady state magnetic field for dusty plasma experiments; ii) irregular stacking, which will allow for a mirror field configuration to study the Critical Ionization Velocity (CIV) in plasmas with imposed E x B rotation (where E is the radial electric field, and B the axial magnetic field). The two configurations will use a cylindrical glass chamber, but with different inner electrode arrangements and diagnostics. The dusty plasma configuration will include a dust dispenser with high reproducibility, a dust collector to retrieve samples between experiments while keeping high vacuum, and periscopes for dust imaging during experiments. The CIV configuration will include a center conductor and a metal liner to impose an E-field through an external high voltage capacitor bank, and periscopes for plasma spectroscopy. [Preview Abstract] |
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GP10.00083: Critical Ionization Velocity Exploration and Diagnostics in Magnetized Rotating Plasmas N. J. Eschbach, K. N. Frost, J. N. Stefancik, C. A. Romero-Talamas The Adjustable Long Pulse High-Field Apparatus (ALPHA), a Bitter-type electromagnet with a 15-cm bore capable of delivering up to 10-T for more than 10 seconds, is being constructed at the Dusty Plasma Laboratory at UMBC. Experiments designed to explore and surpass the Critical Ionization Velocity (CIV) limit in E x B rotating hydrogen plasmas, previously identified both theoretically and experimentally by other groups, are planned with ALPHA. To carry out such experiments, a mirror coil design for ALPHA’s Bitter-plates will be implemented. The mirror field will have about 4.5-T at midplane for steady state operation, and higher in pulsed mode. A mirror ratio slightly above 1 is estimated to be sufficient for confining plasmas that will yield measurable spectroscopic intensities to obtain rotation velocities through Doppler shifts. Optical access at midplane will be challenging since the magnet consists of a single continuous coil. Thus, to capture Doppler-shifted spectra, thin periscopes are designed to fit between the cylindrical vacuum vessel and the magnet. A mirror coil design, optical access system, and different electrode and insulator configurations will be presented. [Preview Abstract] |
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GP10.00084: Application of a Bayesian tomography method to the line-integrated measurements in fusion plasmas Dong Li, Jackob Svensson, Tian Bo Wang, Yun Bo Dong, Wei Deng Bayesian probability theory is now widely used for data analysis in fusion diagnostics where uncertainty analysis and consistency check are of particular importance. In Bayesian formalism, the quantities of interest are expressed in the probabilistic form rather than a single determined solution, as a consequence, the uncertainty of the result can be acquired from the confidence interval of a posterior probability. Moreover, validity of the result can be checked by examining whether the misfits between predicted and measured data are within an assumed data error. In this article, we introduce a Bayesian tomography method using non-stationary Gaussian Processes to adapt locally to the varying smoothness in space, through which the accuracy of reconstructions can be improved significantly. To date, this Bayesian tomography method has been applied to diagnostics in several fusion devices: soft X-ray in W7-AS and HL-2A, bolometer in JET and soft X-ray spectroscopy in WEST, being used favorably for the study of relevant physics. [Preview Abstract] |
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GP10.00085: Equilibrium Reconstruction in LTX-$\beta$ using Response Functions Leonid Zakharov, Paul E. Hughes, Dennis B. Boyle, Chris Hansen The equilibrium reconstruction technique targets reconstruction of plasma profiles using external measurements of magnetic field around the plasma as well as internal measurements inside the plasma core. The essence of the approach is to eliminate from the signals the contribution of of currents oi poloidal field coils in the passive structures, while extracting the pure contribution from the plasma. Then the problem of reconstruction is reduced to adjusting the plasma fitting parameters to the resulting signals from the plasma. Passive conducting structure always represented the problem because of unpredictable eddy currents as well as 3-dimensional geometry, and 3-D perturbations on the local magnetic measurements, especially on short pulse devices like LTX-$\beta$ where decay time of eddy currents is comparable with plasma duration. The presentation explains how the method of Response Functions in combination with the numerical 3-D model of the passive structures, proposed here, resolves this problem and allows to eliminate from signals not only the contribution of the equilibrium coil currents and associated eddy currents, but also the contribution from the eddy currents excited by the plasma itself. [Preview Abstract] |
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GP10.00086: ABSTRACT WITHDRAWN |
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GP10.00087: Model predictive control of plasma profilesat DIII-D William Conlin, Joseph Abbate, Egemen Kolemen, Keith Erickson neural network has been developed to predict future values of plasma profiles in real time using past data and a set of proposed actuator inputs. Thispredictive model offers many opportunities for real time control. The simplest method, which is already possible with the current model, is to simply make multiple predictions using different proposed actuator inputs, and selecting the inputs that lead to the ``best'' predicted profile. This can be improved upon by modifying the model to make over a longer horizon, sinstead of just predicting the next timestep, it can predict the next timesteps. This then allows for longer lookaheads in the control action, which allows us to predict and preemptively mitigate instabilities before they become dangerous, to potentiallyfind new routes to H-mode, which currently requires very strong heating, and can be difficult to achieve. [Preview Abstract] |
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GP10.00088: Linear Quadratic Gaussian (LQG) Shape Control in ITER Josiah Wai, Egemen Kolemen We develop a shape control algorithm for the ITER tokamak using linear quadratic Gaussian (LQG) control techniques. In ITER, the large distance between the plasma and poloidal field (PF) coils introduces a high degree of coupling into the model. Combined with ITER's high performance constraints the shape controller must utilize all coils in an efficient manner. LQG is a type of optimal control that can handle this type of multi-input multi-output design effectively and also in the presence of noise and uncertainty. The algorithm controls the boundary flux at a large number of locations as well as the x-point and strike point positions. The goal that flux errors at the control locations approach zero is treated as a tracking problem for the linearized system. The control design is validated in the 15 MA inductive scenario using a closed loop nonlinear simulation. [Preview Abstract] |
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GP10.00089: Nonlinear Adaptive Burn Control in ITER Based on Two-Temperature Response Model Vincent Graber, Eugenio Schuster By using robust burn control approaches, ITER will be the first reactor capable of confining plasmas with reactivities suitable for energy production. An adaptive nonlinear control scheme that employs planned actuators for ITER is synthesized from a 0-D plasma model with uncertain parameters. Because the temperature of the ions and electrons could differentiate significantly in ITER, the model considers separate ion and electron temperature response models. The control scheme relies on neutral beam injection (NBI) for the bulk of the plasma heating. Electron and ion cyclotron heating are exploited to independently regulate the electron and ion temperatures, respectively. The dynamics of fast NBI beam ions and fusion alpha particles are modeled by including state-dependent thermalization delays and fractional heating to the ions and electrons. The dynamics of the pellet injector, used for density control, is modeled as flight-delayed pellets that are discretely deposited into the plasma. The model contains uncertainty in the particle recycling from the walls, confinement properties, and the heating shared between the ions and electrons. The adaptive control scheme successfully stabilizes the system despite the various time delays, actuator dynamics, and model uncertainties. [Preview Abstract] |
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GP10.00090: The ITER-Core Turbulent Plasma Diagnostics Based on Multi-Frequency-Laser-Beam Thomson Scattering V. Alexander Stefan A novel ITER-core turbulent-plasma-diagnostic method is proposed based on the multi laser beam Thomson scattering, during the nonlinear electron cyclotron plasma heating (with on-axis B-field of \quad 10T).$^{\mathrm{\thinspace }}$\footnote{ G. S. Kurskiev $^{\mathrm{\thinspace }}$et. al., Nuclear Fusion, Volume 55, Number 5 (2015); P. Nielsen, C. Gowers and H. Salzmann,$^{\mathrm{,\thinspace }}$Journal of Instrumentation, Volume 12, July 2017} \footnote{ V. Alexander Stefan, APS-DPP-2009, (K1.00028); APS-DPP-2010, (DPP10-2010-000167); APS-DPP-2014(CP8.00010); V. Stefan, B. I. Cohen, C. Joshi, \textit{Science}, 243, 4890, (1989); V. Alexander Stefan, Unified theory of parametric excitations in magnetized plasma produced by the action of nonmonochromatic driver pump. II. Multiple driver pump, The Physics of Fluids 26, 1797 (1983)} Nonlinear electron cyclotron plasma heating leads to the excitation of electron-Bernstein mode turbulence in the ITER core-plasma, influencing electron temperature and electron density profile. Under these conditions, the use of multi-frequency-laser-beam may prove to be an effective core-plasma diagnostic method for nonstationary plasma turbulence.\footnote{ V. V. Pustovalov and V.P. Silin, Nonstationary plasma turbulence in parametric resonances, Soviet Physics Technical Physics, vol. 20, no. 12, 1976, p. 1544-1548; V. P. Silin and V.T. Tikhonchuk, Parametric plasma turbulence Physics Reports, \textbf{135}, 1, April 1986, Pages 1-46\par \par \par \par } [Preview Abstract] |
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GP10.00091: Electron temperature fluctuation measurements of pedestal fluctuations in improved confinement regimes at ASDEX Upgrade Rachel Bielajew, Garrard Conway, Luis Gil, Amanda Hubbard, Pedro Molina Cabrera, Davide Silvagni, Eleonora Viezzer, Anne White, The ASDEX Upgrade Team Improved confinement regimes, including stationary ELM-free H-mode (similar to the EDA H-mode), QH-mode, and I-mode, have the benefit of high energy confinement without the presence of ELMs. These three regimes all have edge modes thought to be associated with their favorable confinement properties. Temperature fluctuation measurements provide important information on scaling and transitions between L-mode, H-mode, and improved confinement regimes. These measurements can be obtained using Correlation Electron Cyclotron Emission (CECE) with high temporal (4 MHz) and good spatial (4 mm) resolution. This work presents temperature fluctuation measurements in improved confinement regimes at ASDEX Upgrade from a multi-channel CECE radial comb. In this work, edge modes are localized, and the structure of broadband turbulence is compared between L-mode and several types of improved regimes. [Preview Abstract] |
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GP10.00092: Measuring the Dependence of Turbulence Characteristics and Transport Behavior on $\rho $* Through Dimensionless Scaling Experiment in HL-2A Xijie Qin, George Mckee, Lucas Morton, Raymond Fonck, Zheng Yan, Rui Ke, Min Xu, Ting Wu The variation of local long wavelength density fluctuation characteristics with $\rho $* are measured with beam emission spectroscopy (BES) in HL-2A plasmas while other dimensionless quantities are held nearly fixed, to examine how turbulence and transport depend on $\rho $*, the dimensionless size parameter. The ion gyroradius is varied while holding other dimensionless parameters ($\beta $, q, $\kappa $, $\nu $, R$/$a, M\textunderscore A, T\textunderscore i$/$T\textunderscore e) nearly fixed by varying the toroidal field and plasma current and adjusting input power and density accordingly to match profiles. The normalized density fluctuation amplitude (\~{n}/n), poloidal and radial correlation lengths, decorrelation time, and poloidal velocity are calculated from a 16 (radial) by 2 (poloidal) channel array of BES density fluctuation measurement over the minor radial range, $\rho =$0.2-0.8s. Initial results demonstrate localized poloidal velocity flow reversals and variation in correlation properties with $\rho $*. The analysis of turbulence and transport dependence on $\rho $* is aimed at predicting confinement in ITER-like large scale plasmas. [Preview Abstract] |
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GP10.00093: Study of Alfv\'{e}n Eigenmodes (AEs) stability on JET with active antennas and modeling with MHD and gyrokinetic codes. N. Fil, M. Porkolab, A. Tinguely, S. Dowson, M. Fitzgerald, S. E. Sharapov, H Sheik, J. Mailloux, P. Puglia, A. Fasoli, D. Testa, Z. Lin, J. Bao, M. Podesta JET DT campaign planned by the end of 2020 will be a significant opportunity to explore fast ion fusion physics before ITER operations. The AEs Active Diagnostic (AEAD) will play a critical role to study the interaction between alphas and Alfv\'{e}n modes [1]. Recent efforts have been made on JET to develop a scenario to observe unstable TAEs attributed to fusion $\alpha $'s in DT plasma [2]. In preparation of these experiments, a wide range of experimental and theoretical studies have been undertaken with a full range of isotope experiments (DD and TT). We use the synergy between the AEAD and modeling codes [1] such as the MHD code MISHKA and the gyrokinetic code GTC to study AEs stability in various JET plasmas. GTC self-consistently treats bulk ions, fast ions, electrons and fields which allows us to study both unstable AEs observed passively and stable AEs excited resonantly by the AEAD. Good agreement is obtained between simulations and experiments which adds confidence to further predictions (JET-DT and ITER). [1] V. Aslanyan et al., NF \textbf{59} (2019) 026008. [2] R. Dumont et al., NF \textbf{58} (2018) 082005. [Preview Abstract] |
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GP10.00094: Modelling of ICRF heating effects on the evolution of fast ion transport in plasmas with MHD instabilities A. A. Teplukhina, F. M. Poli, M. Podesta, N. Bertelli, Ye. O. Kazakov In the present research work we will analyse fast ion transport under sawtooth activity in case of joint ICRF and NBI heating. JET experiments on the three-ion H-(D-NBI)-D heating scheme (Y. Kazakov, et al, 2017 Nature Phys. 13 973) have been chosen for the analysis as they clearly show strong variations in the sawtooth period depending on the NBI and ICRF heating scenarios. The sawtooth period depends on the fast ions presence in the plasma core, as well as the neutron rate that is strongly affected by the fast ion distribution. The TRANSP code simulates plasmas in interpretative and predictive modes. A reduced model is used to estimate the effect of low-n MHD instabilities like the sawtooth activity on fast ion transport (M. Podesta, et al, 2014 Plasma Phys. Control. Fusion 56 p. 055003). Interaction between fast ions and RF waves is taken into account with the RF 'kick' operator. Also, we will investigate effects of ICRF-NBI synergy on the fast ion distribution. In particular, we will demonstrate simulation results for JET plasmas with increasing complexity of the physical setup. [Preview Abstract] |
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GP10.00095: Exploring the Dependence of Edge/SOL Turbulence Characteristics on Triangularity Using Gas Puff Imaging on TCV Woonghee Han, Nicola Offeddu, Theodore Golfinopoulos, Christian Theiler, James Terry, Earl Marmar Negative triangularity plasmas have been known for years to feature enhanced confinement as compared to standard, D-shaped plasmas. More recently, correlation electron cyclotron emission and phase contrast imaging measurements on the TCV tokamak revealed that the confinement improvement is accompanied by reduced levels of temperature and density fluctuations across most of the confined plasma. In this contribution, we extend these studies towards the edge/SOL region using Gas Puff Imaging (GPI), a diagnostic technique that routinely measures the spatially-resolved edge/SOL fluctuations. A GPI diagnostic has been installed and tested on TCV in the fall of 2018. During the summer 2019 TCV campaign, edge/SOL fluctuations with different triangularities ($\delta$=-0.4 and +0.5) and plasma densities ($2.0\times 10^{19}$ and $5.2\times 10^{19}$ m$^{-3}$) of limited ohmic L-mode plasmas were measured using GPI. Two different gas puff species (He and D$_2$) were used to obtain different levels of gas puff penetration and cover slightly different regions of the edge/SOL. The results will be presented to see the effect of triangularity on turbulence characteristics in the edge/SOL. [Preview Abstract] |
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GP10.00096: The kinetic equilibrium reconstruction in TCV with LIUQE/RAPTOR: first real-time demonstration during plasma operation Francesco Carpanese, Federico Felici, Olivier Sauter, Cristian Galperti, Jean-Marc Moret The Kinetic Equilibrium Reconstruction (KER), coupling self-consistently the free-boundary equilibrium code LIUQE to the 1.5D transport code RAPTOR, has been performed for the first time in real-time during TCV plasma operation. RAPTOR can solve predictively the flux surface averaged Ohm's law, the electron heat Te and the particle ne diffusion equations providing self-consistent p($\rho $,t) and j($\rho $,t) constraints to the free-boundary equilibrium code. An Extended Kalman Filter technique is used to combine the kinetic measurements available in real-time and transport code prediction yielding a robust state estimation of the plasma profiles. During different plasma states (flat central ne, ECCD induced j profile broadening, NTM induced Te drop) the KER is shown to reproduce better the profile modifications when compared to the standard equilibrium reconstruction, where only external magnetic measurements are considered. [Preview Abstract] |
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GP10.00097: Advances in negative-triangularity tokamak physics in TCV Stefano Coda, Stephan Brunner, Matteo Fontana, Zhouji Huang, Aylwin Iantchenko, Antoine Merle, Gabriele Merlo, Antoine Pochelon, Laurie Porte, Holger Reimerdes, Olivier Sauter, Michael Faitsch The TCV tokamak has pioneered negative triangularity since the mid-1990's, reporting enhanced confinement in both Ohmic and ECRH L-mode scenarios. Local gyrokinetic simulations reproduced the effect near the plasma edge but not in the core where triangularity vanishes, motivating more recent global simulations. Experiment also shows a dominant role of the outermost 20{\%} of the minor radius in determining the overall confinement with stiff core profiles. H-mode scenarios have also been developed, with more frequent and less virulent ELMs, explained by the closure of the ballooning second-stability region and a diminished pedestal stability threshold. Negative triangularity however also shrinks the heat-flux profile in the scrape-off layer. More recently, the ITER baseline $\beta_{\mathrm{N}}$ level (1.7) has been reached in L-mode with NBI. Turbulence has now been systematically compared at positive and negative triangularity using correlation ECE and phase-contrast imaging diagnostics, reaching inside mid-radius. Both report a significant reduction of turbulence with negative triangularity everywhere, in both TEM- and ITG-dominated regimes. [Preview Abstract] |
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GP10.00098: Decoupling shape and position control on TCV Federico Pesamosca, Federico Felici, Stefano Coda Shaping of the plasma cross section leads to improved performance in tokamaks. TCV features a digital shape and position controller, which at present relies completely on real time magnetic equilibrium reconstruction. Its computational time, comparable to the vertical instability growth rate of elongated plasmas, limits the margin for stable operations. In recent experiments, the flexibility of TCV was exploited using its independently powered poloidal field coils to improve position and shape control. The full set of 16 coils is numerically optimized to stabilize the plasma while minimizing the input request from power supplies. A new position error signal to be minimized allows decoupling as it combines slow and reliable information on the plasma location (from magnetic equilibrium reconstruction) with analog position observers based on magnetic diagnostics, which are less precise but highly responsive to fast unstable plasma displacements. Orthogonal coil combinations can then be used to control independently the plasma shape, X and strike points. The goal is to robustly stabilize advanced configurations in TCV such as negative triangularity and snowflake plasmas. [Preview Abstract] |
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GP10.00099: Progress of the TCV Divertor Upgrade Holger Reimerdes The TCV tokamak is in the midst of an upgrade to increase its capability to investigate conventional and alternative divertor configurations. The main components of the divertor upgrade are the addition of a gas baffle, enhanced divertor diagnostics and an increased pumping capability. A simultaneous heating systems upgrade increases the power for the divertor to mitigate. The baffle design was guided by simulations with the edge codes SOLPS-ITER and SOLEDGE2D and remains compatible with a wide range of configurations including Snowflake and Super-X. The first baffle version, installed in 2019, is predicted to increase the compression of divertor neutrals by up to a factor of 10 and lead to detachment at lower core electron density. To validate the model predictions the diagnostic array is being augmented with additional baratrons, low-temperature Thomson scattering measurements in the divertor and extending Langmuir probe and IR thermography coverage to the entire poloidal divertor circumference. A prototype non-evaporable high capacity getter pump is installed to evaluate its potential in a tokamak. Planned experiments will test the model predictions and provide guidance for possible revisions of the baffle upgrade. [Preview Abstract] |
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GP10.00100: Doublet Configuration Plasmas on the TCV Tokamak basil duval, Holger Reimerdes, Joyeeta Sinha, Stefano Coda, Jean-Marc Moret Alternative magnetic shapes and plasma operational regimes are being re-examined. A Doublet uses a vertical multiplet, predicted to improve MHD stability, lower vertical instability growth rates and increase plasma beta limits [Jensen 1975]. Plasma exhaust has one plasma core's divertor aimed into a second, up/down symmetric, core. Revisited on TCV, a 260-kA doublet discharge, featuring a spontaneous transport barrier in the separatrix region is presented [Duval 2018].Two separate single core breakdown/plasma ramp up limited configurations top/bottom of the highly open TCV vacuum vessel were run, then two simultaneous nulls were attempted. With Ohmic heating alone, the upper lobe drifted to the lower after 20ms (60kA in each lobe). Independent 0.5-MW X2 ECH gyrotrons aimed at each lobe achieved a total plasma current of 260kA disrupting after 20ms without movement of the lobe centers. Both lobes appeared to heat near equally, independently ECH targeting. A strong temperature gradient in, the separatrix region, (within the mantle), possibly explains why independent lobe EC-heating control was ineffective. The surprising position and plasma equilibrium stability of the TCV doublet indicate a rich future research target where strike point interactions are avoided naturally. [Preview Abstract] |
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GP10.00101: L-mode divertor heat flux profiles on COMPASS Petr Vondracek, Jan Horacek, Jiri Adamek, Michael Komm, Matyas Grof, Miroslav Sos, Martin Hron, Radomir Panek Divertor heat flux was studied for a set of L-mode discharges performed in the COMPASS tokamak. The heat flux profiles were measured by divertor infrared thermography ($\sim$0.6 mm/px, 8 kHz for this experiment) for both the inner and the outer divertor region and by divertor probes (two radial arrays of Langmuir probes and one array of Ball-pen probes, spatial step 4 mm, 4 MHz sampling rate) for the outer divertor region. Dependence of the heat flux decay length $\lambda_q$ on main plasma parameters was studied by varying the toroidal magnetic field, the heating power, the line averaged electron density and the plasma current. Very good agreement of both diagnostics for the sheath heat transmission coefficient $\gamma=11$ is presented. Observed heat flux decay lengths are compared to several existing multi tokamak scalings and modification of these scalings is proposed based on the measured data. [Preview Abstract] |
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GP10.00102: \textbf{HBT-EP Program: MHD Dynamics and Active Control through 3D Fields and Currents} G. A. Navratil, J. Bialek, J. W. Brooks, R. N. Chandra, J. P. Levesque, Bo-ting Li, M. E. Mauel, A. Saperstein, I. G. Stewart, Y. Wei, C. J. Hansen The HBT-EP active mode control research program aims to: (i) understand the physics of scrape-off layer currents (SOLC) and interactions between the helical plasma edge and conducting boundary structures, (ii) test new methods for measurement and mode control that integrate optical and magnetic detector arrays with both magnetic and SOLC feedback, and (iii) understand fundamental MHD issues associated with disruptions, resonant magnetic perturbations (RMP), and SOLC. An extensive set of SOLC sensor tiles attached to the movable outer wall and fixed inner wall have been installed, and used to study SOLC dynamics and current-sharing with the vacuum vessel wall during application of RMPs, kink-mode growth, and disruptions. A biased electrode in the plasma edge was used to induce a strong layer of sheared ExB flow with a transition into a biased H-mode and suppression of edge plasma turbulence. A 64-chord extreme UV/soft X-ray array has been installed to provide detailed internal MHD mode structure information, and was used to demonstrate for the first time kink-mode active feedback control using only non-magnetic sensor input into a GPU-based low latency control system. This GPU control system has also been extended to use driven local plasma current for active MHD mode control. [Preview Abstract] |
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GP10.00103: Mode suppression in HBT-EP using active feedback and a quadrature bias probe array J.W. Brooks, J.P. Levesque, R. Chandra, M.E. Mauel, G.A. Navratil, A. Saperstein, I.G. Stewart, Y. Wei, M.D. Boyer, C. Hansen Suppressing MHD instabilities in Tokamaks is one of the largest challenges facing the fusion community, and to address this, one set of solutions uses sourced electrical currents in the plasma. However, the path these currents take through the plasma and how they couple with MHD modes are not well understood. In this work, we set out to understand the currents' paths and coupling behavior by implementing four directional probes in quadrature in HBT-EP and configuring them to source parallel (co-IP or counter-IP), cross, or perpendicular currents as well as measure plasma rotation. HBT-EP's active feedback system, using a parallelized GPU with 21 \^{A}\textmu s cycle latency, determines the n$=$1 mode phase and then sources phase-locked currents at the probes to couple with and ideally suppress the m$=$2 and m$=$3 modes. Mirnov coils and scrape-off-layer sensors measure the flow of 14 kHz current driven by the probes and provide insight into the current's path, and a resistive Ohm\^{a}\texteuro \texttrademark s law current model is developed and compared to the experimental results. [Preview Abstract] |
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GP10.00104: Inverted Extreme Ultraviolet Emissivity Profiles: Towards Fully Nonmagnetic Active Control of Kink Modes R.N. Chandra, J.P. Levesque, J.W. Brooks, M.E. Mauel, G.A. Navratil Active feedback in tokamaks with magnetic sensors and actuators has been well established, from vertical position to RWM control. However, engineering constraints in reactor-scale devices may push sensor and feedback coils outside of the first wall or blanket, reducing attainable frequency response. Furthermore, previous magnetic feedback studies could not discriminate between disruptive external modes, and potentially benign internal ones without further diagnostic information. As a possible solution, the HBT-EP tokamak has installed a 64 chord extreme ultraviolet fan array (sensitive to 15-1000eV photons) for use as sensors together with a 22 $\mu$S cycle time Nvidia GPU system and voltage probe actuator. The goal of this study is to determine a basis set and discretization for tomographic recovery of the emissivity profile, which best enables real time discrimination and tracking of internal and external modes. Possible candidates such as Fourier-Bessel, local basis functions (the pixel method), and the Singular Value Decomposition of a reference shot are examined. Progress on kink mode control using these methods is reported. [Preview Abstract] |
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GP10.00105: Active Control of Kink Modes Using a Non-magnetic, Extreme Ultraviolet Sensor Array J.P. Levesque, J.W. Brooks, R.N. Chandra, Boting Li, M.E. Mauel, G.A. Navratil, A. Saperstein, I.G. Stewart, Y. Wei, C. Hansen Mode control in tokamaks often utilizes nonaxisymmetric magnetic sensors and actuators near the plasma surface. Placing magnetic coils behind walls would improve their longevity in a reactor at the expense of reducing frequency response, while a light-based detector could still respond quickly and with greater spatial sensitivity. We present the first demonstration of kink mode feedback control using only non-magnetic sensors consisting of extreme ultraviolet (EUV) detector arrays. Sixty-four poloidal views measure internal and external mode dynamics via plasma emissivity. Singular Value Decomposition (SVD) of EUV measurements is used to establish a basis set for calculating amplitude and phase of rotating perturbations. The poloidal spectrum of applied fields can adapt to changing structure of emissivity in real time. Feedback is completed using a graphics processing unit (GPU)-based control system with a total latency of 22$\mu$s [1]. We observe mode suppression and amplification as a function of the applied feedback phase angle relative to measured emissivity fluctuations. The system can directly extend to real-time tomographic reconstructions for mode or equilibrium control using an appropriate series-expansion method.\\ {[1]}N. Rath et al., Rev. Sci. Instr. 85, 045114 (2014) [Preview Abstract] |
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GP10.00106: Tangential Extreme Ultraviolet and Soft X-Ray (EUV/SXR) Diagnostic System on the HBT-EP Tokamak Boting Li, J.P. Levesque, G.A. Navratil, M.E. Mauel Non-invasive measurements of the internal characteristics of MHD mode structures and equilibrium evolution can be done with optical diagnostics. We present the progress on the design of the two-color multi-energy tangential extreme ultraviolet and soft x-ray (EUV/SXR) diagnostic system in the HBT-EP tokamak. The new system will allow reconstruction of temperature profiles and their fluctuations versus time. Two 16-channel diode arrays are used in the system. A filter wheel with four groups of dual-filters is adopted to implement multiple combinations of filters, accomplish easier calibration and protect the filters during discharge cleaning. By using combinations of filters with identical plasma views, it is possible to determine the electron temperature by the ratio of the amplitudes of the signals from different filters. The expected synthetic diagnostic for the emission characteristics of the equilibrium and perturbed temperature profiles of kink and tearing modes is shown. [Preview Abstract] |
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GP10.00107: High Speed Videography on Plasma Periphery in HBT-EP Y. Wei, J.W. Brooks, J.P. Levesque, A. Saperstein, I.G. Stewart, M.E. Mauel, G.A. Navratil Fast cameras have been widely used in identifying ELM filaments, turbulence structures, divertor strike points, as well as in analyzing MHD mode structures and frequencies in the plasma periphery [1]. A Phantom v7.1 high-speed camera has been installed on the HBT-EP tokamak. This camera operates in the visible range, and by mounting an optical filter it can also be used to study emission from a particular transition -- one of special interest is the D-alpha line. Through a zoom lens and a wound fiber bundle the camera observes the poloidal cross section from the inboard limiter to the low field side movable shell at a frequency of 66 kfps and resolution of 128x128 pixels, covering an in-vessel area of 225 square-centimeters at the cross section perpendicular to the central line-of-sight. Using this setup, videos of probe-biased H-mode have been taken and changes in the emission structure at the plasma periphery during L-H mode transition can be identified. In addition, the camera view can also be adjusted to focus on the newly-installed SOL current sensors located on the movable shells, and correlations between SOL current measurements and pixel light intensities have also been investigated. [1] Angelini, et al., Plasma Phys Contr Fusion, 57, 045008 (2015). [Preview Abstract] |
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GP10.00108: Characterizations of Scrape-Off-Layer Currents on HBT-EP Using Movable, Current Sensing Tiles A. Saperstein, J.P. Levesque, Y. Wei, I.G. Stewart, M.E. Mauel, G.A. Navratil Characterizations for the scrape-off-layer currents (SOLCs) have been made at HBT-EP using SOLC sensor tiles attached to movable walls, acting as poloidally segmented limiting surfaces. The segmentation allows for high resolution characterization of the poloidal structure of SOLCs, and the retractable shells allows for fine scans of their radial structure. SOLC measurements during MHD activity correlate well with those made by magnetic diagnostics, and their temporal structure can appear to be more “spikey” than the sinusoidal nature of the magnetics. The response of the SOLC sensors to applied 3D magnetic control coils has also been investigated, comparing the effects of low frequency pulses, static/rotating RMPs, and feedback. Rotating RMPs have also been used to show how the current collected by these tiles depends on the MHD mode frequency and amplitude. Lastly, we report on the installation of new set of inboard SOLC sensors, as well as another set of outboard SOLC sensors that complement the ones installed previously. The HFS sensors allow for better characterization of SOLCs during disruptions, and the designs for the complimenting set of LFS sensors have been modified to improve their performance. Comparisons to theoretical models are also made. [Preview Abstract] |
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GP10.00109: Investigation of turbulence and fast rotating MHD during biased H-mode on HBT-EP I.G. Stewart, J.W. Brooks, J.P. Levesque, M.E. Mauel, G.A. Navratil Suppression of turbulence and turbulent transport by sheared flow has been widely recognized as a means to improve confinement in tokamaks. Using a biased electrode on HBT-EP, a strong layer of sheared ExB flow forms at the plasma edge and an improvement in confinement has been observed in what has been termed a biasing induced H-mode. A scan of potential fluctuations reveals the suppression of turbulence is highest where the shear in the electric field is greatest, yielding a complex radial structure. Power spectra analysis indicates that most of the suppression occurs in the range of frequencies corresponding to the largest eddy size (between 10 and 100 kHz). The poloidal wavenumber-frequency spectrum of the turbulence is also dramatically modified between unbiased and biased periods. Additionally, the dynamics of fast rotating MHD modes, made possible by the relatively large radial extent of the electric field well (20\% of the minor radius), are analyzed. [Preview Abstract] |
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GP10.00110: Observations of the Moving Plasma Edge during Magnetic Perturbations and Instabilities in HBT-EP M.E. Mauel, J.W. Brooks, A. Saperstein, I.G. Stewart, Y. Wei, J.P. Levesque, G.A. Navratil Magnetic perturbations move the boundary of toroidally-confined plasma, which can be detected by local probes, optical videography\footnote{Angelini, \textit{et al.}, \textit{Plasma Phys Contr Fusion}, \textbf{57}, 045008 (2015).}, x-ray emmision, and scrape-off-layer currents (SOLC)\footnote{Levesque, \textit{et al.}, \textit{Nuc Fusion}, \textbf{57}, 086035 (2017).}. This poster summarizes these observations in the HBT-EP device caused by kink and tearing instabilities, resonant magnetic perturbations (RMPs), and disruptions. Particular attention is given to the physical structure of the helical ``filaments'' and ``bubbles'' associated with time-varying magnetic perturbations and the relationship of these structures to the strength and orientation of the magnetic perturbation. A linear relationship appears between the plasma's $n = 1$ helical displacement and the current flowing into the surrounding chamber$^3$, and these currents and plasma edge motion becomes very large during disruptions. As observed elsewhere, the SOLC in HBT-EP are primarily co-aligned with the plasma current and exhibit temporal distortion, consistent with an elevated electron temperature within the ``filament.'' This motivates calculations of the 3D magnetic field-line structure and comparison of this [Preview Abstract] |
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GP10.00111: SPARC Overview and Plans Dennis G Whyte The MIT Plasma Science and Fusion Center is partnering with Commonwealth Fusion Systems to design and build SPARC, a whose goal is to demonstrate net fusion plasma energy gain Q$_{\mathrm{p\thinspace }}=$P$_{\mathrm{fusion}}$/P$_{\mathrm{ext\thinspace }}$\textgreater 2 in a compact (R\textasciitilde 1.65 m) tokamak. The toroidal field coils will use REBCO high-temperature superconductors and R{\&}D progress towards achieving B$_{\mathrm{0}}=$12 tesla will be described. SPARC's detailed design and mission are discussed, including the possible addition of an advanced long-leg divertor. While funded by the private-sector, SPARC provides diverse opportunities for collaborations with the US and international fusion community. Work supported by Commonwealth Fusion Systems. [Preview Abstract] |
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GP10.00112: CFS, the high-field approach, and U.S. Strategic Planning Robert Mumgaard Commonwealth Fusion Systems (CFS) is an investment backed company developing the high-field approach to commercial fusion energy in conjunction with the MIT Plasma Science and Fusion Center through the deployment of SPARC and ARC. The ongoing strategic planning exercise for the FES-funded fusion science program presents an opportunity to bolster the entire bourgeoning U.S. private fusion industry through the advancement of fusion science and technology and the deployment of unique test stands. The following elements of a U.S. strategic plan have been identified as strongly beneficial to the CFS commercialization effort: 1) A volumetric neutron source to test materials at fusion-relevant nuclear conditions. 2) Development of fusion-relevant nuclear materials. 3) A dedicated facility to study advanced divertor topologies in the relevant parameter range. 4) Development of commercially-relevant blanket systems. 5) Advancement of tritium science and technology. 6) Innovative plasma current drive. 7) Additive manufacturing of fusion materials. The role of each of these initiatives in the CFS high-field commercialization plan will be discussed. Work supported by Commonwealth Fusion Systems. [Preview Abstract] |
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GP10.00113: \textsc{STATUS OF HTS MAGNET DEVELOPMENT FOR SPARC} Brandon Sorbom, Daniel Brunner, Jessica Cheng, Vincent Fry, Zach Hartwig, Amanda Hubbard, Brian LaBombard, Robert Mumgaard, Erica Salazar, Rui Vieira, Dennis Whyte The production of high-temperature superconductors (HTS) has recently reached commercial maturity at the scale and performance required to build large bore, high-field magnets, enabling a breakthrough opportunity to accelerate fusion energy. This poster will give an overview of the magnet work performed over the past year towards the development of HTS magnet systems to be used in the SPARC device. Several large-scale cable tests were successfully performed at the SULTAN facility at fields up to 10.9 T carrying SPARC-relevant JxB loads over 1000's of cycles. In addition to the experimental work, the design of a TF model coil has begun with the goal to build and test it within the next two years. The SPARC team has collaborated with HTS centers of excellence around the world to measure the performance of HTS samples from multiple vendors up to 24 T as well as building the facilities to perform this work in-house in the future. Finally, the team has actively engaged the HTS industry, becoming the largest customer of HTS tape in the world and working with suppliers to increase tape performance and rapidly scale the industry to volumes relevant to SPARC and then ARC. [Preview Abstract] |
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GP10.00114: Multiple Choice Tokamak Reactor Design: (a) Steady state (b) Pulsed (c) None of the above J Freidberg, D Segal, A Cerfon The work presented here re-examines the long held US consensus that a tokamak reactor must operate as a steady state, rather than a pulsed, device. There are two reasons motivating this re-examination. First, current drive has proven to be more difficult to achieve than originally believed. The efficiency of the most favorable method, lower hybrid current drive, is just too low. Second, in order for pulsed devices to be able to survive cyclical stresses, earlier designs resulted in relatively large, economically unattractive reactors. Our analysis attempts to reassess these issues by making use of the game changing development of new high temperature superconductors (HTS). The ideas are as follows. Make the reactor pulsed in order to resolve the current drive problem. Make the OH transformer and toroidal field coils out of HTS, achieving maximum B $=$ 22 T. A high toroidal field is expected to improve performance leading to a smaller reactor. Similarly, a high field OH transformer should substantially reduce the reactor size, since the same flux swing is now possible with a smaller coil radius. In addition, advanced technologies involving demountable joints and liquid blankets reduce the major component replacement down time. This allows high average power production even with shorter pulses in compact reactors limited to the same number of stress cycles as larger low field pulsed reactors. Do these ideas make pulsed reactors competitive with steady state reactors? Results will be presented at the meeting. [Preview Abstract] |
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GP10.00115: Solid Standing of the Compact High Field Machine Approach for Fusion D. Carnevale, B. Coppi, Ignitor Program Group Although the value of investigating the physics of plasmas close to or at ignition condition has never been questioned, the ``practical relevance’’ of efforts with this goal [1] has been frequently passed under silence. By now studies of the requirements of power producing reactors have led to conclude that operating at ignition is necessary for this kind of reactors. The confinement scaling laws [2], that were identified when high field compact experiments began to be proposed originally in order to investigate igniting plasmas, have been rediscovered. Both ``Renovatio Memoriae'' [3] and ``Damnatio Memoriae'' [4] episodes have occurred in this context and in reference to the first introduction [2] of high field superconducting magnet technology in fusion research. The record confinement parameters, beginning to approach the ideal ignition conditions, obtained by the Alcator C Mod machine have validated the perspectives of success of the relevant line of experiments [2].\\ $[1]$ B. Coppi, American Institute of Physics, 1721, 1, (2017) 020003.1. \\ $[2]$ B. Coppi, A. Airoldi, R. Albanese, et al., Nucl. Fus., 55, (2015) 053011.\\ $[3]$ A.E. Costley, et al., Nucl. Fus., 56, (2016) 066003.\\ $[4]$ D. Kramer, Physics Today, 71, 8, (2018). [Preview Abstract] |
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GP10.00116: Characteristics of a Low Frequency Edge Oscillation seen during I-Mode on Alcator C-Mod William McCarthy, Brian Labombard, Adam Kuang, Dan Brunner, Amanda Hubbard, Theodore Golfinopoulos The I-mode confinement regime is characterized by H-mode like thermal confinement, L-mode like particle confinement and being ELM free, making it a good candidate for reactor scenarios. The Weakly Coherent Mode, a broad fluctuation ($\sim$200 kHz central frequency on C-mod) localized to the pedestal region is thought to cause the enhanced particle transport. A second mode, with much lower frequency ($\sim$15 kHz), has been observed in I-mode discharges. The mode spans the last closed flux surface and can be seen on divertor Langmuir probes as spikes in ion saturation current, and in a variety of other diagnostics. This mode likely contributes to I-mode transport. A database containing a large number of I-mode discharges has been assemble to investigate key questions: parameter space dependence on mode existence, central frequency and frequency width. Temporal dynamics of the mode have been explored using a scanning Langmuir Mach probe with a Mirror Langmuir probe bias system. Preliminary analysis suggests that thermal and particle transport are driven in opposite directions near the LCFS. [Preview Abstract] |
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GP10.00117: Towards predicting pedestal evolution in fusion plasmas using machine learning methods Abhilash Mathews, Jerry Hughes, Amanda Hubbard, Anne White, David Hatch Edge transport barriers strongly influence energy and particle confinement, and in turn the energy gain of tokamaks, yet a fully predictive model of pedestal formation and transport is lacking. Pedestal structure is constrained by magnetohydrodynamic limits when approaching instability due to edge localized modes (ELM), but a general model for ELM-free regimes is absent. Towards this target, computational methods for predicting edge pedestal evolution by training plasma simulations against two-dimensional (i.e. radius and time) experimental profiles are explored on Alcator C-Mod. Gaussian processes with an adaptive kernel are capable of systematically constructing pedestal evolution training data with quantified uncertainties based upon plasma diagnostic measurements. This pathway can be extended across tokamaks for cross-machine validation to advance predictive capability of pedestal dynamics. [Preview Abstract] |
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GP10.00118: Comparison of Recursive Neural Networks and Random Forests for disruption prediction on C-Mod Jinxiang Zhu, Kevin Montes, Cristina Rea, Robert Granetz A disruption prediction algorithm based on the Recurrent Neural Network method has been developed using a large database of both disruptive and non-disruptive discharges from Alcator C-Mod. The previous C-Mod disruption warning database had a different sampling scheme for the disruptive and non-disruptive shots. This sampling difference made some artificial effects to the training and testing of the Neural Network. To avoid this problem, we repopulated the disruption warning database with the same uniform sampling rate for both disruptive and non-disruptive shots. The Recurrent Neural Network algorithm was trained on flattop data consisting of 10 plasma signals, with a uniform 5 ms sampling rate. However, because of the fast time scale of C-Mod disruptions, only the last 50 ms of the disruptive shots was used in the training process. A shot-by-shot testing scheme has been developed to give a disruption warning alarm using an optimized control threshold, and we successfully achieve over 90\% accuracy on the test dataset. A comparison with the results from a Random Forests algorithm will be shown, and the first results from the sensitivity analysis of a trained neural network will be presented. [Preview Abstract] |
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GP10.00119: Numerical SOLPS-ITER study of the effect of fueling on neutral density profiles in high opacity H-modes on Alcator C-Mod Richard Reksoatmodjo, Saskia Mordijck, Jerry Hughes, Jeremy Lore, Xavier Bonnin The role of neutrals in setting density pedestal structure was evaluated in experiments on Alcator C-Mod, in H-mode regimes approaching ITER-like edge neutral opacities. Enhanced D-alpha H-modes at high plasma and power densities were realized at moderate and high current to give an opacity scan, with wall recycling neutrals only, as well as with supplemental gas puffing. To assess the role of fueling versus transport at the plasma edge, we use the SOLPS-ITER code suite to first calculate the radial and poloidal neutral density profiles for discharges at varying density. We initially match the upstream experimental radial fluxes as well as density and temperature profiles by varying the radial transport coefficients in SOLPS-ITER. A simple ballooning transport model is implemented to break the poloidal symmetry of the neutral density distribution around the vessel. Simulations of the neutral density in the higher opacity discharge exhibit more resistance to the effects of ballooning transport when compared to simulations of the lower opacity discharge. Neutral gas puff sources are being implemented in the model to further probe neutral transport dynamics into and along the opaque SOL, including near the divertor targets, where detachment is approached at high fueling rates. [Preview Abstract] |
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GP10.00120: Validating the SOLPS-ITER drift model using C-Mod data Eric Meier, Xavier Bonnin, Daniel Brunner, Wouter Dekeyser, Jerry Hughes, Adam Kuang, Brian LaBombard, Robert Mumgaard, Richard Pitts, Matthew Reinke, Ilya Senichenkov To develop practical tokamak fusion reactors, accurate edge plasma transport modeling is critical. The need to properly model impurity transport is particularly acute: injected impurities are necessary to dissipate exhaust power to a level that ensures survival of divertor targets but core impurity contamination must be limited. The advanced capability of the SOLPS-ITER code to capture plasma drifts has made it a focal point of the tokamak community. On the Alcator C-Mod tokamak, exceptionally well-diagnosed experiments with impurity injection have been performed. Available diagnostics include upstream Thomson scattering, target Langmuir probes, divertor neutral pressure, multi-channel spectroscopy, and bolometry. SOLPS-ITER is applied to a 5.4 T, q$_{\mathrm{95}}=$4.9 EDA H-mode, a steady state ELM-free regime, with q$_{\mathrm{\vert \vert }}$ up to 0.4 GW m$^{\mathrm{-2}}$, which has steady phases with and without toroidally symmetric private flux region N$_{\mathrm{2}}$ injection. Initial results for the phase without N$_{\mathrm{2}}$ show clearly that drifts are needed to reproduce measured edge plasma profiles. Progress on modeling with drifts and with nitrogen injection will also be reported. [Preview Abstract] |
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GP10.00121: Current Drive Scaling of Local Helicity Injection in the \textsc{Pegasus} Toroidal Experiment G.M. Bodner, M.W. Bongard, R.J. Fonck, J.A. Reusch, N.J. Richner Local Helicity Injection (LHI) is a non-solenoidal startup technique that utilizes electron current injectors at the plasma edge to initiate a tokamak-like discharge. In order to determine the scalability of LHI to MA-class facilities, it is necessary to identify the key parameters that dictate LHI performance$.$ Injection on the high-field-side (HFS) allows for the creation of discharges driven purely by helicity injection. Ohmic and stochastic confinement scalings predict a favorable non-linear relationship between $I_{p} $ and drive voltage $V_{LHI} $. Recent experiments have indicated a linear current drive scaling suggesting a constant impedance. This scaling has been observed over different levels of $B_{T} $ and MHD activity. Thomson measurements at low $B_{T} $ indicate hollow $T_{e} $ profiles that increase in $\left\langle {T_{e} } \right\rangle $ and decrease in $\left\langle \eta \right\rangle $ as the input power is increased. Despite this decrease in $\left\langle \eta \right\rangle $, the current drive scaling remains linear. At higher levels of $B_{T} $, peaked $T_{e} $ profiles ($T_{e,0} \sim 100\;$ eV) and higher $I_{p} $ are observed for the same amount of $V_{LHI} .$ These results have been compared to the first Thomson documentation of Ohmic discharges in \textsc{Pegasus} which feature $T_{e} \le 250$ eV. Calculation of neoclassical resistivity and plasma impedance from equilibrium reconstructions and Thomson data suggest this scaling result may be attributed to an increase in $Z_{eff} .$ [Preview Abstract] |
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GP10.00122: The Coupling of Local Helicity Injection to Ohmic Current Drive on the \textsc{Pegasus} ST C. Pierren, G.M. Bodner, M.W. Bongard, R.J. Fonck, N.J. Richner, C. Rodriguez Sanchez, C.E. Schaefer Ohmic (OH) operations on \textsc{Pegasus} have been restored to provide comparison of MHD activity in OH and local helicity injection (LHI) driven discharges and to provide tests of handoff from LHI to OH current drive. Temperatures of 250 eV and densities of $2\times 10^{19}$ m$^{\mathrm{-3}}$ were measured with Thomson scattering in OH H-mode plasmas. LHI discharges at similar density and $B_{T} $ show $T_{e} \sim 150$ eV and are comparable to OH L-mode plasmas. Insertable radial arrays of magnetic pick-up loops and 3D Hall sensors observe typical $n=1$ internal tearing mode MHD activity and a large reduction in broadband MHD fluctuations relative to LHI plasmas. The magnetic and kinetic boundaries coincide in OH plasmas, in contrast to LHI plasmas, where the kinetic boundary appears to occur several cm inside the magnetic edge. This evidence suggests the presence of a a dual-zone confinement structure during LHI, with an inner tokamak-like plasma and an outer force-free current layer. Spectroscopic measurements of OH plasmas show a reduction in impurity content compared to LHI. LHI-initiated plasmas driven predominantly by helicity injection and non-solenoidal induction have been successfully coupled to pure OH sustainment at $I_{p} \sim 100$ kA. Remaining OH studies are concentrating on optimizing LHI-OH handoff through variations in $J(R),T_{e} ,B_{T} ,n_{e} ,$ etc. [Preview Abstract] |
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GP10.00123: Characterization of Magnetic Structure and Activity in Local Helicity Injection C.E. Schaefer, G.M. Bodner, M.W. Bongard, R.J. Fonck, J.A. Reusch, N.J. Richner Local Helicity Injection (LHI) is a non-solenoidal startup technique that initiates a tokamak-like discharge using electron current injectors at the plasma edge. Comparisons on the \textsc{Pegasus} ST of internal 3D $\mathbf{B}(R,t)$ Hall probe measurements with Thomson pressure profiles show the magnetic boundary is shifted up to 8 cm outward relative to the kinetic pressure edge. In Ohmic-driven discharges this disparity is not present. In comparison to Ohmic, LHI discharges show increased broadband and low-frequency $n=1$ magnetic activity that is localized to the edge region where the injected current streams presumably exist and the kinetic pressure is near zero. The broadband activity exhibits power law behavior resembling Alfv\'{e}nic turbulence, while high-frequency activity ($f\approx 2$ MHz) increases with total LHI drive. These observations, plus earlier reports of anomalous ion heating in the edge region, suggest a two-zone confinement structure during LHI consisting of an inner tokamak-like plasma and an outer force-free region of injected current. The outer region appears to be characterized by strong local magnetic and reconnection activity, poor thermal confinement, and presumably strongly stochastic field structures. These measurements are being applied to studies of the spontaneous reduction of low-frequency MHD activity and consequent improvement of LHI current drive. [Preview Abstract] |
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GP10.00124: Assessment of Impurity Content and Radiated Power in LHI discharges on the \textsc{Pegasus} ST C. Rodriguez Sanchez, G.M. Bodner, R.J. Fonck, M.D. Nornberg, C. Pierren Local Helicity Injection (LHI) is routinely used in the \textsc{Pegasus} ST to produce high-performance discharges with $I_{p} \le 0.24$ MA, $n_{e} \approx 10^{19}$ m$^{\mathrm{-3\thinspace }}$and $T_{e} \approx 100$ eV. Recent experiments show that the impedance of the plasma is independent of the helicity drive as $I_{p} $ scales linearly with $V_{LHI} $. One possible explanation of this behavior would be that $Z_{eff} $ and the plasma resistivity increases with $I_{p} $. To examine this possibility an assessment of radiated power and impurity concentration is being done in \textsc{Pegasus} using a tangential bolometer array, VB spectroscopy and a SPRED VUV spectrometer. Discrete line radiation from N and O are at least 10$\times $ higher during LHI than Ohmic. A broadband spectrum at $\lambda \le 35$ nm is observed during LHI but absent during Ohmic discharges. Radiated power increases rapidly with $V_{LHI} $, suggesting an increase in $Z_{eff}_{\mathrm{\thinspace }}$as $I_{p} $ increases. Initial observations suggest Bremsstrahlung radiation measurements are infeasible during LHI but may be measured during an Ohmic phase in an LHI-Ohmic handoff scenario. To quantify estimates of impurity content, a new multichannel diode bolometer array with full coverage across the plasma is being developed. [Preview Abstract] |
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GP10.00125: Development of Non-Solenoidal Tokamak Startup on the \textsc{Urania} Experiment A.T. Rhodes, M.W. Bongard, S.J. Diem, R.J. Fonck, J.A. Goetz, B.A. Kujak-Ford, B.T. Lewicki, M.D. Nornberg, A.C. Palmer, J.A. Reusch, J.D. Weberski, G.R. Winz Initiation of plasma current without a central solenoid is a critical scientific goal for the spherical tokamak (ST). Following several campaigns on the \textsc{Pegasus} Toroidal Experiment using local helicity injection (LHI), which achieved $I_{p} >0.2$ MA without use of an Ohmic solenoid, the facility is being shut down to upgrade into the Unified Reduced $A$ Non-Inductive Assessment (\textsc{Urania}) experiment. This enhancement to the \textsc{Pegasus} facility will remove the central solenoid entirely, increase the toroidal field a factor of 4, up to 0.6 T, and investigate a variety of non-inductive startup techniques, including LHI, transient and sustained coaxial helicity injection (T- and S-CHI), poloidal field induction, and EBW radiofrequency heating and possibly current drive. A next-generation LHI system is being designed and built for \textsc{Urania}. Unlike the previous circular plasma-cathode electron sources used for LHI, this injector has a curved-slot arc channel of 1 cm width and 16 cm in length that is mounted off of a re-entrant port. This geometry allows for large $A_{inj} $ for helicity input, while keeping $w_{inj} $ low for a high Taylor limit, and matches the plasma edge curvature. A prototype of this design is currently in fabrication for initial testing with $I_{inj} $ from 8--16 kA, and $V_{inj} $ up to 1 kV. [Preview Abstract] |
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GP10.00126: Design Status of the \textsc{Urania} Experiment A.C. Palmer, M.W. Bongard, R.J. Fonck, B.T. Lewicki, J.A. Reusch, G.R. Winz The \textsc{Urania} experiment is a conversion of the \textsc{Pegasus} spherical tokamak experiment designed to support $B_{T} =$ 0.6--0.9 T operation (4$\times $--6$\times $ \textsc{Pegasus} values) for up to 100 ms. It includes new: center rod; outer TF coil system; torque plate assemblies; and integrated divertor coils. The center rod system is comprised of 24 water-cooled, insulated wedge conductors placed inside a new inner vacuum wall. There is no Ohmic solenoid, which allows sufficient Cu for the $\le 72$ kA/turn current while retaining aspect ratios as low as 1.17. The outer TF conductors are 12 pairs of air-cooled, reinforced C-shaped plate conductors. Pairwise crossed conductor links connect the outer C-plates to the central rod conductors to eliminate the need for a toroidal compensation wind-back coil. Torque plate assemblies on top and bottom consist of several insulating plates that mechanically secure both the outer C-conductors and the interconnecting links. This torque plate assembly counteracts torsional magnetic loads and provides compliance for vertical displacement. The assembly accommodates MN magnetic forces and limits the axial excursion of the central assembly to \textless 1 mm. This leads to tolerable tensile and compressive loads on the center rod assembly that arise from magnetic and thermal expansion. This allows a robust, fixed electrical joint design for the TF coils in the critical high-stress core region. That joint employs a belt and compression wedge approach developed on \textsc{Pegasus} to provide 30 MPa of contact pressure for the 72 kA electrical connection. [Preview Abstract] |
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GP10.00127: Diagnostic Neutral Beam and Beam-Based Diagnostic Development to Study Non-Inductive Startup Techniques A.K. Keyhani, M.T. Borchardt, R.J. Fonck, B.T. Lewicki, M.D. Nornberg, G.R. Winz An 80 kV, 4 A, H$^{\mathrm{0}}$ beam is being developed as a plasma diagnostic tool for studying non-inductive tokamak plasma startup methods in the \textsc{Urania} experiment. The new diagnostics will provide measurements of equilibrium magnetic fields and possibly field fluctuations, ion temperatures, and plasma density profiles. These beam-based diagnostics will address important aspects of helicity injection startup and sustainment. These include examining dissipation mechanisms during helicity drive ($e.g.$ anomalous ion heating through magnetic reconnection, anomalous resistivity, etc.), the role of impurities, and plasma magnetic field structures. A washer stack arc plasma source is implemented as the source of H$^{\mathrm{+\thinspace }}$for the DNB. Its plasmas are characterized with a double tip Langmuir probe, a spectrometer, and measurements of the arc voltage and current. Initial results show that stable density plasmas ($n_{e} \approx 10^{17}$m$^{\mathrm{-3}})$ with steady electron temperatures of 0.5--4 eV can be generated by the source, which are expected to produce a full energy species fraction of greater than 80{\%}. A novel 80 kV resonant DC-DC converter power supply has been constructed and is being optimized for minimal output voltage ripple. Initial tests have produced a 10 ms pulse at 40 kV. [Preview Abstract] |
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GP10.00128: CHI Research on \textsc{Urania} R. Raman, J.A. Reusch, M.W. Bongard, F. Ebrahimi, R.J. Fonck, J.A. Goetz, T.R. Jarboe, B.A. Nelson, M. Ono, G.R. Winz The spherical tokamak (ST) may require and the advanced tokamak would considerably benefit from the elimination of the central solenoid. \textsc{Urania} is a ST non-solenoidal startup development station under design and fabrication dedicated to solving the startup problem. On \textsc{Urania}, Transient and Sustained coaxial helicity injection (T- and S-CHI) will be explored, as well as possible synergies of CHI with local helicity injection and EBW heating and current drive. T-CHI has shown promising capability on the HIT-II and NSTX STs. However, in both these machines the vacuum vessel was electrically cut. For reactor applications a simpler biased electrode configuration is required. To develop this capability a single biased electrode is being tested on QUEST, where up to 45 kA of toroidal current has been generated using CHI. \textsc{Urania} will use a more advanced double biased electrode configuration with optimized injector electrodes and injector poloidal field coils that should allow the T-CHI system to generate 0.3 MA of closed flux current, the limit permitted by the equilibrium PF coils. Present design indicates that standard divertor coils will provide sufficient flux for CHI studies but may be enhanced with increased current capabilities if needed. The CHI design and the CHI research plan for \textsc{Urania} will be described. [Preview Abstract] |
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GP10.00129: Suppression of microtearing transport in a diamagnetic well induced at high-beta in the low-aspect-ratio Pegasus spherical torus David R. Smith, M. Bongard, R. Fonck, G. McKee, J. Reusch, P. Terry, Z. Williams, M.J. Pueschel A diamagnetic well and local minimum \textbar B\textbar region is readily accessed in high-$\beta $ plasmas driven by local helicity injection in the $A\sim $ 1 Pegasus ST. This magnetic topology may afford novel, favorable characteristics affecting turbulent transport. $\nabla $B reversal on the low-field-side is stabilizing for drift waves, reduces the trapped particle fraction, and expands the parameter space for fast ion trapping. The high-$\beta $ plasma, however, remains net-paramagnetic with near omnigeneity (\textbar B\textbar $\approx $ \textbar B\textbar ($\psi ))$ in the bad curvature region. Here, we report on the gyrokinetic stability of microtearing modes in the Pegasus minimum \textbar B\textbar regime. Multiple classes of microtearing instabilities at k$_{\mathrm{y}}\rho _{\mathrm{s}}\sim $ 0.1-1 arise in the magnetic well region at $\psi _{\mathrm{N}}\sim $ 0.3-0.9. Collisionless high-k modes (k$_{\mathrm{y}}\rho_{\mathrm{s}}\approx $ 1) with narrow parallel mode structures are destabilized at $\beta_{\mathrm{crit}}\approx $ 3{\%}, and collisional low-k modes (k$_{\mathrm{y}}\rho _{\mathrm{s}}\approx $ 0.3) with extended parallel mode structures are destabilized at $\beta_{\mathrm{crit}}\approx $ 12{\%}. Nonlinear gyrokinetic simulations for a conventional monotonic \textbar B\textbar equilibrium show that the low-k modes produce electromagnetic electron thermal transport, but the transport and low-k instabilities are suppressed in the diamagnetic well configuration. [Preview Abstract] |
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GP10.00130: Impurity and transport measurements in LTX plasmas fully surrounded by liquified lithium surfaces D.P. Boyle, R.E. Bell, P.E. Hughes, M. Lucia, R. Majeski, E. Merino, J.C. Schmitt, R. Kaita, A. Maan, F. Scotti, S, Kubota, C. Hansen, T.M. Biewer, D.B. Elliott, T.K. Gray The first successful operation of a tokamak almost fully surrounded by liquified lithium surfaces was achieved in the Lithium Tokamak Experiment (LTX), prior to its upgrade to LTX-$\beta $. While early attempts at operating with lithium coatings above the lithium melting temperature suffered poor performance due to excessive impurities, improved techniques for lithium evaporation and wall/vacuum-conditioning allowed for operation at 260 ${^\circ}$. Here we present new analysis of lithium, carbon, and oxygen impurity profiles in the experiments with liquified lithium coatings, and compare them to measurements with solid coatings. Analysis shows similar, but modestly higher impurity concentrations with liquified Li. Enhanced diagnostics in LTX-$\beta $, including improved spectroscopy and Thomson scattering systems, now enable detailed measurements in a wider parameter space of plasma and surface conditions. Analysis and comparison of impurity profiles and transport will be presented for LTX, as well as for new experiments in LTX-$\beta $. [Preview Abstract] |
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GP10.00131: First results from the Lithium Tokamak Experiment - $\beta $ R. Majeski, R. E. Bell, D. P. Boyle, P. E. Hughes, T. Kozub, A. LeViness, E. Merino, X. Zhang, J. K. Anderson, W. Capecchi, P. Beiersdorfer, M. A. Dorf, F. Scotti, V. A. Soukhanovskii, T. Biewer, D. B. Elliott, D. Donovan, R. Kaita, A. Maan, C. Hansen, B. Koel, S. Kubota, T. Rhodes, L. E. Zakharov LTX$\beta $, the upgrade to the Lithium Tokamak Experiment, has operated with full lithium coating of the plasma-facing surfaces, at increased toroidal field of 0.3 T. Plasma current has so far been limited to 100 kA. The upgrade includes a neutral beam injector provided by Tri-Alpha Energy Technologies - 600 kW of beam power has been injected so far. Up to 60{\%} of the injected power is deposited in the plasma, in agreement with NUBEAM modeling. Significant beam fueling is observed under some conditions. New insertable lithium evaporators have been installed on LTX$\beta $, which provide full wall coatings, with a 10-15 minute evaporation cycle. LTX$\beta $ retains the same plasma geometry, and the heated high-Z liner featured in LTX. Upgrades to the diagnostic set include active CHERs. New Lyman- $\alpha $ arrays will permit a determination of energy confinement time as a function of recycling. Here we will discuss first results from LTX$\beta $, as well as the research goals. [Preview Abstract] |
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GP10.00132: Improved control and operation of the Lithium Tokamak eXperiment-Beta (LTX-$\beta )$ E. Merino, R. Majeski, D. P. Boyle, P. E. Hughes, G. Tchilinguirian, G. Zimmer, S. Doskoczynski, T. Kozub, D. B. Elliot, A. Maan Start-up of the Lithium Tokamak eXperiment-Beta (LTX-$\beta )$ required the development of new systems for control of the shot cycle, timing of power supplies and diagnostics, and management of database storage. This effort involved the integration of pre-existing (Ohmic heating power supply, neutral beam control) and newly added (beam calorimetry) systems. Additional focus was placed on improving the hardware used for these upgrades. An enterprise level server was recently set up to manage the new database and the data acquisition devices. In addition, the Neutral Beam system saw an upgrade of its computer hardware and software. An overview of all the above will be presented. [Preview Abstract] |
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GP10.00133: Charge exchange recombination spectroscopy on the upgraded Lithium Tokamak eXperiment-$\beta $ Drew Elliott, Theodore M. Biewer, Ronald Bell, Dennis Boyle, Paul Hughes, Richard Majeski, Christopher Hansen, Robert Kaita, Shigeyuki Kubota The Lithium Tokamak eXperiment-Beta (LTX-$\beta )$ has recently begun its initial campaign. LTX-$\beta $ has been operated with typical plasma currents \textgreater 85 kA and line averaged densities above 10$^{\mathrm{18}}$ m$^{\mathrm{-3}}$. A major part of the upgrade was the addition of neutral beam injection (NBI); NBI during the plasma has been achieved with extracted power of \textgreater 500 kW, more than 3x the average ohmic heating power. To capitalize on the NBI we have installed a new optical system for active charge exchange recombination spectroscopy (CHERS). The new CHERS system, intended to focus on lithium, will measure local impurity concentration, ion temperature, and toroidal velocity. The system has 52 total views, split into 4 groups of 13. The multi-view setup allows for major radii between 26 cm and 59 cm to be sampled with a \textasciitilde 2 cm spatial resolution. The plasma major radius has been 34-38 cm with a minor radius of \textasciitilde 20 cm. Half of the views face away from the beam so that the background can be simultaneously subtracted. The cameras used for CHERS have a typical sampling rate of 2.0-2.5 ms while the NBI duration is 5 ms and the plasma duration is \textgreater 40 ms. Beam optimization, initial charge exchange emission, and comparisons between the different spectrometers and emission lines will be highlighted. [Preview Abstract] |
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GP10.00134: A neutral particle analyzer for fast ion physics studies in LTX-b W. J. Capecchi, J. K. Anderson, R. Majeski, R. Kaita, D. P. Boyle, P. E. Hughes, D. B. Elliot, C. Hansen, L. E. Zakharov The LTX-b device is unique in its capability to produce tokamak plasmas with nearly zero wall recycling. A 0.7MW neutral beam injector serves as a source of heating and fueling, as well as~super-Alfvenic ions, whose non-thermal pressure may approach a level sufficient to destabilize the TAE or other Alfvenic eigenmodes found in the spherical tokamak. The low recycling environment results in a flat electron temperature profile, is expected to result in flat ion temperature profiles, and a very low background neutral density, reducing the drag on beam-induced plasma rotation and possibly affecting the damping of fast ion driven instabilities. As such, the study of the NBI-heated LTX-b plasma is valuable both for quantifying the effect of fast ions on a lithium-covered first wall, and for general understanding of instabilities that may arise in a future burning plasma. This work describes initial calculations of expected instability drive and damping in LTX-b, as well as the design of a neutral particle analyzer with sufficient sensitivity and time resolution to study instability-induced changes to the fast ion distribution. [Preview Abstract] |
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GP10.00135: Initial Measurements of Electron Density Fluctations on LTX-$\beta$ Using Microwave/Millimeter-Wave Diagnostics S. Kubota, R. Majeski, D.P. Boyle, P.E. Hughes, R. Kaita, R. Lantsov, W.A. Peebles, T.L. Rhodes, C. Hansen The mechanisms for fluctuation suppression due to low-recycling lithium walls and their impact on the confinement properties of the LTX-$\beta$ device are key research topics for exploring the potential of lithium as a surface coating for plasma facing components in future fusion devices. UCLA operates a suite of microwave/millimeter-wave diagnostics for electron density profile and fluctuation measurements on LTX-$\beta$. Upgrades to existing systems will increase the sensitivity of the 288 GHz interferometer and the repetition rate of the 13.5$-$33 GHz profile reflectometers (sweep time down to 4 $\mu$s). Measurements from both diagnostics will be presented, as well as first results from the new two-channel tunable-frequency quadrature reflectometer (13.5$-$20.5 and 27$-$40 GHz individually), specifically designed for measuring low-$k$ electron density fluctuations over a wide spectral bandwidth (up to 5 MHz). Quantitative results from the fluctuation measurements (e.g.\ spatial localization, amplitude, wavenumber spectra) will require synthetic diagnostics and analysis tools. This will include modeling far-forward scattering of the interferometer beam as well as backscattering from the profile reflectometers. Discussion of these tools will also be presented. [Preview Abstract] |
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GP10.00136: Surface characterization and local recycling measurement for evaporative lithium coatings on LTX-$\beta$ A. Maan, E. Ostrowski, R. Kaita, D. Donovan, D.P. Boyle, P.E. Hughes, R. Majeski, E. Merino, F. Scotti, V. Soukhanovskii, B.E. Koel, T.M. Biewer, D.B. Elliott, C. Hansen Evaporative coatings on LTX and LTX-$\beta$ have been used to improve plasma performance, including higher confinement times and flat temperature profiles. This has been attributed to a reduced recycling boundary, which retains a high fraction of incoming hydrogen. LTX-$\beta$ recently employed a vacuum suitcase called the Sample Exposure Probe (SEP), to transfer samples of plasma-facing components (PFCs) under vacuum after plasma exposure to an analysis station for X-Ray Photoelectron Spectroscopy (XPS). The XPS indicates that the lithium coatings on PFCs oxidize to form Li2O as well as LiOH. It was initially hypothesized that elemental lithium is needed to provide a low recycling boundary by retaining incoming hydrogen as LiH. It appears, however, that a low recycling boundary is possible even in the presence of oxidized lithium. Using Langmuir probe and filterscope data to estimate the recycling, we correlate how it varies with surface lithium species as identified through analysis of PFC samples. [Preview Abstract] |
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GP10.00137: Physics Results from the LTX-$\beta$ Magnetics Upgrade P.E. Hughes, C. Hansen, F. Scotti, D.P. Boyle, R. Majeski Lithium Tokamak eXperiment--Beta (LTX-$\beta$) has begun operating with increased magnetic fields, lithiated walls, and neutral beam core-fueling, allowing a new series of experiments into the low-recycling, high-performance regime previously observed in LTX [D.P. Boyle \textit{et al. PRL} July 2017]. Using the upgraded magnetic diagnostic suite [P.E. Hughes \textit{et al. RSI} Oct. 2018], we investigate the effect of the neutral beam on stored energy, plasma $\beta$, and MHD instability drive, and compare against previously observed LTX performance. Analysis techniques are explored to identify $n\leq5$ and $m\leq10$ MHD activity on the TA and REPA, and the SESA is studied for comparison to the eigenmode modeling of predicted shell eddy currents. Reconstructions in the PSI-Tri equilibrium code [C. Hansen \textit{et al. PoP} Apr. 2017], employing the full upgraded magnetic diagnostics suite now including a newly compensated diamagnetic loop, are compared against fast camera data and prior LTX reconstructions. [Preview Abstract] |
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GP10.00138: Development and implementation of an external diamagnetic loop on LTX-$\beta $ Alexandra LeViness, Dennis Boyle, Chris Hansen, Paul Hughes, Richard Majeski The diagnostic set for LTX-$\beta $, the upgrade to the Lithium Tokamak Experiment, already includes a compensated diamagnetic loop within the vacuum vessel for determination of the plasma pressure---key for measuring stored energy and energy confinement time. We describe the development and implementation of a second compensated diamagnetic loop, external to the vacuum vessel, for confirmation of the internal loop's measurements. This second loop makes use of an empty cooling channel within the center stack and is more easily accessed for alignment, given its external position. One additional challenge of this external loop is the inclusion of a small amount of toroidal flux between the Ohmic solenoid coil and its return conductor in the center stack, which must be compensated for. The measurements from this loop are compared against those from the internal loop and kinetic diagnostics, and will be added to equilibrium reconstructions. Additionally, as LTX-$\beta $ now includes a neutral beam, the loop can help to diagnose plasma heating during beam injection. [Preview Abstract] |
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GP10.00139: Grid-based Solenoid-free Start-up Modeling of 2$^{\mathrm{nd}}$ Harmonic Electron Cyclotron Heating and Current Drive Masayuki Ono, Nicola Bertelli, Hroshi Idei, Kazuaki Hanada, Shinichiro Kojima, Takumi Onchi, Hatem Elserafy The QUEST ECH solenoid-free start-up experiment utilizing the 28 GHz gyrotron at 2$^{\mathrm{nd}}$ harmonic frequency has demonstrated remarkable efficiency and record start-up current values. A grid-based modeling code where the plasma parameters, generated plasma currents, and resulting changing poloidal magnetic fields are evolved from the vacuum fields. A new feature of this model is the inclusion of the neutral collisions and ionization terms in the model. Formation of hot trapped particles, which then generates processional current, provides improved confinement limited only by collisions even in the open field line configuration. The ECH heated grad-B drift driven current together with the processional currents can then create a closed flux surface configuration where the bootstrap current can further enhance the plasma current. Once plasma temperature is sufficiently high \textgreater 1 keV, a single-pass absorption can rise sufficiently for ECCD to become dominant. This entire start-up process is a self-amplifying ``explosive'' non-linear problem, where a very rapid plasma current rise can be expected. [Preview Abstract] |
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GP10.00140: Nonlinear Simulations of Global Alfv\'{e}n Eigenmodes Excitation and Stabilization in NSTX-U. Elena Belova, Eric Fredrickson, Jeff Lestz, Neal Crocker Nonlinear simulations using the HYM code have been performed to study the excitation and stabilization of GAEs in the NSTX-U right before and shortly after the additional off-axis beam injection. The numerical study has been motivated by the experimental discovery of a strong stabilizing effect that large pitch beam ions from the new beam sources have on these modes. The simulations reproduce experimental finding, namely it is shown that off-axis neutral beam injection reliably and strongly suppresses all unstable GAEs. Before additional beam injection, the simulations show unstable counter-rotating GAEs with toroidal mode numbers and frequencies that match the experimentally observed modes. Additional off-axis beam injection has been modelled by adding beam ions with large pitch, and varying density. The complete stabilization occurs at less than 7{\%} of the total beam ion inventory. New analytic theory of GAE (de)stabilization has also been derived, predicting a range of most unstable mode frequencies, and suggesting a different interpretation for GAE stabilization mechanism compared to previous studies. [Preview Abstract] |
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GP10.00141: Simulating Heat Loads onto 3D PFC Geometries in NSTX-U Thomas Looby, Matthew Reinke, Andreas Wingen, David Donovan, Mike Messineo, Jonathan Klabacha High power operation of future NSTX-U scenarios may be constrained by plasma facing component engineering limits. These constraints motivated the development of a comprehensive software package to couple 3D plasma effects to 3D PFC geometries for the goals of pre-shot heat flux prediction, post-shot heat flux validation, and design optimization. Axisymmetric heat flux assumptions are violated when 3D plasma effects (RMP lobes, error fields, gyro orbits, etc.) load PFCs non-uniformly in the toroidal direction, or when the PFCs are designed with inherent 3D geometries (fish-scaling, chamfering, castellations, etc.). The software development roadmap is presented with requirements for interfacing directly to multiphysics and engineering toolkits to enable CAD importation and thermal / stress analysis, and for including synthetic diagnostic tools to enable validation. The present capabilities are outlined, with results demonstrating axisymmetric plasma effects applied to 3D PFCs. [Preview Abstract] |
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GP10.00142: Post-mortem depth-profile analysis of graphite tiles in NSTX-U and comparison to day-to-day XPS data from MAPP Hanna Schamis, Heather Sandefur, Jean Paul Allain, Felipe Bedoya, Robert Kaita Graphite tiles from four different regions of NSTX-U (center stack, lower inboard divertor, lower outboard divertor, and upper divertor) were removed and cored after the 2015-2016 experimental campaign. These cored samples were analyzed at the IGNIS facility at UIUC using x-ray photoelectron spectroscopy (XPS) depth profile, and are shown to contain between 10 and 25\% at.\% boron. The uneven spatial distribution of boron concentrations within the tokamak suggest a nonuniform distribution of boron deposition during boronization and/or of boron erosion and redeposition during operations. In addition, ATJ graphite samples were inserted into the machine and studied with the Material Analysis and Particle Probe (MAPP). MAPP is a plasma facing component (PFC) diagnostic that was commissioned during the 2015-2016 NSTX-U experimental campaign. It has the capability of studying materials using XPS and other techniques without exposing the samples to atmospheric conditions. The first MAPP results have shown what occurs to boron layers on graphite and molybdenum surfaces on a day-to-day basis when exposed to NSTX-U plasmas. [Preview Abstract] |
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GP10.00143: Validation of deuterium neutral density measurements in NSTX-U using the Monte Carlo neutral transport code DEGAS 2 Filippo Scotti, D.P. Stotler, R.E. Bell, M.D. Boyer, B.P. LeBlanc, S.A. Sabbagh, V.A. Soukhanovskii In NSTX-U, deuterium neutral density $n_D$ profiles are inferred at the outboard midplane from deuterium Balmer-$\alpha$ (D-$\alpha$) emission measured with a filtered camera (Edge Neutral Density Diagnostic, ENDD). $n_D$ measurements obtained from D-$\alpha$ emissivity are validated using the Monte Carlo neutral transport code DEGAS 2. Contributions to emissivity due to electron impact excitation and molecular processes are estimated with DEGAS 2 towards assessing the direct use of ENDD for $n_D$ estimates. Experimental measurements and DEGAS 2 simulations were compared over a database of L-mode discharges, showing good agreement in D-$\alpha$ emissivity profiles. D-$\alpha$ emissivity generally peaked at larger radii in simulations, while far-SOL emission was under predicted. A scan in far SOL plasma parameters in DEGAS 2 showed that a small increase in $T_e$, $n_e$ improved the agreement with experimental profiles, pointing to the possible importance of intermittent transport. One-way coupling of DEGAS 2 with UEDGE fluid simulations is underway to study fueling and neutral penetration in NSTX-U discharges. [Preview Abstract] |
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GP10.00144: Reconstructions and inferred Te-measurements using a versatile multi-energy SXR pinhole camera at Madison Symmetric Torus (MST) Luis F. Delgado-Aparicio, Patrick VanMeter, Daniel DenHartog, Novimir Pablant, Ken Hill, Brentley Stratton A multi-energy soft x-ray (SXR) pinhole camera has been designed, built and deployed for the Madison Symmetric Torus (MST) Reversed Field Pinch (RFP) to aid the study of particle and thermal transport, as well as MHD stability physics. This novel imaging diagnostic technique employs a pixelated x-ray detector in which the lower energy threshold for photon detection can be adjusted independently on each pixel. First estimates of the local electron temperature will be presented based on line-integrated brightness profiles as well as from ratios of the available local emissivities obtained from a 1D Abel-inversion procedure. Analysis is complicated by the presence of bright He- and H-like lines of Aluminum at $\sim$2 keV, but an adequate treatment of the multi-energy data can circumvent these difficulties; the technique was challenged also using strong He- and H-like Ar line-emission at 3-3.5 keV. Data is presented from an improved confinement scenario (PPCD) at MST where $T_{e}$ can reach up to 2 keV. [Preview Abstract] |
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GP10.00145: TRANSP: status and plans to bring the golden standard into the silicon age Francesca Poli, Joshua Breslau, Laszlo Glant, Marina Gorelenkova, Jai Sachdev, Garrett Wright, xingqiu yuan TRANSP is a time-dependent 1.5D equilibrium and transport solver, used for modeling of tokamak plasma discharges and experimental planning. TRANSP incorporates state of the art heating/current drive sources and transport models, implemented in a solver (PT-SOLVER) that is especially suited to treat stiff turbulence transport. With increasing number of users worldwide and with the upcoming ITER era, TRANSP is facing a new challenging: taking down the simulation walltime without compromising physics fidelity. Recent development plans include self-consistent treatment of fast ion transport, synergy between RF sources and MHD calculations. While upgrading physics capabilities is still a priority, the focus of the development team is shifting towards the modernization of the code and the re-factoring of its modules for new computer architectures. We describe the plans forward for making TRANSP attractive to a new generation of users, including capabilities for running the code on one own laptop. We describe the plans for developing a whole device model that provides at the same time high fidelity physics models and computational efficiency, as required for long pulse operation and for the upcoming ITER operation. [Preview Abstract] |
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GP10.00146: Error Analysis on Filament Data from Stereoscopic Fast Cameras on MAST Ryan Chaban, Tom Farley, Saskia Mordijck, Nick Walkden, Fulvio Militello, James Harrison, Andrew Kirk Fast camera images are tomographically inverted using a magnetic field line based technique to generate data on filamentary structures as they interchange across the separatrix and propagate through the SOL. This technique has been employed and developed on MAST over many years for a one camera setup. In 2009 MAST ran a campaign in which two stereoscopic fast cameras were mounted on the vessel collecting images. We use this data to quantify the error by comparing the data and analysis across the cameras and by combining the information between the two cameras. Our procedure concerns two analysis checkpoints for quantifying error: direct image comparison after the inversion process, and statistical comparison of recognized filaments after applying the blob finder to the inversions both separately and combined. Using the difference between images as an error metric, our optimization shows a consistent shift between the cameras implying an uncertainty in a single camera of $+$/- 0.5cm radially and $+$/- 5 cm toroidally reducing the error on average 15{\%}. The statistical tests conducted on recognized filaments show that similar blobs in both camera frames score better on the Kolmogorov-Smirnov test comparing them to distributions dictated by the assumptions of theory. [Preview Abstract] |
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GP10.00147: Status and Plans for the NSTX-U Recovery Project Stefan Gerhardt, R.J. Hawryluk, Leslie Hill, Charles Neumeyer At the previous APS-DPP meeting, Menard et al.$^{\mathrm{1}}$ described the eight major scope items in the NSTX-U Recovery Project including:~ (1) six redesigned inner PF coils, (2) redesigned upper and lower polar region structures, (3) redesigned select plasma facing components, (4) improved bake-out, (5) additional component stress/strain trending instrumentation, (6) enhanced test cell shielding, (7) implementation of the accelerator safety order, and (8) reassembly of NSTX-U components with improved alignment.~ Since then, the design maturity of the Recovery scope has increased such that by this meeting \textgreater 90{\%} of the scope will have completed a preliminary design review and \textgreater 70{\%} a final design review. In addition, a comprehensive analysis of the TF bundle has been conducted to ensure that it meets the project requirements. The design, cost and schedule will be reviewed this summer and presented. Progress, status, and plans for the NSTX-U Recovery Project will be described.~ 1) Jonathan Edward Menard et al., 60$^{\mathrm{th}}$ Annual Meeting of the APS Division of Plasma Physics, Abstract: Y05.0000 [Preview Abstract] |
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GP10.00148: Characterization of tearing modes in NSTX Jeong-hun Yang, Mario Podesta, Eric Fredrickson, Kevin Tritz Tearing mode (TM) instabilities are analyzed experimentally in NSTX plasmas. 2-D soft X-ray measurements are used to infer the magnetic island width, phase and radial location, by mapping the perturbed emissivity on the flux surfaces reconstructed from the equilibrium emissivity with Abel inversion. Fitting results are complemented by the data, on mode frequency and number from Mirnov coils and on mode frequency and location from plasma rotation. The TM parameters are then included in interpretive TRANSP simulations to test and validate two models for TM physics. The first model aims at predicting TM stability based on an analytic representation of the island (Poli et al., Nucl. Fusion 2018). The second model is then used to assess energetic particle transport caused by the magnetic islands (Bardoczi et al., Plasma Phys. Control. Fusion 2019). Validated TM models in TRANSP will enable predictive studies of the role of TM instabilities in integrated simulations. [Preview Abstract] |
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GP10.00149: Coupled Core-Edge Simulation for Particle and Energy Transport Xin Zhang, Francesca Poli The dynamic interplay between the core and edge plasma has important consequences in the confinement and heating of fusion plasma. Fast particles produced by Neutral Beam Injection (NBI) have drift orbits that extend into the Scrape-Off-Layer (SOL). The confinement of these fast particles will therefore be determined by both the core and the SOL plasma. Similarly, the coupling of radio-frequency waves in the range of Ion Cyclotron and Lower Hybrid is sensitive to the density in front of the antenna and these waves may suffer significant power loss before reaching the plasma core. Here we aim to develop a self-consistent, time-dependent simulation of coupled core and edge transport, to allow for better understanding of these phenomena and to facilitate future experimental design. The simulations will be performed using NSTX/NSTX-U plasma profiles and the core transport solver TRANSP coupled with the multi-fluid edge transport code UEDGE. [Preview Abstract] |
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GP10.00150: Plasma radiation transport effects in MAST-U tokamak advanced divertor configurations V. A. Soukhanovskii, A. I. Khrabry, H. A. Scott, D. Moulton, J. Harrison Divertor experiments are planned in MAST Upgrade tokamak to improve understanding of divertor geometry effects and detachment processes in standard, Super-X, and snowflake divertor configurations. This work aims at assessing plasma radiation transport effects in these divertor configurations using the CRETIN code for radiation transport and collisional-radiative modeling, and the SOLPS/EIRENE and UEDGE codes for predictive divertor plasma and neutrals modeling. Plasma opacity to line radiation can lead to enhanced photo-ionization, resulting in modified ionization, recombination and radiation rates in the divertor and an increased divertor detachment threshold in density. Hydrogen Lyman and Balmer series line and continuum opacities are evaluated in typical one- and two-dimensional divertor geometries. Line transfer calculations include complex line shapes due to Doppler and Stark broadening and Zeeman splitting. Lyman photon mean free paths can become comparable to spatial plasma scales in nearly detached Super-X divertor plasmas with high neutral density and large poloidal flux expansion. [Preview Abstract] |
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GP10.00151: 2D Structures of Toroidicity-induced Alfv\'{e}n Eigenmodes (TAEs) Measured with Beam Emission Spectroscopy (BES) in the MegaAmp Spherical Tokamak (MAST) Henry H. Wong, Neal A. Crocker, Clive Michael, Troy Carter, Anthony R. Field, Nicolas Fil, Zhihong Lin, Hongyu Wang, Daniel Dunai Measurements of the 2D poloidal structure of density fluctuations associated with TAEs were obtained for the first time from analysis of with BES data from MAST. TAEs have been observed to reduce the effectiveness of beam heating and current drive in tokamak experiments, including MAST [1]. In this study data from BES and Mirnov coils diagnostics on MAST are used to isolate the contributions of TAEs to the density fluctuations using a cross-correlation analysis. The BES view window spans 6cm vertically and 14cm horizontally, covering 1/3 of the minor radius with 32 channels. The 2D structures at specific times and frequencies obtained from Fourier transforms of short data series indicate a strong radial localization of density fluctuations associated with TAE chirps at the time of peak amplitude. The poloidal phase variations of the structures yield k$_{\mathrm{\theta }}$. Details of these structures are to be compared with the linear and nonlinear eigenmode simulation results from the Gyrokinetic Toroidal Code [2]. The time dependent structures can be used to validate nonlinear physics models for TAE chirping such as energetic particle mode and hole-clump instabilities. [1] Jones, O. M , et al. (2015). PPCF, 57(12), 125009. [2] Z. Lin, et al. (1998) White. Science 281, 1835 [Preview Abstract] |
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GP10.00152: Branching of heated electron energy during plasma current start-up using 2nd harmonics ECCD on QUEST Shinichiro Kojima, Kazuaki Hanada, Hiroshi Idei, Takumi Onchi, Ryuya Ikezo, Yoshihiko Nagashima, Makoto Hasegawa, Kengoh Kuroda, Kazuo Nakamura, Hatem Elserafy, Masaharu Fukuyama, Taiichi Shikama, Nao Yoneda, Sadayoshi Murakami, Ryota Yoneda, Tsuyoshi Kariya, Akira Ejiri, Yuichi Takase, Masayuki Ono In Q-shu university steady-state spherical tokamak (QUEST), 70 kA of plasma current start-up driven mostly by energetic electrons has been demonstrated by using the 2nd harmonic of 28 GHz electron cyclotron wave (ECW). The existence of energetic electrons will be a crucial issue in devices like ITER. In experiment, the branching of the heated electron energy from the bulk electron heating to the energetic electron heating is observed. Moreover, the increase of energetic electrons is caused by the degradation of fuel particles is found. The power balance between absorbed ECW power and power loss by collision showed that the branching of the heated electron energy such as once energetic electrons having a few keV exist the bulk electron is cooled. In electron confinement time calculation by each particle collision time and modelled connection length, the electron confinement time of a few keV is increased by the degradation of fuel particles. [Preview Abstract] |
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GP10.00153: Scoping studies for a sustained high power density next-step tokamak facility Jonathan Menard, Tom Brown, Walter Guttenfelder, Peter Titus, Yuhu Zhai A sustained high power density (SHPD) facility may be an important bridge to a Compact Pilot Plant in the U.S. fusion energy development program. A SHPD facility would integrate: (1) sustainment -- for very long pulses from high to full non-inductive operation, (2) high power density -- high core and edge plasma pressure without transients, and (3) an exhaust solution compatible with sustainment and high pressure. Lower aspect ratio (A $=$ 1.8-2.5) tokamaks are considered potentially advantageous for SHPD by maximizing plasma performance per unit magnet cost. In this work, SHPD performance characteristics are studied as a function of device size, aspect ratio, and stability and confinement assumptions. Realistic shaping poloidal field coils based on free-boundary equilibrium calculations and superconducting magnet/conductor layout and stress analysis are also investigated. The impacts of incorporating liquid metal divertor and first-wall components are also considered. Physics and engineering design progress for a lower-A tokamak SHPD facility are described. [Preview Abstract] |
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GP10.00154: Two-dimensional quasilinear relaxation of fast ions V Duarte, N Gorelenkov, M Podesta, R White Quasilinear models promise to be time-efficient and practical tools to assess the fast ion transport induced by the Alfven eigenmodes in tokamaks. We report on the progress in the development and verification of the Resonance Broadened Quasilinear (RBQ) code, which is~capable of modeling the fast ion distribution function while self-consistently evolving the amplitudes of the modes.~RBQ employs realistic~eigenstructures, damping rates and wave-particle interaction matrices pre-computed by the NOVA-K code.~Rigorous verification exercises are undertaken in limiting cases in which there exist analytical solutions for single-mode saturation levels. The effects of the resonant particle pitch angle scattering on the quasilinear dynamics of Alfven eigenmodes are addressed, in both single-mode and multi-mode cases. Progress on whole device modeling via the TRANSP code is also described. Finally, implementation of the extension of the implicit scheme to two dimensions are described. [Preview Abstract] |
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GP10.00155: \textbf{Ohmic Operation of the ST40 Spherical Tokamak} Paul Thomas, Steven McNamara Tokamak Energy is aiming to exploit the combination of the high $\beta $ capability of Spherical Tokamaks and the high Toroidal Field that can be produced by High Temperature Superconducting magnets as a route to fusion energy production. In parallel to the HTS development being undertaken by the company, a high field, copper coil ST, ST40 (R$=$0.4m, R/a$=$1.6-1.8, I$_{\mathrm{p}}=$2MA, B$_{\mathrm{t}}=$3T, $\kappa =$2.5, $\tau _{\mathrm{pulse}}$\textasciitilde 1 sec, 2MW NBI) is being operated, primarily to test ST energy confinement at low collisionality. The results of the first experimental campaign, testing Merging/Compression start-up, were reported at the 60$^{\mathrm{th}}$ DPP meeting. Starting June 2018, ST40 was disassembled and moved to a new, larger facility, able to accommodate NBI and neutron shielding. Plasma operations restarted almost exactly one year later. Initially, the maximum TF will be 1.5T, increasing to 3T by the end of 2019. The diagnostic set has been substantially expanded and includes a diagnostic neutral beam for charge exchange spectroscopy. The first 1MW heating beam will be installed at the end of 2019 and the second early in 2020. The experimental results with Ohmic heating and future plans will be presented. [Preview Abstract] |
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GP10.00156: Modeling of chirping toroidal Alfven eigenmodes in NSTX Roscoe White, Vinicius Duarte, Nikolai Gorelenkov, Eric Fredrickson, Mario Podesta, Herb Berk Modulation of mode amplitude and frequency of TAE modes, observed experimentally and referred to as chirping, is investigated using a guiding center code and a $\delta $f formalism. Chirping is observed as the development in time of Fourier sidebands that move above and below the nominal mode frequency. Subsequent doubling of the sidebands is also sometimes observed. Equilibria with conventional positive magnetic shear are used, as well as NSTX reversed shear cases. The onset of chirping can be triggered by a sudden increase in mode damping, as can occur by the mode contacting the continuum. [Preview Abstract] |
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GP10.00157: Three-dimensional plasmoid-mediated reconnection and the effect of toroidal guide field in simulations of start-up Helicity Injection Fatima Ebrahimi Physics of three-dimensional plasmoid-mediated magnetic reconnection during transient Coaxial Helicity Injection (CHI) plasma start-up is investigated using nonlinear MHD NIMROD simulations in a spherical tokamak. We numerically examine i) the role of three-dimensional magnetic fluctuations arising from neighboring outer edge n $\neq$ 0 current-sheet instabilities on the formation of plasmoid-mediated closed flux surfaces, and ii) the effect of toroidal guide field on the MHD stability during transient CHI. Consistent with NSTX experiments, we find that even in the presence of non-axisymmetric edge magnetic fluctuations, current-carrying axisymmetric (n=0) plasmoids are rapidly formed while twisted open field lines are being injected, and are merged to form a large current-carrying magnetic bubble for plasma startup in a tokamak. It is also found that the 3-D physics response is drastically different for simulations at higher toroidal field and complete stabilization of non-axisymmetic fluctuations were achieved at higher toroidal flux. MHD analysis for URANIA and QUEST ST's and optimization, at higher poloidal injection flux up to 1 Wb to achieve MA current startup will be explored. Supported by DOE grants DE-SC0010565, DE-AC02-09CH11466. [Preview Abstract] |
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GP10.00158: Prediction of high-k electron temperature fluctuation in an NSTX H-mode plasma Xiang Chen, Juan Ruiz Ruiz, Nathan Howard, Walter Guttenfelder, Jeff Candy, Jerry Hughes, Robert Granetz, Anne White High-k temperature fluctuations $\tilde{T}_e$ in fusion plasmas are associated with electron-scale electromagnetic drift-wave type turbulence. High-k $\tilde{T}_e$ might contribute significantly to heat loss in a fusion reactor. However, no direct measurements of high-k $\tilde{T}_e$ have ever been made in any experiments and thus no validation work involves the quantity $\tilde{T}_e$. In this work, we run electron temperature gradient(ETG)-scale nonlinear gyrokinetic simulations of an NSTX H-mode with CGYRO. We will do a synthetic projection of high-k $\tilde{T}_e$ from simulation in NSTX which has never been done before. Comparison with high-k electron density fluctuation $\tilde{n}_e$ are made to demonstrate the advantages of predicted $\tilde{T}_e$ diagnostics and how critical it is for validation of transport models. The impact of $\tilde{T}_e$ on electron thermal transport are studied to answer the question whether ETG is the dominant turbulence mechanism or not. [Preview Abstract] |
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GP10.00159: Modeling of Snowflake Divertor Plasmas in MAST-U Tokamak using UEDGE code Alexander Khrabry, Vsevolod Soukhanovskii, Thomas Rognlien, Maxim Umansky, David Moulton, James Harrison In a snowflake (SF) divertor configuration, a second-order poloidal magnetic field null can potentially lead to spreading heat and particle fluxes over additional strike points. SF divertor experiments are planned in the spherical tokamak MAST U. Numerical simulations of the edge and divertor plasmas are performed for standard and SF divertor configurations using the two-dimensional code UEDGE with carbon sputtering, charge-state resolved carbon transport, and a fluid neutrals model. A good agreement is obtained between our UEDGE standard divertor model and the previously developed SOLPS/EIRENE model using transport coefficients for MAST tokamak. The present work is aimed at understanding the heat and particle flux distribution in the SF divertors as a function of MAST-U tokamak operating parameters (edge plasma density, input power) with a focus on transition to plasma detachment regime. In the SF divertor model, transport coefficient profiles in the near-null region are varied with radial distance and with poloidal field strength to simulate the theoretically predicted SF null-region plasma mixing. [Preview Abstract] |
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GP10.00160: Effect of multiple electron cyclotron harmonics on current drive in QUEST spherical tokamak Takumi Onchi, Hiroshi Idei, Masaharu Fukuyama, Makoto Hasegawa, Kengoh Kuroda, Ryuya Ikezoe, Kazuaki Hanada, Akira Ejiri, Tsuyoshi Kariya, Atsushi Fukuyama, Masayuki Ono Multiple harmonic resonances, from second to fourth for 28 GHz radio frequency (RF), of electron cyclotron wave coexist in QUEST spherical tokamak, and their heating is effective especially for energetic electrons with oblique injection of RF beam. Under the resonance condition with parallel refractive index $N_{\vert \vert }>$ 0.75 and energetic electron temperature $T_{e}>$ 50 keV in the geometry of QUEST device, the effect of relativistic shift is significantly large. The higher harmonic down-shifted resonance overlaps with the lower harmonic up-shifted resonance owing to Doppler-shift effect. The lower harmonic heating is stronger than higher harmonic heating, and hence the efficient current drive is expected of the multiple harmonic up-shifted resonance with oblique RF injection from the low field side. Such multiple harmonic heating may be a mechanism of the current drive of $I_{p}>$ 80 kA solely by obliquely injected X-mode beam, where $N_{\vert \vert }=$ 0.75 and RF power is 120 kW. [Preview Abstract] |
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GP10.00161: The CULART Experiment Celso Ribeiro The Compact device with Ultra-Low Aspect-Ratio Tokamak plasmas (CULART) is a medium-size device [1]. The major objective of this proposal is twofold. First, to explore very high beta limits under the relatively low-medium toroidal field (TF). Secondly, as a proof-of-concept, to use these high beta plasmas as a target for applying the adiabatic compression (AC) technique [2] in order to raise temperature and density thus to attain substantial neutron yields and fusion power. CULART is intended to benefit from present day technology, the achievements of tokamak fusion research, and the advantage of using ohmic and AC heating regimes exclusively. Using the AC technique in CULART sets a pathway to study the potential for a high efficiency, ultra-compact, repetitive-pulsed neutron source based on the spherical tokamak (ST) concept and serve as a benchmark for appropriate scaling towards a fusion reactor and related material studies. It can also be used for more immediate applications in broad areas of physical science beyond fusion energy. Simulations of AC up to 1MW in D-T fuel plasmas, MHD equilibrium and stability will be presented. [1] C. Ribeiro, Proc. 28th Symposium of Fusion Engineering (SOFE), Jacksonville, FL, US, June, 2-6 (2019). [2] H. P. Furth and S. Yoshikawa, Phys. of Fluids, 13, 2593 (1970). [Preview Abstract] |
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GP10.00162: ArbiTER studies of scrape-off layer instability in double-null geometry D. A. Baver, J. R. Myra, Fulvio Militello, David Moulton The ArbiTER$^{\mathrm{1}}$ code has been used in the past to study a wide variety of plasma instabilities. In particular, the use of perturbed density profiles is used to study filamentary structures (i.e. blobs and ELM's) by creating an environment in which linear instabilities mirror the properties of nonlinear blob-filaments. It is also is used to study divertor-leg instabilities. However, the transition from single-null to double-null geometry creates a number of numerical issues related to the use of field-line following coordinates, such as integrated magnetic shear. Solutions have been developed to these problems, and the radial resolution required to overcome these issues can be quantified. By applying these techniques to profiles based on SOLPS simulations of the MAST-U experiment, the efficacy of these solutions will be tested, and insights into the properties of scrape-off layer instabilities in this geometry discussed. 1. D. A. Baver, J. R. Myra and M. V. Umansky, Comm. Comp. Phys. 20, 136 (2016). Work supported by the U.S. Department of Energy Office of Science, Office of Fusion Energy Sciences, under Award Number DE-SC0019270. [Preview Abstract] |
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GP10.00163: High Capacitor Bank Utilization with a \'{C}uk Converter Alex Henson, Timothy Ziemba, Kenneth E. Miller, James Prager, Satbeer Singh Eagle Harbor Technologies (EHT), Inc. is developing a \'{C}uk converter for local helicity injection and magnet driving and control for the Pegasus Toroidal Experiment at the University of Wisconsin -- Madison. A \'{C}uk converter has low output ripple; high efficiency; voltage gain greater than one, allowing for deeper energy storage utilization; continuous power flow that lowers output EMI, reducing noise generation; continuous input and output current -- energy flow from the series capacitor allows for greater control of the injector currents. Additionally, this configuration allows for series arrangements that isolate individual switch modules, so a failure does not potentially damage all solid-state switches. EHT has completed a Phase I program to design and build a high-frequency \'{C}uk converter, which was tested at Pegasus. EHT will present Phase I results showing increase capacitor bank utilization with a \'{C}uk converter. In a potential Phase II program, EHT will design, build, and test a bidirectional \'{C}uk converter that will reduce the heat load on electromagnet coils. [Preview Abstract] |
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GP10.00164: Propagation of magnetic fluctuation driven by plasmoid reconnection in HIST-CHI experiments Masayoshi Nagata, Hideaki Miyamoto, Youhei Ibaraki, Takashi Kanki, Yusuke Kikuchi, Naoyuki Fukumoto Multiple plasmoid reconnection required for the flux closure in the transient-coaxial helicity injection (T-CHI) start-up process has been demonstrated in the Helicity Injected Spherical Torus (HIST) device. Two or three plasmoids are generated after the tearing instability of an elongated Sweet-Parker current sheet during the T-CHI. Here, we report that in the T-CHI start-up plasmas (H, D and He) with the strong toroidal (guide) field ($I_{\mathrm{TF}}=$140 kA), (i) the frequency of regular oscillations of reconnecting magnetic field decreases as the mass number increases, i.e., 250 kHz (H), 150 kHz (D) and 60 kHz (He). (ii) the fluctuation propagates radially with 30 km/s (H), 20 km/s (D) and 12 km/s (He) from $R=$ 0.25 m at the X-point toward the outboard side. It has been found that the propagation speed agrees with the Alfven speed. The small-size plasmoids cannot move radially and are staying between the elongated current sheet. Consequently, the plasmoid reconnection could be related to the excitation of Alfven wave, leading to the ion heating in the T-CHI discharge. [Preview Abstract] |
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