Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session GP12: Poster Session III (Plasma Accelerators and Radiation: NSTX-U, ST, and International MFE)Poster
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Room: Exhibit Hall A |
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GP12.00001: PLASMA ACCELERATORS \& RADIATION |
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GP12.00002: High quality electron bunch generation using a longitudinal density-tailored plasma-based accelerator Warren Mori, Xinlu Xu, Fei Li, Wei Lu, Chan Joshi The feasibility of generating high quality electron bunches (high brightness, low energy spread and short duration) from a plasma-based accelerator that utilizes an appropriately tailored plasma density profile is demonstrated with three-dimensional particle-in-cell simulations. The underlying physical mechanism that leads to generation of high quality electrons is uncovered by tracking the particle trajectories of the electrons as they cross the sheath and are trapped by the wake. The intensity of the driver and the steepness of density profile select the electrons to be injected and determines the quality of the injected beam. In the given example, a peak brightness of $10^{19}$~A/m$^{2}$/rad$^{2}$ and a slice energy spread of $\sim 0.5~\mathrm{MeV}$ can be achieved by exciting a wake as the laser pulse traverses a plasma density downramp with a nominal density of $10^{19}$~cm$^{-3}$. Work supported by NSF and DOE. [Preview Abstract] |
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GP12.00003: Exact phase space matching for staging plasma and traditional accelerator components using longitudinally tailored plasma profiles Yipeng Wu, Xinlu Xu, Wei Lu, Warren Mori, Mark Hogan, Chan Joshi Phase space matching between two plasma-accelerator (PA) stages and between a PA and a traditional accelerator component is a critical issue for the generation and utilization of high energy electron beams produced by plasma-based accelerators. Catastrophic emittance growth in the presence of a finite energy spread and lack of proper matching will occur as the beam propagates through different stages and components due to the drastic differences of the transverse focusing strengths. We propose using the ideal focusing forces from nonlinear wakes in longitudinally tailored plasma density profiles to provide exact phase space matching components to properly transport the beam through two such stages with negligible emittance growth. Theoretical analysis and full 3-dimensional OSIRIS particle-in-cell simulations are carried out to show how these structures may work in four different scenarios. Good agreement between theory and simulation is obtained. Work supported by NSF and DOE. [Preview Abstract] |
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GP12.00004: Laser-plasma acceleration with multi-color pulse stacks: Designer electron beams for advanced radiation sources Serge Kalmykov, Bradley Shadwick, Isaac Ghebregziabher, Xavier Davoine Photon engineering [S. Y. Kalmykov et al., New J. Phys. 14, 033025 (2012); Phys. Plasmas 22, 056701 (2015)] offers new avenues to coherently control electron beam phase space on a femtosecond time scale. It enables generation of high-quality beams at a kHz-scale repetition rate. Reducing the peak pulse power (and thus the average laser power) is the key to effectively exercise such control. A stepwise negative chirp, synthesized by incoherently stacking collinear sub-Joule pulses from conventional CPA, affords a micron-scale bandwidth. It is sufficient to prevent rapid compression of the pulse into an optical shock, while delaying electron dephasing. This extends electron energy far beyond the limits suggested by accepted scalings (beyond 1 GeV in a 3 mm plasma), without compromising beam quality. In addition, acceleration with a stacked pulse in a channel favorably modifies electron beam on a femtosecond time scale, controllably producing synchronized sequences of 100 kA-scale, quasi-monoenergetic bunches. These comb-like, designer GeV electron beams are ideal drivers of polychromatic, tunable inverse Thomson $\gamma$-ray sources. [Preview Abstract] |
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GP12.00005: Laser-induced wakefield acceleration by using density-tapered gas-cell Minseok Kim, Inhyuk Nam, Seungwoo Lee, Hyyong Suk The plasma sources with upward density gradient can be used to increase a dephasing length and an accelerating field in laser wakefield acceleration (LWFA) mechanism. As a result, the electron energy accelerated is expected to be increased and we developed a density-tapered gas-cell on this account. Using a 20 TW Ti:Sapphire laser constructed at GIST, we performed the acceleration experiments with the gas-cell and gas-jet with density-gradient. In this presentation, the results of acceleration experiments will be presented in detail. [Preview Abstract] |
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GP12.00006: Hybrid Laser Wakefield and Direct Laser Plasma Accelerator in the Plasma Bubble Regime Xi Zhang, Vladimir Khudik, Alexander Pukhov, Gennady Shvets The concept of hybrid laser wakefield and direct laser plasma accelerator in plasma bubble regime was recently [1] proposed. The advantage of this approach is two-fold: (a) electrons' energy gains from the laser and from the wake add up, and (b) dephasing is slowed down. Using 2D VLPL simulations, we will demonstrate that two conditions must be met by the electrons injected into the hybrid accelerator: (1) strong spatial overlap with the laser field, and (2) large initial transverse energy. The firstcondition is met by employing two laser pulses: one to produce a plasma bubble, and the second time-delayed pulse to interact with the injected electrons. We will show that there are two approaches to meeting the second condition: self-injection using an engineered density bump [1] and ionization-injection. The criteria for direct laser acceleration of ionization-injected electrons will be discussed. Combinations of laser pulses with different wavelengths will also be considered. This work is supported by the US DOE grant DE-SC0007889 and the AFOSR grant FA9550-14-1-0045. \\[4pt] [1] X. Zhang, V. N. Khudik and G. Shvets, Phys. Rev. Lett. , 184801 (2015). [Preview Abstract] |
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GP12.00007: Constant-gradient resonant laser acceleration of electrons in the plasma bubble regime Gennady Shvets, Xi Zhang, Vladimir Khudik We present a new mechanism of steady electron acceleration resulting from the interplay of direct laser acceleration (DLA) and the deceleration by the longitudinal wakefield which takes place in the plasma bubble regime. The unusual aspect of such arrangement that sets it apart from the earlier considered case of synergistic laser wakefield/DLA [1] is that the plasma wake removes the energy from the electrons while at the same time increasing the amplitude of their betatron oscillations. Using PIC simulations, we demonstrate that such regime can be realized through external injection of electrons into the decelerating phase of the plasma bubble. It is also found that electrons can be accelerated via resonant interaction of the laser with high harmonics of the betatron motion. We show that the two key parameters determining the maximum energy gain are the ratio of the laser field to the longitudinal field, and the difference of the phase velocity of the laser wave from the speed of light. A similarity with the pendulum motion is revealed and used to explain how the acceleration is terminated. This work was supported by DOE grants DESC0007889 and DE-SC0010622, and by an AFOSR grant FA9550-14-1-0045.\\[4pt] [1] X. Zhang et al., Phys. Rev. Lett.,114,184801 (2015). [Preview Abstract] |
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GP12.00008: Universal Scalings for Direct Laser Acceleration of Relativistic Electrons in Ion Channels Vladimir Khudik, Alexey Arefiev, Xi Zhang, Gennady Shvets Direct Laser Acceleration (DLA) of electrons in ion channels is investigated in the general case when the laser phase velocity is greater or equal to the speed of light, and the electrons execute a fully three-dimensional trajectory inside the focusing channel. In the paraxial limit of electron motion (mostly forward), we develop an analytic theory that provides an accurate estimate of the maximum possible energy gain of the electrons as a function of their initial conditions and laser parameters. Some of the counter-intuitive predictions validated via particle simulations include the emergence of the phase space barriers that prevent electrons from getting accelerated, and the threshold-like dependence of the energy on the initial conditions. The predictive power of the theory is demonstrated by identifying the laser-plasma parameters for the electron acceleration through the resonant interaction between the third harmonic of betatron oscillations and the laser wave. Possible experimental signatures of the high-order resonances will be discussed. This work was supported by DOE grants DESC0007889 and DE-SC0010622, and by an AFOSR grant FA9550-14-1-0045. [Preview Abstract] |
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GP12.00009: High Energy Electron Acceleration from Underdense Plasmas with the OMEGA EP Laser Thomas Batson, Anthony Raymond, Karl Krushelnick, Louise Willingale, Phil Nilson, Dustin Froula, Dan Haberberger, Andrew Davies, Wolfgang Theobald, Jackson Williams, Hui Chen, Alex Arefiev Experiments performed using the OMEGA EP laser system studied channeling through an underdense CH plasma, as well as the energy spectra, pointing, and divergence of a direct laser accelerated (DLA) electron beam. An intense, ps scale laser pulse propagating through an underdense plasma results in the expulsion of electrons from along the laser axis to form a channel [1]. Electrons can then be injected from the channel walls into the laser path, which results in the DLA of these electrons and the occurrence of a high energy electron beam [2]. The 4 omega optical probe diagnostic was used to characterize the density of the plasma plume and channel density, while proton radiography was used to observe the electromagnetic fields of the channel formation. 2D particle-in-cell simulations are used to investigate the effects of the plasma density and laser parameters on the channel behavior and electron spectra. This work was supported by the National Laser Users' Facility (NLUF), DOE.\\[4pt] [1] Willingale et al., Physical Review Letters, 106, 105022 (2011)\\[0pt] [2] Willingale et al., New Journal of Physics, 15, 025023 (2013) [Preview Abstract] |
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GP12.00010: Waveguide assisted wakefield acceleration in near critical density plasmas Linus Feder, George hine, Fatholah Salehi, Bo Miao, Andy Goers, Howard Milchberg We demonstrate the generation of multi-MeV electron beams with stable pointing and improved divergence using a sub-mJ prepulse to form a channel for the main 10-300 mJ accelerating pulse. An ultrashort, low energy pre-pulse a few nanoseconds before the main pulse ionizes a column through a thin near critical density gas jet. The column then expands to form a guiding structure for the main pulse, which then accelerates electrons through laser wakefield acceleration. [Preview Abstract] |
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GP12.00011: Beatwave acceleration in a plasma metamaterial A. Lopes, E.P. Alves, R.A. Fonseca, L.O. Silva These media can be engineered to produce negative indices of refraction, which support the propagation of unusual light waves. In addition, the interaction of these unusual light waves with charged particles in the medium can lead to unusual features like negative radiation pressure. It is well known that two counter propagating chirped lasers in a positive response medium can lead to energy transfer from the waves to the particles leading to its acceleration. In this work, we explore the combination of two co-propagating EM pulses which, under the right conditions, achieve the same results as the previous scheme, but now in a plasma metamaterial. Depending on the chosen frequency, one of the pulses can experience a positive response medium (n\textgreater 0) and the other a negative one (n\textless 0), leading to anti-parallel phase velocities. This setup is capable of creating a beat wave which enables the acceleration of charged particles via the radiation tension. Our results are supported by numerical simulations using meta-OSIRIS, which combines a solver to deal with dielectric and magnetic materials with arbitrary EM linear properties with the standard PIC algorithm. Our simulations addressing this new setup provide results consistent with the theoretical predictions. [Preview Abstract] |
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GP12.00012: X-ray wakefield acceleration in a nanotube Xiaomei Zhang, Toshiki Tajima, Deano Farinella, Yongmin Shin, Gerard Mourou, Jonathan Wheeler, Peter Taborek Electrons can be accelerated to high energies in the wakefield when a short pulse laser or beam passes through a plasma [1]. Experiments have shown that the GeV energy can be obtained over centimeter acceleration in gas plasma with the optical laser. Laser wakefield theory shows that for a given laser the energy gain is inversely proportional to the plasma density and low density implies a much longer acceleration length, which means the scheme of ultrahigh energy gain wakes the acceleration length longer. The recent proposed generation of the X-ray laser pulse provides us an attractive way to get ultrahigh energy [2]. Due to the much higher critical density for the X-ray laser pulse, solid density materials can be chosen, which causes stronger wakefield and ultrahigh energy gain with a compact structure [3]. Motivated by this, we explore the X-ray wakefield accelerator in a nanotube and get the scalings of acceleration.\\[4pt] [1] T. Tajima, J.M. Dawson, Phys. Rev. Lett. \textbf{43}, 267 (1979).\\[0pt] [2] G. Mourou, et al., Eur. Phys. J. \textbf{223}, 1181 (2014).\\[0pt] [3] T. Tajima, Eur. Phys. J. \textbf{223}, 1037 (2014). [Preview Abstract] |
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GP12.00013: Ultrafast probing of transient electric fields from optical field ionized plasmas using picosecond electron deflectometry Zhaohan He, John Nees, Bixue Hou, Karl Krushelnick, Alec Thomas Femtosecond bunches of electrons with relativistic to ultra-relativistic energies can be robustly produced in laser plasma wakefield accelerators (LWFA). Scaling the electron energy down to sub-relativistic and MeV level using a millijoule laser system will make such electron source a promising candidate for ultrafast electron diffraction (UED) the applications due to the intrinsic short bunch duration and perfect synchronization with optical pump. Electrons with sub-relativistic ($\sim$100 keV) energies can be used to probe transient electric field generated in laser plasmas with very high sensitivity. In a proof-of-principle experiment, we measured field evolution from plasma produced by focusing femtosecond laser pulses into a gas jet at intensities up to $10^{17}$ W/cm$^2$. Due to the energy spread in laser plasma generated electrons, dipole magnets are used to record a streaked electron image such that the temporal evolution can be mapped in a single shot. This technique allows for probing irreversible processes such as melting of crystalline samples. [Preview Abstract] |
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GP12.00014: Transverse phase-space properties of beams produced via ionization injection in laser-plasma accelerators Carlo Benedetti, Carl Schroeder, Stepan Bulanov, Cameron Geddes, Eric Esarey, Wim Leemans Ionization injection has become a commonly used technique to inject electrons in plasma wakefields. In ionization injection, electrons are ionized by an intense laser in the plasma wakefield, reducing the wakefield amplitude required for trapping. The lower trapping threshold allows operation at lower plasma densities, enabling higher beam energy gains. However, ionization injection can also result in poor quality of the trapped electron bunch, compared to self-injection. Here, we investigate, analytically and by means of numerical modeling, the trapping threshold, in 3D, for a relativistically intense ionizing laser driver. We characterize, as a function of wake phase velocity and laser intensity, the transverse phase-space properties at injection for the beam produced via ionization injection, and we compare it to that obtained in the case of self-injection. Techniques to improve the phase-space quality of the bunch produced via ionization injection will be discussed. [Preview Abstract] |
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GP12.00015: Studies of a hybrid Trojan Horse wakefield accelerator with high transformer ratio Nathan Cook, David Bruhwiler, Bernhard Hidding, Jean-Luc Vay, Stephen Webb Plasma wakefield acceleration uses relativistic high-charge electron bunches to generate a plasma blowout supporting intense electric fields for trapping and acceleration. Dramatic improvements in emittance, peak current and brightness are achievable through laser-controlled ionization in the plasma blowout, which is the premise of the Trojan Horse approach. The hybrid Trojan Horse concept extends this approach to use the output beam from a laser plasma accelerator to drive a Trojan Horse, resulting in a compact system that can produce higher brightness bunches with order-of-magnitude lower energy spread. We are exploring the use of multiple, shaped laser pulses to resonantly inject a shaped electron drive bunch. The resulting output bunch could generate wakes in PWFA or beam-driven dielectric structures with transformer ratios of 5 to 10 or larger. Hence, a hybrid Trojan Horse accelerator with bunch shaping may provide a compact source of nC bunches that can drive a variety of systems for studying high-gradient wakefields and lepton acceleration. Initial work will use previously simulated electron bunches from a laser plasma accelerator to drive the plasma wakefield stage. We present preliminary results from simulations using the parallel, particle-in-cell framework Warp. [Preview Abstract] |
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GP12.00016: Ionization Injection and Acceleration of Electrons in Meter Scale Plasma at FACET Navid Vafaei-Najafabadi, C.E. Clayton, K.A. Marsh, W. An, W. Lu, W.B. Mori, C. Joshi, E. Adli, J. Allen, C.I. Clarke, S. Corde, J. Frederico, S. Gessner, S.Z. Green, M. Litos, D. Walz, M.J. Hogan, V. Yakimenko, P. Muggli Generation and acceleration of a bright beam in a plasma wakefiled accelerator using ionization injection is explored in experiment and supporting simulation. The plasma is generated via field ionization of a column of lithium vapor by the 3nC, 20.35 GeV electron beam at FACET. The electron beam then drives a wakefield in the singly ionized lithium plasma, which rapidly evolves to the blowout regime. The column of lithium vapor is bounded by cold helium gas, which result in density ramps on either side of the lithium vapor. Electrons belonging to helium, on the lithium density ramp, can therefore be ionized by the combined fields of the wakefield and the electron beam as it undergoes betatron oscillations. The resulting injected beam can gain over 25 GeV of energy, with energy spread on the order of 10{\%}, and emittance that is much smaller than that of the drive beam. Solutions for reducing energy spread of this beam based on OSIRIS simulations will be presented. [Preview Abstract] |
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GP12.00017: QuickPIC Simulations on Recent PWFA Experiments at FACET Weiming An, Erik Adli, James Allen, Christine Clarke, Chris Clayton, Sebastien Corde, Joel Frederico, Spencer Gessner, Selina Green, Mark Hogan, Chan Joshi, Mike Litos, Wei Lu, Ken Marsh, Warren Mori, Navid Vafaei-Najafabadi, Vitaly Yakimenko The plasma wake field accelerator (PWFA) is a promising advanced accelerator concept for making a more compact and cheaper future high energy accelerator. We present QuickPIC simulations of three PWFA experiments at FACET, SLAC. The first one demonstrated high efficiency, high gradient acceleration of electrons with low energy spread through the ``two-bunch'' scheme, in which an electron drive bunch excites a wake while and a second trailing bunch surfing the wake gets accelerated. The second one demonstrates high energy gain of positrons with low energy spread through a ``self-load'' scheme, in which a single positron bunch propagating in the plasma drives a wake wave and the rear part of the positron bunch loads and surfs the wake to high energy. The third one is electron acceleration in a plasma hollow channel, in which an electron bunch propagates through a plasma hollow channel that is pre-ionized by a Bessel laser beam. [Preview Abstract] |
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GP12.00018: Plasma undulator based on laser excitation of wakefields in a plasma channel Carl Schroeder, Sergey Rykovanov, Eric Esarey, Cameron Geddes, Wim Leemans A novel plasma undulator based on the wakefields excited by a laser pulse in a plasma channel is described. Generation of the undulator fields is achieved by inducing centroid oscillations of the laser pulse in the channel. The period of such a plasma undulator is proportional to the Rayleigh length of the laser pulse and can be sub-millimeter, with an effective undulator strength parameter of order unity. The undulator period can further be controlled and reduced by beating laser modes or using multiple colors. Analytic expressions for the electron trajectories in the plasma undulator and the synchrotron radiation are compared to numerical modeling. Examples of short-period laser-driven plasma undulators are presented based on available laser and plasma channel parameters. [Preview Abstract] |
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GP12.00019: Narrow bandwidth Thomson photon source development using Laser-Plasma Accelerators C.G.R. Geddes, S. Steinke, H.-E. Tsai, S.G. Rykovanov, J.-L. Vay, A. Bonatto, C. Benedetti, C.B. Schroeder, E. Esarey, A. Friedman, D.P. Grote, W.P. Leemans Compact, high-quality photon sources at MeV energies are being developed based on Laser-Plasma Accelerators (LPAs). Simulations are presented on production of controllable narrow bandwidth sources using the beam and plasma capabilities of LPAs. An independent scattering laser, combined with appropriate pulse shaping and laser guiding is important to realize high photon yield. Plasma optics are described to tailor beam divergence in cm-scale distances, reducing photon source bandwidth. The LPA can further be used to de-accelerate the electron beam after photon production to reduce undesired radiation. Combination of laser driven and beam driven deceleration is presented to reduce residual beam energy, as is important for a laboratory or field operable source. Design of experiments and laser capabilities to combine these elements will be presented, towards a compact photon source system. [Preview Abstract] |
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GP12.00020: High energy photon emission from wakefields and its signatures in astrophysical Blazars Deano Farinella, Xiaomei Zhang, Calvin Lau, Sam Taimourzadeh, Yoonwoo Hwang, James Koga, Toshikazu Ebisuzaki, Toshiki Tajima Episodic eruptions of accretion disks of AGNs (and Blazars) due to the Magneto-Rotational-Instability are related to the excitation of intense Alfven waves and their subsequently mode converted EM pulses. These intense pulses are related to the emission of bursts of gamma rays and extreme high energy cosmic ray (EHECR) genesis in AGN and Blazars [1]. Wakefield acceleration and pondermotive acceleration [2] of electrons give rise to gamma ray emissions of the above through synchrotron radiation which can undergo inverse-compton scattering to attain high x-ray energies. We study additional emissions of gamma rays by the betatron oscillations and QED radiative processes in the intense accelerating fields.\\[4pt] [1] T. Ebisuzaki and T. Tajima, Astropart. Phys. \textbf{56}, 9(2014). \newline [2] C. Lau et al., Phys. Rev. STAB \textbf{18}, 024401(2015) [Preview Abstract] |
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GP12.00021: Clarification of THz Electromagnetic Radiation mechanism from the Laser Produced Plasma Masataka Hideta, Yusuke Hyuga, Yasuhiko Sentoku, Noboru Yugami Conical forward Terahertz radiation from ultra-short pulse laser produced plasma has been observed [1] [2]. The radiation frequency is smaller than the plasma frequency that is estimated by the initial gas density and laser intensity. This radiation mechanism has not been clarified. To study the radiation mechanism, 2D PIC code is used. The radiation is described by the following equation, \[ \left( {\nabla^{2}-\frac{1}{c^{2}}\frac{\partial^{2}}{\partial t^{2}}+\frac{\omega_{p}^{2} }{c^{2}}} \right)B =\mu_{0} e\nabla n\times v \] where, $\omega_{p}$, $n$ and \textbf{\textit{v}} represent the plasma frequency, the plasma density and the electron velocity, respectively. The right hand side is considered as the radiation source which strongly depend on the gradient of the plasma density and the electron velocity. In the experiment, the laser propagates with creating the plasma, the plasma density profile is a function of the radial direction. Therefore, the strong gradient is at the edge of the plasma column, not the center of the plasma, the radiation is expected to generate there and its frequency is also equal to the local plasma frequency. The 2D calculation shows the EM wave is generated around the edge of the plasma column and its frequency is lower than the plasma frequency. [1] N. Yugami \textit{et al}., Jpn. J. Appl. Phys., \textbf{45,} L1051 (2006). [2] C. D'Amico \textit{et al}., Phys. Rev. Lett., \textbf{98}, 235002 (2007). [3] H.-C. Wu \textit{et al}., Phys. Rev. E \textbf{83}, 036407 (2011). [Preview Abstract] |
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GP12.00022: Noninvariance of Energy-Momentum Scale Ranges in Vlasov Simulations of Relativistic Interactions and Warm Wavebreaking of Relativistic Plasma Waves Alec Thomas For certain classes of relativistic plasma problems, using a Lorentz boosted frame can be even more advantageous for gridded momentum space-position space-time simulations than Vay [Vay PRL 2007] showed was the case for position space-time simulations, resulting in speed up proportional to $\gamma_{boost}^6$. The technique was applied using a Spectral Vlasov code to the problem of warm wavebreaking limits in relativistic plasma and demonstrates numerical results consistent with the analytic conclusions of Schroeder et al. [Schroeder PRE 2005]. By appropriate normalization, a self-similar behavior for the Vlasov equation in different Lorentz frames is found. These results are relevant to beam and laser driven plasma based accelerators and the potential for Vlasov simulation of them. [Preview Abstract] |
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GP12.00023: Recent advances in the modeling of plasmas with the Particle-In-Cell methods Jean-Luc Vay, Remi Lehe, Henri Vincenti, Brendan Godfrey, Patrick Lee, Irv Haber The Particle-In-Cell (PIC) approach is the method of choice for self-consistent simulations of plasmas from first principles. The fundamentals of the PIC method were established decades ago but improvements or variations are continuously being proposed. We report on several recent advances in PIC related algorithms, including: (a) detailed analysis of the numerical Cherenkov instability and its remediation, (b) analytic pseudo-spectral electromagnetic solvers in Cartesian and cylindrical (with azimuthal modes decomposition) geometries, (c) arbitrary-order finite-difference and generalized pseudo-spectral Maxwell solvers, (d) novel analysis of Maxwell's solvers' stencil variation and truncation, in application to domain decomposition strategies and implementation of Perfectly Matched Layers in high-order and pseudo-spectral solvers.\\[4pt] Work supported by US-DOE Contracts DE-AC02-05CH11231 and the US-DOE SciDAC program ComPASS. Used resources of NERSC, supported by US-DOE Contract DE-AC02-05CH11231. [Preview Abstract] |
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GP12.00024: Envelope Model Simulation of Laser Wakefield Acceleration with Realistic Laser Pulses from the Texas Petawatt Kathleen Weichman, Adam Higuera, Dan Abell, Ben Cowan, Neil Fazel, John Cary, Michael Downer In a laser wakefield accelerator (LWFA), diffraction of an over-focused laser pulse can provide localized electron injection, leading to the production of a monoenergetic electron bunch. While electron energies up to several GeV have been reported at the Texas Petawatt Laser facility, near-Gaussian beam simulations predict energies higher than have been observed. Experimentally measured laser profiles are non-Gaussian, indicating that closer agreement with experimental conditions is needed to predictively model this experiment. The implementation of the envelope model in the particle-in-cell code VORPAL lowers the computational cost of capturing injection dynamics during the early evolution of laser wakefields. We compare VORPAL envelope model simulations using laser pulses based on experimentally measured profiles versus a corresponding a two-Gaussian approximation.\\[4pt] We acknowledge DOE Grants No. DE-SC0011617 and DE-SC0012444, DOE/NSF Grant No. DE-SC0012584, and the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. KW is supported by the DOE CSGF under Grant No. DE-FG02-97ER25308. [Preview Abstract] |
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GP12.00025: Applying Boundary Conditions Using a Time-Dependent Lagrangian for Modeling Laser-Plasma Interactions J. Paxon Reyes, B.A. Shadwick Describing a cold-Maxwell fluid system with a spatially-discrete, unbounded Lagrangian is problematic for numerical modeling since boundary conditions must be applied after the variational step. Accurate solutions may still be attained, but do not technically satisfy the derived energy conservation law. The size of the numerical domain, the order accuracy of the discrete approximations used, and the type of boundary conditions applied influence the behavior of the artificially-bounded system. To encode the desired boundary conditions of the equations of motion, we include time-dependent terms into the discrete Lagrangian. Although some foresight is needed to choose these time-dependent terms, this approach provides a mechanism for energy to exit the closed system while allowing the conservation law to account for the energy loss. Results of a spatially-discrete, time-dependent Lagrangian system (with approximations of second-order accuracy in space and fourth order in time) will be presented. The fields and total energy will be compared with models of the same accuracy using a time-independent variational approach as well as a non-variational approach. [Preview Abstract] |
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GP12.00026: Challenges of PIC Simulations at High Laser Intensity Scott V. Luedtke, Alexey V. Arefiev, Toma Toncian, Bjorn Manuel Hegelich New lasers with very high intensity pulses ($I > 10^{22}$ W/cm$^2$) are being commissioned to explore new regimes of laser-matter interactions. These lasers require accurate particle-in-cell (PIC) simulations, which may require new computational approaches to efficiently produce physically accurate results. We examine the constraints on PIC simulations at high field intensity imposed by both the particle pusher and field solver. As proposed by Arefiev, et al. (Physics of Plasmas 22, 013103 (2015)), we implement adaptive sub-cycling in the Boris pusher of the EPOCH code and demonstrate its effectiveness in efficiently reducing errors from the pusher. It is well know that the use of a finite-difference scheme also modifies the electromagnetic wave dispersion relation. We examine the effect of the resulting discrepancy in the phase velocity on electron acceleration, and demonstrate that relatively small errors in the phase velocity lead to substantial changes in the electron energy gain from the laser pulse. We discuss the corresponding conditions for the field solver. These results are relevant to direct laser acceleration and underdense ionization experiments. [Preview Abstract] |
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GP12.00027: MAGNETO-INERTIAL FUSION |
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GP12.00028: Diagnostic Suite for HyperV Coaxial Plasma Gun Development for the PLX-$\alpha$ Project Andrew Case, Sam Brockington, F. Douglas Witherspoon We present the diagnostic suite to be used during development of the coaxial guns HyperV will deliver to LANL in support of the ARPA-E Accelerating Low-Cost Plasma Heating And Assembly (ALPHA) program. For plasma jet diagnostics this includes fast photodiodes for velocimetry, a ballistic pendulum for measuring total plasmoid momentum, interferometry for line integrated plasma density, deflectometry for line integrated perpendicular density gradient measurements, and spectroscopy, both time resolved high resolution spectroscopy using a novel detector developed by HyperV and time integrated survey spectroscopy, for measurements of velocity and temperature as well as impurities. In addition, we plan to use fast pressure probes for stagnation pressure, a Faraday cup for density, fast imaging for plume geometry and time integrated imaging for overall light emission. A novel low resolution long record length camera developed by HyperV will also be used for plume diagnostics. For diagnostics of gun operation, we will use Rogowski coils to measure current, voltage dividers for voltages, B-dot probes for magnetic field, and time resolved fast photodiodes to measure plasmoid velocity inside the accelerator. [Preview Abstract] |
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GP12.00029: Effects of real viscosity on plasma liner formation and implosion from supersonic plasma jets Kevin Schillo, Jason Cassibry, Scott Hsu The PLX-$\alpha$ project endeavors to study plasma liner formation and implosion by merging of a spherical array of plasma jets as a candidate standoff driver for magneto-inertial fusion (MIF). Smoothed particle hydrodynamics (SPH) is being used to model the liner formation and implosion processes. SPH is a meshless Lagrangian method to simulate fluid flows by dividing a fluid into a set of particles and using a summation interpolant function to calculate the properties and gradients for each of these particles. The SPH code was used to simulate test cases in which the number of plasma guns and initial conditions for the plasma were varied. Linear stabilizations were observed, but the possibility exists that this stabilization was due to the implementation of artificial viscosity in the code. A real viscosity model was added to our SPHC model using the Braginskii ion viscosity. Preliminary results for test cases that incorporate real viscosity are presented. [Preview Abstract] |
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GP12.00030: The PLX-$\alpha $ project: Radiation-MHD Simulations of Imploding Plasma Liners Using USim Kristian Beckwith, Peter Stoltz, Madhusudhan Kundrapu, Scott Hsu USim is a tool for modeling high energy density plasmas using multi-fluid models coupled to electromagnetics using fully-implicit iterative solvers, combined with finite volume discretizations on unstructured meshes. Prior work has demonstrated application of USim models and algorithms to simulation of supersonic plasma jets relevant to the Plasma Liner Experiment (PLX) and compared synthetic interferometry to that gathered from the experiment [1]. Here, we give an overview of the models and algorithms included in USim; review results from prior modeling campaigns for the PLX; and describe plans for radiation magnetohydrodynamic (MHD) simulation efforts focusing on integrated plasma-liner implosion and target compression in a fusion-relevant regime using USim for the PLX-$\alpha $ project.\\[4pt] [1] E. C. Merritt, A. L. Moser, S. C. Hsu, J. Loverich, and M. Gilmore; Phys. Rev. Lett. 111, 085003 (2013) [Preview Abstract] |
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GP12.00031: Diagnostics for PLX-alpha Mark Gilmore, Scott Hsu The goal of the Plasma Liner eXperiment PLX-alpha at Los Alamos National Laboratory is to establish the viability of creating a spherically imploding plasma liner for MIF and HED applications, using a spherical array of supersonic plasma jets launched by innovative contoured-gap coaxial plasma guns. PLX-$\alpha $ experiments will focus in particular on establishing the ram pressure and uniformity scalings of partial and fully spherical plasma liners. In order to characterize these parameters experimentally, a suite of diagnostics is planned, including multi-camera fast imaging, a 16-channel visible interferometer (upgraded from 8 channels) with reconfigurable, fiber-coupled front end, and visible and VUV high-resolution and survey spectroscopy. Tomographic reconstruction and data fusion techniques will be used in conjunction with interferometry, imaging, and synthetic diagnostics from modeling to characterize liner uniformity in 3D. Diagnostic and data analysis design, implementation, and status will be presented. [Preview Abstract] |
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GP12.00032: Compression of a fast MHD jet upon impact with a heavy gas cloud: an analog to liner compression Amelia Greig, Paul Bellan The Caltech plasma jet experiment was previously configured to have a fast MHD-driven hydrogen plasma jet collide with a neutral argon cloud located in the path of the jet [1]. By changing to the jet frame, this compression of the light hydrogen jet by impact with the heavy cloud is seen to be equivalent to the Magnetized Inertial Fusion situation of a heavy liner compressing a low-density, magnetized plasma. In the previous experiments it was observed that the hydrogen jet quickly ionized the heavy argon cloud and, being relative low mass compared to the cloud, the hydrogen jet became scrunched up by the impact. Magnetic probe measurements indicated that this nearly tripled the jet magnetic field while Stark broadening showed a simultaneous substantial density increase. We will repeat these measurements for a greater range of parameters and with better diagnostics with the ultimate goal of determining an equation of state characterizing the actual observed behavior. Preliminary results will be presented. \\[4pt] [1] A. L. Moser and P. M. Bellan, Astrophysics and Space Science 337, 593 (2012) [Preview Abstract] |
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GP12.00033: MHD simulation of a magnetized target in an imploding conical cavity Cheng Li Conical imploding magnetic target fusion (MTF) is a new concept, in which the compression comes from both fast mechanic implosion and synchronized theta-pinching or Z-pinching. The compressed magnetized target has a moving end, an increasing external current, and an accumulating high density. Magneto-hydrodynamics (MHD) simulation could help revealing the details of the evolving plasma and finding the parameters (imploding speed, fuel amount, theta-pinch or Z-pinch current profile, etc.) required to reach Lawson Criterion. Preliminary 2D MHD simulation results of a conical imploding theta-pinch are presented. [Preview Abstract] |
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GP12.00034: Conical imploding magnetized target fusion Yian Lei Liquid conical implosion can achieve very high ratio compression of gas or plasma. By magnetizing the plasma at the end of the implosion with very high pulse current, the mechanic compression as well as the current (Z) or magnetic (theta) pinch would compress and heat the magnetized plasma to fusion temperature. The initial thin fusion fuel is prepared by expansion, as the driving liquid sinks from the top of the cone and leaves a large space ($\sim$ 1 m$^{3}$). The gas is preheated by microwave to about a few thousand Kelvin. As the implosion of the liquid goes on, the gas will be ionized. At the top of the cone, a current will produce a magnetic field matching plasma temperature to confine the energy. As the implosion is self-accelerating, the current will spike up near the end of the implosion, creating a magnetic pinch to further heat up the dense plasma. [Preview Abstract] |
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GP12.00035: Development of 1D Liner Compression Code for IDL Akihisa Shimazu, John Slough, Anthony Pancotti A 1D liner compression code is developed to model liner implosion dynamics in the Inductively Driven Liner Experiment (IDL) where FRC plasmoid is compressed via inductively-driven metal liners. The driver circuit, magnetic field, joule heating, and liner dynamics calculations are performed at each time step in sequence to couple these effects in the code. To obtain more realistic magnetic field results for a given drive coil geometry, 2D and 3D effects are incorporated into the 1D field calculation through use of correction factor table lookup approach. Commercial low-frequency electromagnetic fields solver, ANSYS Maxwell 3D, is used to solve the magnetic field profile for static liner condition at various liner radius in order to derive correction factors for the 1D field calculation in the code. The liner dynamics results from the code is verified to be in good agreement with the results from commercial explicit dynamics solver, ANSYS Explicit Dynamics, and previous liner experiment. The developed code is used to optimize the capacitor bank and driver coil design for better energy transfer and coupling. FRC gain calculations are also performed using the liner compression data from the code for the conceptual design of the reactor sized system for fusion energy gains. [Preview Abstract] |
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GP12.00036: Long-length, long-lived flow-shear stabilized Z-pinches: Background and Experimental plans for scaling studies B.A. Nelson, U. Shumlak, R.P. Golingo, E.L. Claveau, H.S. McLean, A.E. Schmidt The ZaP experiment produces long-lived sheared-flow-stabilized Z-pinch plasmas up to 126~cm in length for several flow-through times, and up to thousands of Alfv\'{e}n times. Experimental measurements of the magnetic structure along the full length of the plasma column show an axially uniform Z-pinch plasma during the observed quiescent period. Interferometry, fast-framing images, and Rogowskii coils corroborate the existence of a pinched plasma during this quiescent period of time. Detailed two-dimensional non-linear magnetohydrodynamic (MHD) calculations have been performed showing the formation and assembly of long-length, long-lived Z-pinches. Experimentally-observed plasma lifetimes and velocity-shear profiles are shown to be consistent with calculations of viscous-damping timescales based on the measured plasma parameters. A newly-funded ARPA-E ALPHA project, the Fusion Z-pinch Experiment ``FuZE'' is being constructed at the University of Washington, in collaboration with the Lawrence Livermore National Laboratory. FuZE will study scaling and stability of the successful ZaP experiment to higher pinch currents. The FuZE experimental design, goals, and plans, based on ZaP experimental results, will be presented. [Preview Abstract] |
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GP12.00037: Scaling the Shear-flow Stabilized Z-pinch to Reactor Conditions H.S. McLean, A. Schmidt, U. Shumlak, B.A. Nelson, R.P. Golingo, E. Cleveau We present a conceptual design along with scaling calculations for a pulsed fusion reactor based on the shear-flow-stabilized Z-pinch device. Experiments performed on the ZaP device [1], at the University of Washington, have demonstrated stable operation for durations of 20 usec at $\sim$100kA discharge current for pinches that are $\sim$1 cm in diameter and 100 cm long. The inverse of the pinch diameter and plasma energy density scale strongly with pinch current and calculations show that maintaining stabilization durations of $\sim$7 usec for increased discharge current ($\sim$15x) in a shortened pinch (10 cm) results in a pinch diameter of $\sim$200 um and plasma conditions that approach those needed to support significant fusion burn and energy gain (Ti$\sim$30keV, density$\sim$3e26/m$^{3}$, ntau$\sim$1.4e20 sec/m$^{3}$). Compelling features of the concept include operation at modest discharge current (1.5 MA) and voltage (40kV) along with direct adoption of liquid metals for at least one electrode---technological capabilities that have been proven in existing, commercial, pulse power devices such as large ignitrons.\\[4pt] [1] U. Shumlak, et. al., Nucl. Fusion 49 (2009) 075039. [Preview Abstract] |
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GP12.00038: Design of the Fusion Z-Pinch Experiment - FuZE U. Shumlak, H.S. McLean, B.A. Nelson, R.P. Golingo, A. Schmidt, E.L. Claveau Based on the successful results of the sheared flow stabilized (SFS) Z-pinch from ZaP and ZaP-HD, a new experiment FuZE is designed to scale the plasma performance to fusion conditions. The SFS Z-pinch is immune to the instabilities that plague the conventional Z-pinch yet maintains the same favorable radial scaling. The plasma density and temperature increase rapidly with decreasing plasma radius, which naturally leads to a compact configuration at fusion conditions. The SFS Z-pinch is being investigated as a novel approach to a compact fusion device in a new collaborative ARPA-E ALPHA project with the University of Washington and Lawrence Livermore National Laboratory. The project includes an experimental effort coupled with high-fidelity physics modeling using kinetic and fluid simulations. Along with scaling law analysis, computational and experimental results that have informed the design and development of the FuZE apparatus are presented. [Preview Abstract] |
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GP12.00039: The FRCHX Plasma Injector System Chris Grabowski, James Degnan, Matthew Domonkos, Edward Ruden, Glen Wurden, Thomas Weber The Field-Reversed Configuration Heating Experiment (FRCHX) has been developed in collaboration between the Air Force Research Laboratory (AFRL) and Los Alamos National Laboratory (LANL) to form high density field-reversed configuration (FRC) plasmas intended for adiabatic compression to high energy density conditions. The FRC is first formed via reversed-field theta pinch in Deuterium background plasma. Once formed it is translated a short distance and trapped by a magnetic well within an aluminum solid liner, where it is diagnosed and/or compressed by implosion of the liner. Lifetime of the FRC's poloidal flux affects peak density, temperature, and neutron yield during compression. Despite recent improvements, a significantly longer lifetime is still needed. The merging of two counter-propagating high density FRC plasmas within a central trapping/compression region is proposed. Poloidal flux lifetimes 2 to 3 times longer with embedded fields of 4-5 T, densities $> 1 \times 10^{17}$ cm$^{-3}$, and temperatures (Te+Ti) $>$ 500 eV are projected. These parameters surpass any achieved previously with uncompressed FRC plasmas. An overview of the proposed FRC merging system will be given with further details of projected FRC parameters anticipated. [Preview Abstract] |
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GP12.00040: Magnetized Inertial Fusion (MIF) Research at the Shiva Star Facility James Degnan, C. Grabowski, M. Domonkos, E.L. Ruden, D.J. Amdahl, W.M. White, M.H. Frese, S.D. Frese, G.A. Wurden, T.E. Weber The AFRL Shiva Star capacitor bank (1300 $\mu$F, up to 120 kV) used typically at 4 to 5 MJ stored energy, 10 to 15 MA current, 10 $\mu$s current rise time, has been used to drive metal shell (solid liner) implosions for compression of axial magnetic fields to multi-megagauss levels, suitable for compressing magnetized plasmas to MIF conditions. MIF approaches use magnetic field to reduce thermal conduction relative to inertial confinement fusion (ICF). MIF substantially reduces required implosion speed and convergence. Using profiled thickness liner enables large electrode apertures and field-reversed configuration (FRC) injection. Using a longer capture region, FRC trapped flux lifetime was made comparable to implosion time and an integrated compression test was conducted. The FRC was radially compressed a factor of ten, to 100x density \textgreater 10$^{18}$ cm$^{-3}$ (a world FRC record), but temperatures were only 300-400 eV, compared to intended several keV. Compression to megabar pressures was inferred by the observed liner rebound, but the heating rate during the first half of the compression was less than the normal FRC decay rate. Principal diagnostics were soft x-ray imaging, soft x-ray diodes, and neutron measurements. [Preview Abstract] |
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GP12.00041: Window decompression in laser-heated MagLIF targets Daniel Woodbury, Kyle Peterson, Adam Sefkow The Magnetized Liner Inertial Fusion (MagLIF) concept requires pre-magnetized fuel to be pre-heated with a laser before undergoing compression by a thick solid liner. Recent experiments (M. R. Gomez, et. al., Phys. Plasmas 22, 056306 (2015)) and simulations (A. B. Sefkow, et. al., Phys. Plasmas 21, 072711 (2014)) suggest that yield has been limited to date by poor laser preheat and laser-induced mix in the fuel region. In order to assess laser energy transmission through the pressure-holding window, as well as resultant mix, we modeled window disassembly under different conditions using 1D and 2D simulations in both Helios and HYDRA. We present results tracking energy absorption, time needed for decompression, risk of laser-plasma interaction (LPI) that may scatter laser light, and potential for mix from various window thicknesses, laser spot sizes and gas fill densities. These results indicate that using thinner windows (0.5-1 $\mu$m windows) and relatively large laser spot radii (600 $\mu$m and above) can avoid deleterious effects and improve coupling with the fuel. [Preview Abstract] |
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GP12.00042: Exploring magnetized liner inertial fusion with a semi-analytic model* R.D. McBride, S.A. Slutz, D.B. Sinars, R.A. Vesey, M.R. Gomez, A.B. Sefkow, S.B. Hansen, K.R. Cochrane, P.F. Schmit, P.F. Knapp, M. Geissel, A.J. Harvey-Thompson, C.A. Jennings, M.R. Martin, T.J. Awe, D.C. Rovang, D.C. Lamppa, K.J. Peterson, G.A. Rochau, J.L. Porter, W.A. Stygar, M.E. Cuneo In this presentation, we explore magnetized liner inertial fusion (MagLIF) [S. A. Slutz et al., Phys. Plasmas 17, 056303 (2010)] using a semi-analytic model [R. D. McBride and S. A. Slutz, Phys. Plasmas 22, 052708 (2015)]. Specifically, we present simulation results from this model that: (a) illustrate the parameter space, energetics, and overall system efficiencies of MagLIF; (b) demonstrate the dependence of radiative loss rates on the radial fraction of the fuel that is preheated; (c) explore some of the recent experimental results of the MagLIF program at Sandia National Laboratories [M. R. Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)]; (d) highlight the experimental challenges presently facing the MagLIF program (as MagLIF is first being tested using the infrastructure presently available at the Z pulsed-power facility); and (e) demonstrate how these challenges could change as various system upgrades are made to the Z facility over the next three to five years and beyond. *Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
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GP12.00043: Proton Probing using the T-Cubed Laser Peter Kordell, Paul Campbell, Louise Willingale, Anatoly Maksimchuk, Karl Krushelnick, Eleanor Tubman, Nigel Woolsey The University of Michigan's 20 TW, 400 fs pulse T-cubed laser can produce proton beams of up to 7.2 MeV through target normal sheeth acceleration. The proton flux at 4 MeV produces sufficient signal on Radiochromic Film for use as an ultrafast, electromagnetic field diagnostic. A two beam experiment has been set-up to enable co-timed, pump-probe relativistic intensity interactions. We present an evaluation of the flux, uniformity, energy and laminar flow of the proton probe for future use in imaging of a simple wire target interaction. This work was supported by the DOE (Grant No. DE-SC0012327). [Preview Abstract] |
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GP12.00044: The first experimental campaign on the Laser Megajoule Facility: characterization of plasma transparency in radiatively-heated slots Veronique Tassin, Andre Dulieu, Cedric Courtois, Alexis Casner, Rudolf Rosch, Tony Caillaud, Clement Trosseille, Olivier Henry, Frederic Seguineau, Frederic Durut The Laser Megajoule Facility [1] has been commissioned in October 2014 with performing the first experimental campaign. The goal of this first experimental campaign was to study the evolution of the plasma transparency in slots machined within radiatively-heated samples. The plasma was produced using a radiation hohlraum drive. The evolution of the plasma transparency was radiographed with a 2D time-resolved imager consisting in grazing incidence X-ray microscopes and pinholes coupled to an X-ray framing camera. We have conducted a series of experiments to study the effect of the slot width, the material thickness and the material nature (either tantalum-oxide aerogel or gold). Experimental results will be compared with 2D and 3D radiation hydrodynamics codes. \\[4pt] [1] LMJ-PETAL user guide: http://www-lmj.cea.fr [Preview Abstract] |
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GP12.00045: X-ray diffraction at Matter in Extreme Conditions endstation Zhou Xing, Eric Galtier, Hae Ja Lee, Bob Nagler Understanding dynamic response at the atomic level under extreme conditions is highly sought after goal to science frontiers studying warm dense matter, high pressure, geoscience, astrophysics, and planetary science. Thus it is of importance to determine the high pressure phases or metastable phases of material under shock compression. In situ X-ray diffraction technique using LCLS free electron laser X-ray is a powerful tool to record structural behavior and microstructure evolution in dense matter. Shock-induced compression and phase transitions of material lead to changes of the lattice spacing or evolution of new X-ray diffraction patterns. In this talk, we describe a platform dedicated for the X-ray diffraction studies at Matter in Extreme Conditions (MEC) [1], which can be used to reconstruct a complete diffraction pattern from numerous detectors, optimize detector positioning in a timely manner, extract the lattice spacing profiles and texture features. This platform is available to the user community for real-time analysis. We will also discuss experimental results, using this platform, on the crystalline silicon phase transitions up to 60 GPa. \\[4pt] [1] B. Nagler et al. J. Synchrotron Rad. 22 (2015) [Preview Abstract] |
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GP12.00046: Collision-less Coupling between Explosive Debris Plasmas and Magnetized Background Plasmas Anton Bondarenko, Derek Schaeffer, S. Eric Clark, Erik Everson, Bo Ram Lee, Carmen Constantin, Christoph Niemann The explosive expansion of debris plasma into magnetized background plasma characterizes a variety of astrophysical and space environments, including supernova remnants, interplanetary coronal mass ejections, and ionospheric explosions. In these and other related phenomena, collision-less electro-magnetic processes rather than Coulomb collisions typically mediate the transfer of momentum and energy from the debris to the background. A unique experiment that jointly utilizes the Large Plasma Device (LAPD) and the Phoenix laser facility at UCLA has investigated the super-Alfv\'{e}nic, quasi-perpendicular expansion of a laser-produced carbon (C) debris plasma through a preformed, magnetized helium (He) background plasma via a variety of diagnostics, including emission spectroscopy, wavelength-filtered imaging, and magnetic field probes. Collision-less coupling is directly observed via Doppler shifts in the He II 468.6 nm spectral line, which indicate that the He II ions are accelerated by the laminar electric field that develops due to the expanding C debris. By utilizing an early-time model of the C debris density and velocity, the laminar electric field is calculated and used in combination with the measured magnetic field to simulate He II ion trajectories and velocities. A synthetic Doppler-shifted wavelength spectrum of the He II 468.6 nm spectral line is generated from the simulated He II ion velocities and found to agree well with the measurements. [Preview Abstract] |
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GP12.00047: Copper Conductivity Model Development and Validation Using Flyer Plate Experiments on the Z-machine L. Riford, R.W. Lemke, K. Cochrane Magnetically accelerated flyer plate experiments done on Sandia's Z-machine provide insight into a multitude of materials problems at high energies and densities including conductivity model development and validation. In an experiment with ten Cu flyer plates of thicknesses 500-1000 $\mu$m, VISAR measurements exhibit a characteristic jump in the velocity correlated with magnetic field burn-through and the expansion of melted material at the free surface. The experiment is modeled using Sandia's shock and multiphysics MHD code ALEGRA. Simulated free surface velocities are within 1\% of the measured data early in time, but divergence occurs at the feature, where the simulation indicates a slower burn through time. The cause was found to be in the Cu conductivity model's compressed regime. The model was improved by lowering the conductivity in the region 12.5-16 g/cc and 350-16000 K with a novel parameter based optimization method using the velocity feature as a figure of merit. \\[4pt] *Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
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GP12.00048: Entrainment and acceleration of ambient plasma in a magnetized, laser-produced plasma Jeffrey Bonde, Stephen Vincena, Walter Gekelman Collisionless momentum coupling of a high energy density plasma expansion to a magnetized, ambient plasma is studied with a laser produced plasma expanding at speeds comparable to the background Alfv\'{e}n speed, $v_{\exp } =1.2\times 10^{7}$cm/s $\approx v_{A} $. These expansions form diamagnetic cavities in which the background field is fully expelled. A moving Rosenbluth sheath forms at the boundary carrying a charge layer electrostatic sheath and inductive electric field. The total field in the lab frame was derived from emissive probe and magnetic probe measurements in the azimuthally symmetric experiment. Particle orbit tracing of an initially cold, stationary plasma tracked the evolution of the distribution of particles in these fields. A laser-induced fluorescence (LIF) diagnostic captured the resultant flows in the ambient argon plasma. The bulk flow fields from the orbit solvers and LIF are compared and found to agree $v_{orbit} \approx v_{LIF} \approx 3\times 10^{5}$ cm/s while the distributions are highly non-Maxwellian. The orientation and magnitude of the flows show that the electrostatic sheath of the rapidly expanding plasma mostly entrains a tenuous background plasma, accelerating ions against the expansion. Orbit solvers show the effect has a significant dependence an ambient ion mass. [Preview Abstract] |
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GP12.00049: Spectroscopic Study of a Pulsed High-Energy Plasma Deflagration Accelerator Keith Loebner, Thomas Underwood, Theodore Mouratidis, Mark Cappelli Observations of broadened Balmer lines emitted by a highly-ionized transient plasma jet are presented. A gated CCD camera coupled to a high-resolution spectrometer is used to obtain chord-averaged broadening data for a complete cross section of the plasma jet, and the data is Abel inverted to derive the radial plasma density distribution. This measurement is performed over narrow gate widths and at multiple axial positions to provide high spatial and temporal resolution. A streak camera coupled to a spectrometer is used to obtain continuous-time broadening data over the entire duration of the discharge event (10-50 microseconds). Analyses of discharge characteristics and comparisons with previous work are discussed. [Preview Abstract] |
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GP12.00050: NSTX, ST AND INTERNATIONAL EXPERIMENT |
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GP12.00051: Bifurcation to Enhanced Performance H-mode on NSTX D.J. Battaglia, C.S. Chang, S.P. Gerhardt, S.M. Kaye, R. Maingi, D.R. Smith The bifurcation from H-mode (H$_{98}$ \textless 1.2) to Enhanced Performance (EP)H-mode (H$_{98} = $ 1.2 - 2.0) on NSTX is found to occur when the ion thermal ($\chi_{\mathrm{i}})$ and momentum transport become decoupled from particle transport, such that the ion temperature (T$_{\mathrm{i}})$ and rotation pedestals increase independent of the density pedestal. The onset of the EPH-mode transition is found to correlate with decreased pedestal collisionality ($\nu $*$_{\mathrm{ped}})$ and an increased broadening of the density fluctuation (dn/n) spectrum in the pedestal as measured with beam emission spectroscopy. The spectrum broadening at decreased $\nu $*$_{\mathrm{ped}}$ is consistent with GEM simulations that indicate the toroidal mode number of the most unstable instability increases as $\nu $*$_{\mathrm{ped}}$ decreases. The lowest $\nu $*$_{\mathrm{ped}}$, and thus largest spectrum broadening, is achieved with low pedestal density via lithium wall conditioning and when Z$_{\mathrm{eff}}$ in the pedestal is significantly reduced via large edge rotation shear from external 3D fields or a large ELM. Kinetic neoclassical transport calculations (XGC0) confirm that Z$_{\mathrm{eff}}$ is reduced when edge rotation braking leads to a more negative E$_{\mathrm{r}}$ that shifts the impurity density profiles inward relative to the main ion density. These calculations also describe the role kinetic neoclassical and anomalous transport effects play in the decoupling of energy, momentum and particle transport at the bifurcation to EPH-mode. This work was sponsored by the U.S. Department of Energy. [Preview Abstract] |
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GP12.00052: Study of the L-H transition using GPI in NSTX* A. Diallo, S. Banerjee, S. Zweben The study on a physical mechanism for the L-H transitions is still an active research area. The work presented here is motivated by recent results from Cziegler [1] on C-Mod that show that the time sequence of the L-H transition can be described by the peaking of the Reynolds work, then the collapse of the turbulence, and finally the rise of the diamagnetic electric field shear. In the present work, we apply and extend this analysis using the GPI data on NSTX. The studies focus on about 17 L-H transitions in Ohmic, RF, and NBI discharges. Using an orthogonal programming based particle image velocimetry [2], the GPI images are analyzed to determine the time resolved 2D flow field in the plasma edge of NSTX, assuming the turbulence motion follows the fluid flow. The 2D flow in the radial and vertical directions are used to compute the turbulent kinetic energy transfers in the framework of the K-epsilon model [3]. Detailed discussions of the analysis as well as the analysis of the time resolved Reynolds stress and its impact on turbulence for the set of discharges will be presented. Work supported by the US DoE under DE-AC02-09CH11466.\\[4pt] [1] Cziegler et al. PPCF, \textbf{56}075013 (2014)\textbf{;}\\[0pt] [2] Banerjee et al. RSI \textbf{86}, 033505 (2015);\\[0pt] [3] Manz et al., PoP \textbf{1}, 012309 (2012) [Preview Abstract] |
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GP12.00053: Blob Structure and Motion in the Edge of NSTX Stewart Zweben, James Myra, William Davis, Daniel D'Ippolito, David Russell The structure and motion of discrete plasma blobs (a.k.a. filaments) in the edge and scrape-off layer (SOL) of NSTX is studied for representative Ohmic and H-mode discharges. Individual blobs were tracked in the radial vs. poloidal plane using data from the gas puff imaging (GPI) diagnostic at 400,000 frames/sec. A database of blob amplitude, size, ellipticity, tilt, and velocity was obtained for about 45,000 blobs. The blob velocities are compared with theoretical estimates and analytic blob models [1], and the blob shapes are compared with the edge and SOL flow shear [2]. Empirical relationships between various blob properties will be described, e.g. radial speed vs. amplitude, tilt vs. ellipticity, etc. Limitations of the blob tracking algorithm will also be discussed.\\[4pt] [1] D.A. D'Ippolito et al, Phys. Plasmas 18, 060501 (2011).\\[0pt] [2] J.R. Myra et al Nucl. Fusion 53, 073013 (2013). [Preview Abstract] |
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GP12.00054: Investigating small-scale edge turbulence with the NSTX-U GPI diagnostic Noah Mandell, Stewart Zweben, Walter Guttenfelder, Yang Ren, Steve Sabbagh The Gas Puff Imaging (GPI) diagnostic on NSTX has previously been used to measure medium-scale edge turbulence with correlation lengths $L_{pol}, L_{rad}$ $\sim 2\ $-$\ 20$ cm, corresponding to $k_\perp \rho_s \sim .1\ $-$\ 1$. Some smaller-scale structures down to $\sim1$ cm were occasionally observed, but not very clearly. Therefore the GPI optics have been upgraded using a new zoom lens system to investigate smaller-scale structures down to a scale length of ~1 mm for NSTX-U. We present the previous best measurements of small-scale structure in GPI, and compare them with prior observations from the high-k scattering diagnostic on NSTX, and with calculations of ETG and microtearing modes in NSTX. We also present details on the new optics, and describe the effects of field line curvature on limiting the spacial resolution of the GPI system. [Preview Abstract] |
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GP12.00055: ELM elimination in high triangularity discharges as a function of Lithium dose in NSTX Rajesh Maingi, John Canik ELMs were eliminated gradually with increasing lithium dose in NSTX high triangularity, high performance discharges, similar to results from medium triangularity discharges [1-4]. Analysis of the recycling and edge profiles was done with the SOLPS code to ascertain the divertor recycling coefficient with increasing Li pre-discharge deposition in highly shaped plasmas. The divertor peak heat flux, peak D-alpha intensity and measured upstream profiles of electron and ion density and temperature are used to constrain fits. Modeling indicates that the minimum divertor recycling coefficient at maximum pre-discharge lithium deposition is in the 0.85-0.90 range, and that the cross-field transport is strongly induced in a region just inside the separatrix, radially inward of the recycling source deposition region. Edge stability analysis indicates that the profile changes correlated with lithium conditioning increase the current-driven kink/peeling mode stability limit, allowing access to improved stability. \\[4pt] [1] R. Maingi, et. al., Phy. Rev. Letts. 103 (2009) 075001\\[0pt] [2] J.M. Canik, et. al., Phys. Plasmas 18 (2011) 056118\\[0pt] [3] D.P. Boyle, et. al., Plasma Phys. Contr. Fusion 53 (2011) 105011\\[0pt] [4] R. Maingi, et. al., Nucl. Fusion 52 (2012) 083001 [Preview Abstract] |
(Author Not Attending)
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GP12.00056: Evolution patterns and parameter regimes in edge localized modes on the National Spherical Torus Experiment David R. Smith, R.J. Fonck, G.R. McKee, A. Diallo, S.M. Kaye, B.P. LeBlanc We implement unsupervised machine learning techniques to identify characteristic evolution patterns and associated parameter regimes in edge localized mode (ELM) events observed on the National Spherical Torus Experiment. Multi-channel, localized measurements spanning the pedestal region capture the complex evolution patterns of ELM events on Alfven timescales. Some ELM events are active for less than 100 micro-s, but others persist for up to 1 ms. Also, some ELM events exhibit a single dominant perturbation, but others are oscillatory. Clustering calculations with time-series similarity metrics indicate the ELM database contains at least two and possibly three groups of ELMs with similar evolution patterns. The identified ELM groups trigger similar stored energy loss, but the groups occupy distinct parameter regimes for ELM-relevant quantities like plasma current, triangularity, and pedestal height. Notably, the pedestal electron pressure gradient is not an effective parameter for distinguishing the ELM groups, but the ELM groups segregate in terms of electron density gradient and electron temperature gradient. The ELM evolution patterns and corresponding parameter regimes can shape the formulation or validation of nonlinear ELM models. Finally, the techniques and results demonstrate an application of unsupervised machine learning at a data-rich fusion facility. [Preview Abstract] |
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GP12.00057: Gyrokinetic turbulence simulations of the pedestal region at various lithium doses in NSTX Mireille Coury, Walter Guttenfelder, David R. Mikkelsen, John M. Canik, Ahmed Diallo, Rajesh Maingi It is shown that lithium-coated walls alter the pedestal structure by, for instance, improving the energy confinement and reducing recycling.\footnote{R. Maingi \textit{et al.}, \textit{Nucl. Fus.}, \textbf{52}(2012)}$^,$\footnote{J.M. Canik \textit{et al.}, \textit{Nucl. Fus.}, \textbf{53} (2013)} Recent work\footnote{R. Maingi \textit{et al.}, \textit{ J. Nucl. Mat.}, \textbf{463} (2015)} shows improved discharge characteristics with increasing lithium doses in highly shaped discharges. Edge-localized modes triggered by large edge pressure and current gradients are altered, even suppressed with increasing lithium doses. In this work, the plasma edge characteristics under increasing lithium doses are investigated with GS2 gyrokinetic code.\footnote{M. Kotschenreuther \textit{et al.}, \textit{Comp. Phys. Commun.}, \textbf{88} (1995)} Using experimental discharges as input parameters, microinstabilities are investigated in the pedestal region and the effect of increasing lithium doses on these microinstabilities is discussed. [Preview Abstract] |
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GP12.00058: Field line reconstruction for edge transport modeling in non-axisymmetric tokamaks configurations Heinke Frerichs, Oliver Schmitz, Ian Waters, Todd Evans, Yuhe Feng, Vlad Soukhanovskii Symmetry breaking effects such as resonant magnetic perturbations (RMPs) present a challenge for the numerical analysis of divertor operation, because they require three dimensional models. One such model is provided by the EMC3-EIRENE code, which is based on a finite flux tube grid for field line reconstruction that allows to account for realistic, three dimensional configurations. We present the Field Line Analysis and Reconstruction Environment (FLARE) - a collection of tools for the analysis of the magnetic field structure. It includes a flexible grid generator which allows to set up plasma transport simulations for single and double null configurations (both disconnected and connected). This includes the ``snowflake minus'' topology, and we present an application for a ``near-exact snowflake'' configuration at NSTX-U. Recent edge plasma simulations for DIII-D and ITER include plasma response effects as calculated by the M3D-C1 code, and it is found that these configurations require a local adjustment of radial/poloidal resolution in order to maintain a reasonable level of magnetic flux conservation. [Preview Abstract] |
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GP12.00059: Edge Transport Modeling using the 3D EMC3-Eirene code on Tokamaks and Stellarators J.D. Lore, J.W. Ahn, A. Briesemeister, N. Ferraro, B. LaBombard, A. McLean, M. Reinke, M. Shafer, J. Terry The fluid plasma edge transport code EMC3-Eirene has been applied to aid data interpretation and understanding the results of experiments with 3D effects on several tokamaks. These include applied and intrinsic 3D magnetic fields, 3D plasma facing components, and toroidally and poloidally localized heat and particle sources. On Alcator C-Mod, a series of experiments explored the impact of toroidally and poloidally localized impurity gas injection on core confinement and asymmetries in the divertor fluxes, with the differences between the asymmetry in L-mode and H-mode qualitatively reproduced in the simulations due to changes in the impurity ionization in the private flux region. Modeling of NSTX experiments on the effect of 3D fields on detachment matched the trend of a higher density at which the detachment occurs when 3D fields are applied. On DIII-D, different magnetic field models were used in the simulation and compared against the 2D Thomson scattering diagnostic. In simulating each device different aspects of the code model are tested pointing to areas where the model must be further developed. The application to stellarator experiments will also be discussed. Work supported by U.S. DOE: DE-AC05-00OR22725, DE AC02-09CH11466, DE-FC02-99ER54512, and DE-FC02-04ER54698. [Preview Abstract] |
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GP12.00060: Studies of NSTX L and H-mode Plasmas with Global Gyrokinetic Simulation Y. Ren, W. Wang, W. Guttenfelder, S.M. Kaye, S. Ethier, R.E Bell, B.P. LeBlanc, E. Mazzucato, D.R. Smith, C.W. Domier, H. Yuh Plasma turbulence is considered one of the main mechanisms for driving anomalous thermal transport in magnetic confinement fusion devices. Based on first principle model, gyrokinetic simulations play an important role in studying the relation between plasma turbulence and anomalous thermal transport. In order to predict the confinement performance of future devices, it is crucial to validate gyrokinetic codes against experiments. Nonlinear local gyrokinetic simulations have been used to asses turbulence-driven transport in NSTX L and H-mode plasmas [1,2], and agreement in thermal transport with experiments has only been observed in limited cases. Due to the larger $\rho^*$ of NSTX compared to conventional tokamaks, global effects may be important in determining thermal transport. Here, we present nonlinear global gyrokinetic simulations of NSTX L and H-mode plasmas using global gyrokinetic code GTS [3] and comparisons with experimental transport analysis. Comparisons with nonlinear local gyrokinetic simulations will be also be presented.\\[4pt] [1] Y. Ren et al., Phys. Plasmas 19, 056125 (2012)\\[0pt] [2] Y. Ren et al., Nucl. Fusion 53, 083007 (2013)\\[0pt] [3] W.X. Wang et al., Phys. Plasmas 17, 072511 (2010) [Preview Abstract] |
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GP12.00061: Experimental Study of Density Gradient Stabilization Effects on High-k Turbulence in NSTX J. Ruiz Ruiz, W. Guttenfelder, Y. Ren, A. White, S.M. Kaye, B.P. Leblanc, E. Mazzucato, K.C. Lee, C.W. Domier, D.R. Smith, H. Yuh Electron scale (high-k) ETG-turbulence is diagnosed in NSTX using a high-k microwave scattering system. We report on the stabilization effects of electron density gradient on electron-scale density fluctuations in a set of neutral beam injection (NBI) heated H-mode plasmas. The absence of high-k density fluctuations is correlated with large equilibrium density gradient, consistent with linear stabilization of ETG modes due to density gradient using the ETG linear threshold. The observed scattered power is anti-correlated with equilibrium density gradient. Corresponding linear gyrokinetic simulations using GS2 show that larger equilibrium density gradient leads to higher wavenumbers at the maximum linear growth rate. Real frequencies calculated by GS2 and experimental Doppler-subtracted plasma frame frequencies both decrease with density gradient. Nonlinear electron-scale gyrokinetic simulations were carried out with GYRO: high electron density gradient is shown to reduce electron density fluctuations, heat flux and stiffness, and to increase the ETG nonlinear threshold, reinforcing the experimental observations of density gradient stabilization of high-k turbulence. [Preview Abstract] |
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GP12.00062: Extracting physical quantities from BES data Michael Fox, Anthony Field, Alexander Schekochihin, Ferdinand van Wyk We propose a method to extract the underlying physical properties of turbulence from measurements, thereby facilitating quantitative comparisons between theory and experiment. Beam Emission Spectroscopy (BES) diagnostics record fluctuating intensity time series, which are related to the density field in the plasma through Point-Spread Functions (PSFs). Assuming a suitable form for the correlation function of the underlying turbulence, analytical expressions are derived that relate the correlation parameters of the intensity field: the radial and poloidal correlation lengths and wavenumbers, the correlation time and the fluctuation amplitude, to the equivalent correlation properties of the density field. In many cases, the modification caused by the PSFs is substantial enough to change conclusions about physics. Our method is tested by applying PSFs to the ``real'' density field, generated by non-linear gyrokinetic simulations of MAST, to create synthetic turbulence data, from which the method successfully recovers the correlation function of the ``real'' density field. This method is applied to BES data from MAST to determine the scaling of the 2D structure of the ion-scale turbulence with equilibrium parameters, including the ExB flow shear. [Preview Abstract] |
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GP12.00063: Development of a Novel Method for Determination of Momentum Transport Parameters Michael J. Peters, Walter Guttenfelder, Filippo Scotti, Stanley M. Kaye, Wayne M. Solomon The toroidal momentum pinch velocity V$_{\varphi}$ and diffusivity $\chi_{\varphi}$ in NSTX were previously determined from the transient response of the toroidal rotation $\Omega $ following applied n$=$3 magnetic perturbations that brake the plasma [1,2]. Assuming $\Pi =$nmR$^{2}$(-$\chi_{\phi}\nabla \Omega +$V$_{\phi}\Omega$), where the momentum flux $\Pi $ is determined using TRANSP, these local analyses used fits to $\Omega $ and $\nabla \Omega $ to obtain $\chi_{\phi}$ and V$_{\phi}$ one flux surface at a time. This work attempts to improve the accuracy of the inferred $\chi_{\phi}$(r) and V$_{\phi}$(r) profiles by utilizing many flux surfaces simultaneously. We employ nonlinear least-squares minimization that compares the entire perturbed rotation profile evolution $\Omega $(r,t) against the profile evolution generated by solving the momentum transport equation. We compare the local and integrated approaches and discuss their limitations. We also apply the integrated approach to determine whether an additional residual stress contribution (independent of $\Omega$ or $\nabla \Omega$) can be inferred given experimental uncertainties.\\[4pt] [1] W. Solomon et al., Phys. Rev. Lett. \textbf{101}, 065004 (2008).\\[0pt] [2] Kaye et al., Nucl. Fusion \textbf{49}, 045010 (2009). [Preview Abstract] |
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GP12.00064: Modifications to Ideal Stability by Kinetic Effects for Disruption Avoidance J.W. Berkery, S.A. Sabbagh, Y.S. Park, R.E. Bell, S.P. Gerhardt, B.P. LeBlanc, J.E. Menard Marginal stability points of global modes during high beta operation in NSTX can be found by computing kinetic modifications to ideal magnetohydrodynamic limits on stability. Calculations with the \verb"DCON" code for nearly five thousand experimental equilibria show that the no-wall beta limit decreased with increasing aspect ratio and increasing broadness of the pressure profile, which has implications for NSTX-U. Kinetic modification to ideal limits calculations for several discharges as computed using the \verb"MISK" code predict a transition from damping of the mode to growth as the time approaches the experimental time of marginal stability to the resistive wall mode. The main stabilization mechanism is through rotational resonances with the ion precession drift motion of thermal particles in the plasma, though energetic particles also contribute to stability. To determine RWM marginal stability for use in disruption avoidance, ideal stability limits need to be modified by kinetic effects in order to reproduce experimental marginal stability points. Guided by the full calculations, reduced stability models are investigated to inform automated disruption characterization and prediction analyses presently being developed using NSTX data for application to NSTX-U. [Preview Abstract] |
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GP12.00065: Idiosyncratic Features of 3D Field Produced by Current-Carrying Flux Tubes in the Scrape-Off-Layer (SOL) of Tokamaks Hiro Takahashi Magnetic field produced by toroidally asymmetric Scrape-Off-Layer Currents (SOLCs) can mimic on superficial levels field observed with the Mirnov diagnostic that may be interpreted as MHD modes of low toroidal harmonic numbers, such as RWMs, ELMs, NTMs, EHOs, and locked modes. This work focuses instead on idiosyncratic spatial features that SOLC-generated field also possesses with the possibility in mind of exploiting them in experiments for determining whether or not some or all of field traditionally attributed to MHD modes in fact originates from SOLCs. While the ion saturation current density and the field-line incident angle where current-carrying flux tubes meet solid surfaces limit the magnitude of currents that can flow in the SOL under stationary conditions, the overall structure of the flux tubes determines the effectiveness of such currents per unit magnitude in generating an asymmetric field component. This work will calculate 3D field generated by currents in flux tubes at an ion saturation current density limit, identify its idiosyncratic spatial structures, and quantify its magnitude. [Preview Abstract] |
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GP12.00066: Analysis of fast-ion D-alpha data from NSTX E. Ruskov, W Heidbrink, D. Liu, A. Bortolon, E. Fredrickson, M. Podesta Measured fast-ion D-alpha (FIDA) data from an extensive NSTX database are compared to ``classical'' predictions that neglect transport by instabilities. Even in the absence of detectable MHD, in virtually all cases, the measured radiance is lower, the profile peaks at smaller major radius, and the profile is broader than the predictions. Abrupt large-amplitude MHD events flatten the FIDA profile, as do most toroidal Alfven eigenmode (TAE) avalanche events. Generally, the onset of a long-lived mode also flattens the FIDA profile. There is a shortfall of high-energy ions at large major radius in discharges with repetitive TAE bursts. [Preview Abstract] |
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GP12.00067: Effect of low frequency MHD instability on fast ion distribution in NSTX G. Hao, D. Liu, W.W. Heidbrink, M. Podesta, E.D. Fredrickson, A. Bortolon, R. White, D. Darrow, G.Y. Fu, Z.R. Wang, G.J. Kramer, Y.Q. Liu, K. Tritz In NSTX spherical tokamak plasmas, the onset of low-frequency MHD modes cause a rapid $\sim$ 25{\%} reduction in the fast-ion D-alpha (FIDA) signal. These, 5-20 kHz instabilities are commonly observed in the early phase of neutral beam heated plasmas that often have reversed magnetic shear in the plasma core. The collapse of the core fast ion density is measured by the vertical FIDA diagnostic. Although the profile flattens, changes in spectral shape are modest, suggesting that much of the distribution function is affected. Meanwhile, a modest increase of fast-ion losses is indicated by the measurements from neutron and fast-ion loss detectors. Moreover, this mode is always accompanied by Compressional Alfven Eigenmode (CAE). This suggests that low-f MHD instabilities can cause the redistribution of fast ions in both real and velocity space. Preliminary simulation results from the MARS-F code suggest that the low-f instability is a coupled infernal-peeling mode. The dependence of the mode's onset on the equilibrium parameters and its effect on the fast ion distribution will be computed, and compared with experimental measurements. [Preview Abstract] |
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GP12.00068: Nonlinear Dynamics of Beam-driven TAEs in NSTX Guoyong Fu, Deyong Liu, Feng Wang Energetic particle modes and Alfv\'enic modes driven by super-Alfv\'enic beam ions were routinely observed in neutral beam heated plasmas on NSTX. These modes can significantly impact beam-ion transport, thus causing beam-ion redistribution and losses. Recent simulation results of TAEs show mode radial structure consistent with the reflectometer measurements of electron density fluctuations [1]. In this paper we report on new simulations of multiple TAEs in NSTX plasmas using the M3D-K code. The results show strong interaction between TAEs and fishbone that either enhances or reduces saturation level of individual modes depending on mode number and other parameters. As beam ion beta increases beyond a threshold, mode saturation levels are found to increases sharply. Correspondingly the locally flattening regions merge together resulting in global particle transport and substantial particle loss. These results are similar to the TAE avalanche observed in NSTX.\\[4pt] [1] D. Liu et al, Phys. Plasmas 22, 042509 (2015) [Preview Abstract] |
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GP12.00069: Nonlinear Fishbone Dynamics in Spherical Tokamaks with Toroidal Rotation Feng Wang, G.Y. Fu Fishbone is ubiquitous in tokamak plasmas with fast ions. A numerical study of nonlinear dynamics of fishbone has been carried out in this work. Realistic parameters of NSTX are used to understand instability and nonlinear frequency chirping in tokamak plasmas. First, the effects of shear toroidal rotation are considered for fishbone instability. It's shown that with low $q_{min}$, it has small effects on the mode; while with high $q_{min}$, a new unstable region with a strong ballooning feature in mode structure appears. Second, a detailed study of nonlinear frequency chirping and energetic particles' dynamics is carried out. Linearly, the mode is driven by both trapped and passing particles, with dresonance condition $\omega_{d}\simeq\omega$ for trapped particles and $\omega_{\phi}+\omega_{\theta}\simeq\omega$ for passing particles. As the mode grows, resonance particles oscillate and move outward in $P_{\phi}$ space, which reduces particles' frequency. We believe that this is the main reason for the mode frequency chirping down. Finally, as the mode frequency chirping down, particles with lower orbit frequencies, which are non-resonant linearly, can turn into resonant particles in the nonlinear regime. This effect can sustain a quasi-steady state mode amplitude. [Preview Abstract] |
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GP12.00070: Hybrid MHD/particle simulation study of sub-cyclotron Alfv\'{e}n Eigenmodes in NSTX Jeff Lestz, Elena Belova, N.N. Gorelenkov Low toroidal mode number, high frequency compressional (CAE) and global (GAE) Alfv\'{e}n Eigenmodes are often driven unstable by super-Alfv\'{e}nic beam ions in NSTX. These modes have been identified as part of an energy channeling mechanism that may explain observed anomalous electron temperature profile flattening in beam-heated NSTX discharges [1]. 3D hybrid simulations using the HYM code are conducted to study the excitation and stability properties of such CAE and GAE modes in NSTX and NSTX-like plasmas. HYM allows for the self-consistent simulation of these modes with a delta-f particle treatment of the energetic beam ions coupled to a single fluid resistive MHD model of the thermal plasma. Particular attention is paid to the sensitivity of CAE/GAE excitation on parametric changes in the equilibrium beam ion distribution function, among other factors.\\[4pt] [1] E.V. Belova, et al., Phys. Rev. Lett. \textbf{115}, 015001 (2015). [Preview Abstract] |
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GP12.00071: Application of the Finite Orbit Width Version of the CQL3D Code to NBI$+$RF Heating of NSTX Plasma Yu.V. Petrov, R.W. Harvey The CQL3D bounce-averaged Fokker-Planck (FP) code [1] has been upgraded to include Finite-Orbit-Width (FOW) effects. The calculations can be done either with a fast Hybrid-FOW option or with a slower but neoclassically complete full-FOW option. The banana regime neoclassical radial transport appears naturally in the full-FOW version by averaging the local collision coefficients along guiding center orbits, with a proper transformation matrix from local ($R$, $Z)$ coordinates to the midplane computational coordinates, where the FP equation is solved. In a similar way, the local quasilinear rf diffusion terms give rise to additional radial transport of orbits. The full-FOW version is applied to simulation of ion heating in NSTX plasma. It is demonstrated that it can describe the physics of transport phenomena in plasma with auxiliary heating, in particular, the enhancement of the radial transport of ions by RF heating and the occurrence of the bootstrap current. Because of the bounce-averaging on the FPE, the results are obtained in a relatively short computational time. A typical full-FOW run time is 30 min using 140 MPI cores. Due to an implicit solver, calculations with a large time step (tested up to \textit{dt }$=$ 0.5 sec) remain stable. \\[4pt] [1] R.W. Harvey and M. McCoy, ``The CQL3D Fokker Planck Code,'' www.compxco.com/cql3d [Preview Abstract] |
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GP12.00072: RF power deposition explorations in tokamaks using cameras and Langmuir probes J. Hosea, R. Perkins, N. Bertelli, G. Taylor, C. Qin, L. Wang, J. Yang, X.J. Zhang, B.H. Park, S.J. Wang On NSTX the HHFW RF power deposition in the SOL can be large and follows the magnetic field lines from in front of the antenna to an RF heat deposition spiral on the divertor regions [1]. The strong SOL deposition and the spiral formation appear to be properties of the HHFW regime for edge densities above the cutoff density in front of the antenna as simulated with the AORSA RF code [2]. AORSA simulations indicate there is much lower RF deposition in the SOL in the minority ICRF heating regimes of conventional aspect ratio tokamaks [3]. Here, the RF divertor deposition characteristics for NSTX as gleaned from visible cameras and probes are compared to those for EAST and KSTAR. To date no spirals have been confirmed for EAST and KSTAR: most of the RF deposition on the divertor as indicated with probes is peaked near the outer vessel strike radius (OVSR), with a broad deposition pattern outside the OVSR. Cameras indicate considerable RF deposition inside the divertor trenches and some RF power deposition outside them. Work continues to quantify RF power deposition in the SOL at increasing levels of applied RF power.\\[4pt] [1] R. Perkins et al., PRL \textbf{109} (2012) 045001;\\[0pt] [2] N. Bertelli et al., Nucl. Fus. \textbf{54} (2014) 083004;\\[0pt] [3] N. Bertelli et al., 25 IAEA FES Conf. (2014) TH/P4-14. [Preview Abstract] |
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GP12.00073: Nonaxisymmetric effects in strongly driven Coaxial Helicity Injection in simulations of NSTX E.B. Hooper, C.R. Sovinec Nonaxisymmetric effects become important in strongly-driven CHI in NSTX simulations using NIMROD. An n?1, high m mode excited in simulations of injection and flux closure can significantly impact the injected poloidal flux evolution and closure [1,2]. In nonlinear simulations, the mode velocity and magnetic perturbations occur in ``bursts;'' in previous, lower temperature work the mode was weak and not bursting, with little effect on the injection [1]. The mode is excited just outside the poloidal flux bubble with axes of poloidal velocity vortexes and magnetic flux surfaces aligned along the magnetic field. Their width is approximately that of the current layer in the surface of the bubble. The instability significantly broadens the current layer and apparently is driven in part by currents resulting from expansion of the injected poloidal flux. Linear simulations starting from nonlinear, purely axisymmetric simulations or from the axisymmetric parts of nonaxisymmetric simulations yield the linear eigenmodes and sensitivity to plasma parameters. Ongoing analysis to identify the driving mechanism(s) for the instability is constrained by these linear results.\\[4pt] [1] E B Hooper, et al., Phys. Plasmas 20, 092510 (2013)\\[0pt] [2] F Ebrahimi, et al., Phys. Plasmas 20, 090702 (2013) [Preview Abstract] |
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GP12.00074: Progress toward commissioning and plasma operation in NSTX-U Masayuki Ono NSTX is undergoing a major device upgrade as well as an addition of a second more tangential Neutral Beam Injection (NBI) heating and current drive system. NSTX upgrade will double the toroidal field from 0.5 T to 1 T, the plasma current from 1 MA to 2 MA, the NBI heating and current drive power from 7 MW to 14 MW, and increase the peak field plasma pulse length from 1 sec to 7 sec. More tangential NBI system is designed to achieve 100{\%} non-inductive operation needed for the high beta non-inductive operations. Innovative plasma start-up and ramp-up techniques without the central solenoid operation which is needed for a compact FNSF design will be explored. A major physics/technology goal for NSTX-U is to develop an attractive divertor solution for the very high steady-state divertor heat flux expected for future reactors. With doubling of the heat flux and plasma current, the peak divertor heat flux in NSTX-U could quadruple to about 40 MW/m2 compared to up to 10 MW / m2 of NSTX. For divertor heat mitigation, snow-flake divertor configuration and liquid lithium divertor are being considered. The first plasma operation of NSTX-U is planned in August 2015. [Preview Abstract] |
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GP12.00075: Design and simulation of control algorithms for stored energy and plasma current in non-inductive scenarios on NSTX-U Mark Boyer, Robert Andre, David Gates, Stefan Gerhardt, Jonathan Menard, Francesca Poli One of the major goals of NSTX-U is to demonstrate non-inductive operation. To facilitate this and other program goals, the center stack has been upgraded and a second neutral beam line has been added with three sources aimed more tangentially to provide higher current drive efficiency and the ability to shape the current drive profile. While non-inductive start-up and ramp-up scenarios are being developed, initial non-inductive studies will likely rely on clamping the Ohmic coil current after the plasma current has been established inductively. In this work the ability to maintain control of stored energy and plasma current once the Ohmic coil has been clamped is explored. The six neutral beam sources and the mid-plane outer gap of the plasma are considered as actuators. System identification is done using TRANSP simulations in which the actuators are modulated around a reference shot. The resulting reduced model is used to design an optimal control law with anti-windup and a recently developed framework for closed loop simulations in TRANSP is used to test the control. Limitations due to actuator saturation are assessed and robustness to beam modulation, changes in the plasma density and confinement, and changes in density and temperature profile shapes are studied. [Preview Abstract] |
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GP12.00076: Modeling and control of plasma rotation and $\beta_n$ for NSTX-U using Neoclassical Toroidal Viscosity and Neutral Beam Injection Imene Goumiri, Clarence Rowley, Steven Sabbagh, David Gates, Stefan Gerhardt, Mark Boyer A model-based system is presented allowing control of the plasma rotation profile in a magnetically confined toroidal fusion device to maintain plasma stability for long pulse operation. The analysis, using NSTX data and NSTX-U TRANSP simulations, is aimed at controlling plasma rotation using momentum from six injected neutral beams and neoclassical toroidal viscosity generated by three-dimensional applied magnetic fields as actuators. Based on the momentum diffusion and torque balance model obtained, a feedback controller is designed and predictive simulations using TRANSP will be presented. Robustness of the model and the rotation controller will be discussed. [Preview Abstract] |
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GP12.00077: Performance Assessment of Model-Based Optimal Feedforward and Feedback Current Profile Control in NSTX-U using the TRANSP Code Z. Ilhan, W.P. Wehner, E. Schuster, M.D. Boyer, D.A. Gates, S. Gerhardt, J. Menard Active control of the toroidal current density profile is crucial to achieve and maintain high-performance, MHD-stable plasma operation in NSTX-U. A first-principles-driven, control-oriented model describing the temporal evolution of the current profile has been proposed earlier by combining the magnetic diffusion equation with empirical correlations obtained at NSTX-U for the electron density, electron temperature, and non-inductive current drives. A feedforward $+$ feedback control scheme for the requlation of the current profile is constructed by embedding the proposed nonlinear, physics-based model into the control design process. Firstly, nonlinear optimization techniques are used to design feedforward actuator trajectories that steer the plasma to a desired operating state with the objective of supporting the traditional trial-and-error experimental process of advanced scenario planning. Secondly, a feedback control algorithm to track a desired current profile evolution is developed with the goal of adding robustness to the overall control scheme. The effectiveness of the combined feedforward $+$ feedback control algorithm for current profile regulation is tested in predictive simulations carried out in TRANSP. [Preview Abstract] |
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GP12.00078: Initial Development of the NSTX-U Snowflake Divertor Control Patrick Vail, Egemen Kolemen, Anders Welander, Matthew Lanctot A feedback control system has been implemented at NSTX-U for real-time detection and manipulation of snowflake divertor (SFD) magnetic configurations. The SFD is an alternative magnetic divertor concept that is characterized by a second-order null formed by two x-points in close proximity. The SFD is an attractive option for heat flux mitigation for NSTX-U in which unmitigated peak heat fluxes in standard divertor operation near 20 MW/m$^{2}$ may compromise plasma-facing components. The real-time control system at NSTX-U is capable of simultaneous control of multiple SFD parameters, such as the separation between the two x-points in the divertor region and their orientation. Control of SFD configurations in NSTX-U has been simulated in TOKSYS using the upgraded sets of poloidal field coils in both the upper and lower divertor regions. Performance of the real-time control system and its effect on plasma performance will be assessed experimentally as an initial step toward the development of the SFD concept at NSTX-U. [Preview Abstract] |
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GP12.00079: High-k Scattering and FIReTIP Diagnostic Upgrades for NSTX-U Robert Barchfeld, Evan Scott, Calvin Domier, Christopher Muscatello, Paul Riemenschneider, Mohammad Sohrabi, Neville Luhmann, Yang Ren, Robert Kaita A major upgrade to the High-k Scattering system is underway on NSTX-U, which is being transformed from a primarily toroidal detection geometry (for k$_{\mathrm{r}}$ measurements) to a poloidal detection geometry (for k$_{\mathrm{\theta }}$ measurements) in which a probe beam is launched from Bay G and collected on Bay L. Combined with an increase in probing frequency to 693 GHz, the poloidal wavenumber sensitivity has been extended from k$_{\mathrm{\theta }} \quad =$ 7 cm$^{\mathrm{-1}}$ up to 40 cm$^{\mathrm{-1}}$. The system will be installed and commissioned in 2016 with an initial 4-channel receiver, with plans to eventually upgrade to an 8x2 configuration, which can probe the plasma from the core out to the edge of the pedestal region. The Far Infrared Tangential Interferometer/Polarimeter (FIReTIP) system is being upgraded with field programmable gate array (FPGA) electronics to support real time feedback density control, and will be installed on Bay G this fall. Design and implementation details regarding both diagnostics will be presented. [Preview Abstract] |
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GP12.00080: Status and Plans for the TRANSP Interpretive and Predictive Simulation Code Stanley Kaye, Robert Andre, Gorelenkova Marina, Xingqui Yuan, Richard Hawryluk, Steven Jardin, Francesca Poli TRANSP is an integrated interpretive and predictive transport analysis tool that incorporates state of the art heating/current drive sources and transport models. The treatments and transport solvers are becoming increasingly sophisticated and comprehensive. For instance, the ISOLVER component provides a free boundary equilibrium solution, while the PT\textunderscore SOLVER transport solver is especially suited for stiff transport models such as TGLF. TRANSP also incorporates such source models as NUBEAM for neutral beam injection, GENRAY, TORAY, TORBEAM, TORIC and CQL3D for ICRH, LHCD, ECH and HHFW. The implementation of selected components makes efficient use of MPI for speed up of code calculations. TRANSP has a wide international user-base, and it is run on the FusionGrid to allow for timely support and quick turnaround by the PPPL Computational Plasma Physics Group. It is being used as a basis for both analysis and development of control algorithms and discharge operational scenarios, including simulation of ITER plasmas. This poster will describe present uses of the code worldwide, as well as plans for upgrading the physics modules and code framework. Progress on implementing TRANSP as a component in the ITER IMAS will also be described. [Preview Abstract] |
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GP12.00081: Initial applications of the non-Maxwellian extension of the full-wave TORIC v.5 code in the mid/high harmonic and minority heating regimes N. Bertelli, E.J. Valeo, C.K. Phillips A non Maxwellian extension of the full wave TORIC v.5 code in the mid/high harmonic and minority heating regimes has been revisited. In both regimes the treatment of the non-Maxwellian ions is needed in order to improve the analysis of combined fast wave (FW) and neutral beam injection (NBI) heated discharges in the current fusion devices. Additionally, this extension is also needed in time-dependent analysis where the combined heating experiments are generally considered. Initial numerical cases with thermal ions and with a non-Maxwellian ions are presented for both regimes. The simulations are then compared with results from the AORSA code, which has already been extended to include non-Maxwellian ions. First attempts to apply this extension in a self-consistent way with the NUBEAM module, which is included in the TRANSP code, are also discussed. [Preview Abstract] |
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GP12.00082: Comparison of fast ion confinement during on-axis and off-axis neutral beam experiments on NSTX-U D. Liu, W.W. Heidbrink, G.Z. Hao, M. Podesta, D.S. Darrow, E.D. Fredrickson, S.S. Medley A second and more tangential neutral beam line is a major upgrade component of the National Spherical Torus Experiment -- Upgrade (NSTX-U) with the purpose of improving neutral beam current drive efficiency and providing more flexibility in current/pressure profile control. Good fast ion confinement is essential to achieve the anticipated improvements in performance. In a planed ``sanity check'' experiment, various short and long (relative to fast ion slowing-down time) neutral beam (NB) pulses with different source mixes will be injected into quiescent L-mode plasmas to characterize the fast ion confinement and distribution function produced by the new and the existing NBI lines. The neutron rate decay after the turn-off of short NB pulses will be used to estimate the fast ion confinement time and to investigate its dependence on NB source/geometry, injection energy, and plasma current. The newly installed Solid State Neutral Particle Analyzer (SSNPA) and Fast-Ion D-Alapha (FIDA) diagnostics will be described and will be used to measure fast ion slowing-down distribution function and spatial profile during the injection of relatively long NB pulses. Fast ion prompt losses will be monitored with a scintillator Fast Lost Ion Probe (sFLIP) diagnostic. The experimental techniques, measurements of fast ion confinement time and distribution function, and comparisons with classical predictions from NUBEAM modeling will be presented in detail. *\textit{Work supported by US DOE.} [Preview Abstract] |
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GP12.00083: Core heat convection in NSTX-U via modification of electron orbits by high frequency Alfv\'{e}n eigenmodes N.A. Crocker, K. Tritz, R.B. White, E.D. Fredrickson, N.N. Gorelenkov New simulation results demonstrate that high frequency compressional (CAE) and global (GAE) Alfv\'{e}n eigenmodes cause radial convection of electrons, with implications for particle and energy confinement, as well as electric field formation in NSTX-U. Simulations of electron orbits in the presence of multiple experimentally determined CAEs and GAEs, using the gyro-center code ORBIT, have revealed substantial convective transport, in addition to the expected diffusion via orbit stochastization. These results advance understanding of anomalous core energy transport expected in high performance, beam-heated NSTX-U plasmas. The simulations make use of experimentally determined density perturbation ($\delta $n) amplitudes and mode structures obtained by inverting measurements from 16 a channel reflectometer array using a synthetic diagnostic. Combined with experimentally determined mode polarizations (i.e. CAE or GAE), the $\delta $n are used to estimate the ExB displacements for use in ORBIT. Preliminary comparison of the simulation results with transport modeling by TRANSP indicate that the convection is currently underestimated. [Preview Abstract] |
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GP12.00084: Experimental investigation of the physics controlling core-localized compressional and global Alfv\'{e}n eigenmode spectra, structure, and amplitude in NSTX-U Shawn Tang, N.A. Crocker, T.A. Carter, E.D. Fredrickson, W. Guttenfelder, N.N. Gorelenkov Electron thermal confinement in NSTX was observed to deteriorate with increasing toroidal field and beam power, which potentially has significant implications for NSTX-U. The leading candidates for this anomalous electron transport are high-frequency Alfv\'{e}n eigenmodes excited through Doppler-shifted cyclotron resonance with beam ions [D. Stutman, et al., \textit{PRL} 2009]. These modes were identified as compressional (CAE) and global (GAE) Alfv\'{e}n eigenmodes [N.A. Crocker, et al., \textit{NF} 2013]. The potential impact of these modes motivates the investigation of the physics controlling their spectra, structure, and amplitude. A database of neutral beam heated NSTX shots spanning a broad range of plasma parameters has been compiled to investigate fast-ion driven mode activity [E.D. Fredrickson, et al., \textit{NF} 2014]. This database is extended to include measures of CAE/GAE activity in order to statistically investigate the physics parameters controlling the characteristics of these modes and how they contribute to anomalous electron transport. We show preliminary results of this investigation. [Preview Abstract] |
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GP12.00085: Modeling of potential TAE-induced beam ion loss from NSTX-U plasmas Douglass Darrow, Eric Fredrickson, Mario Podesta, Roscoe White, Deyong Liu NSTX-U will add three additional neutral beam sources, whose tangency radii of 1.1, 1.2, and 1.3 m, are significantly larger than the 0.5, 0.6, and 0.7 m tangency radii of the neutral beams previously used in NSTX. These latter beams will also be used in NSTX-U. Here, we attempt to formulate an estimate of the propensity of the beam ions from all the various sources to be lost under a range of NSTX-U plasma conditions. This estimation is based upon TRANSP calculations of beam ion deposition in phase space, and the location of the FLR-corrected loss boundary in that phase space. Since TAEs were a prominent driver of beam ion loss in NSTX, we incorporate their effects through the following process: NOVA modeling of TAEs in the anticipated NSTX-U plasma conditions gives the mode numbers, frequencies, and mode structures that are likely to occur. Using this information as inputs to the guiding center ORBIT code, it is possible to find resonant surfaces in the same phase space along which beam ions would be able to diffuse under the influence of the modes. The degree to which these resonant surfaces intersect both the beam deposition volume and the orbit loss boundary should then give a sense of the propensity of that beam population to be lost from the plasma. [Preview Abstract] |
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GP12.00086: Design study for a spatial heterodyne Doppler coherence imaging system for flow measurements on NSTX-U Jacob Schwartz, M.A. Jaworski, A. Diallo, R. Kaita, J.H. Nichols Measuring the flow of impurities in the SOL of NSTX-U can lead to understanding of main ion flow and heat transport. Spatial heterodyne Doppler coherence imaging is a technique that allows a single camera frame to record both the brightness and Doppler shift of an emitted spectral line over the entire field of view. With a tangential view on NSTX-U it is possible to tomographically reconstruct 2d (r-z) profiles of emissivity and flow velocity for an imaged impurity ion by assuming axisymmetry and field-aligned flow. One can derive the main ion parallel flow velocity by making four measurements and using additional assumptions. Imaging of two spectral lines each from two ion species allows solving for $n_e$, $T_e$, and the density of the two ion species by using ADAS emissivity tables. Since measurements of the velocity of two impurities are planned, it is possible to derive a main ion parallel velocity by using a reconstructed $n_i$ and $T_i$ (from other diagnostics), a 1d conduction-limited SOL model, and a 1d model of forces on impurities. With fewer than four measurements, it is possible to derive the main ion velocity if the impurities are entrained in the flow. A design study for such a spatial heterodyne Doppler coherence imaging system on NSTX-U will be presented. [Preview Abstract] |
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GP12.00087: MGI and CHI Research on NSTX-U R. Raman, T.R. Jarboe, B.A. Nelson, D. Mueller, F. Ebrahimi, S.C. Jardin, M. Ono, J.E. Menard Results from NSTX Transient Coaxial Helicity Injection (CHI) experiments have demonstrated generation of 300kA start-up currents, and when these discharges were coupled to induction they attained 1MA of plasma current consuming 65{\%} of the inductive flux of standard inductive-only discharges in NSTX. The NSTX-U device will have numerous improvements to enhance transient CHI capability, and simulations suggest that the NSTX-U coil configuration supports CHI plasma currents in excess of 400kA. Design studies of CHI for a ST-FNSF have identified viable reactor installation concepts. In support of disruption mitigation studies, three ITER-type MGI valves in a J X B torque-cancelling configuration have been built and tested in an off-line test stand, including in the presence of externally imposed magnetic field. FY16 research on NSTX-U will study the MGI gas assimilation efficiency, and the advantages of gas injection into the private flux region. This work is supported by U.S. DOE Contracts: DE-AC02-09CH11466, DE-FG02-99ER54519 AM08, and DE-SC0006757. [Preview Abstract] |
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GP12.00088: Impact of the NCC on NSTX-U edge ballooning stability, and the implications for edge transport J.M. Canik, S.P. Gerhardt, J.E. Menard, J.-K. Park A new Non-axisymmetric Control Coil (NCC) set is being considered to augment various studies in NSTX-U involving applied 3D magnetic perturbations. These in-vessel, off-midplane coils offer a significantly broader spectral range, useful for studies of ELM control. The VMEC 3D equilibrium code and the COBRA stability code have been used to investigate the impact of the NCC on ideal ballooning stability. For axisymmetric cases that are stable but near the ballooning boundary, the NCC can strongly destabilize ballooning modes over a large range of plasma radius. Profile variations indicate that, compared to the axisymmetric case, the NCC causes the stability boundary to shift by $\sim$ 10-20{\%} in both magnetic shear and pressure gradient. Viewed as a proxy for the kinetic ballooning mode, these results indicate that activating the NCC set may reduce the pressure gradient by a similar amount. The non-axisymmetry in the radial heat flux can be expected to produce divertor striations similar to those observed in various experiments. Comparisons of how the impact on stability varies with kink and resonant response will be presented. [Preview Abstract] |
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GP12.00089: Impact of Resonant Magnetic Perturbation Fields on NSTX-U Advanced Divertor Topologies Ian Waters, Heinke Frerichs, Oliver Schmitz, Joon-Wook Ahn, Gustavo Canal, Todd Evans, Vlad Soukhanovskii Explorations are under way to optimize the magnetic topology in the plasma edge of NSTX-U with the goal of improving neutral and impurity fueling and exhaust. The use of magnetic perturbation fields is being considered to spread heat and particle fluxes in the divertor, adjust plasma refueling, control impurity transport, and improve coupling to the exhaust systems. Also, advanced divertor configurations are being considered to improve peak heat loads on divertors. An assessment is made of the topologies of a number of representative NSTX-U advanced divertor configurations: lower single null, exact snowflake, and snowflake minus. Wall to wall magnetic connection lengths for each configuration are assessed in both their perturbed and axisymmetric configurations with perturbation coil currents of 1kA and 3kA. The magnetic perturbations yield complex strike patterns on divertor elements that are expected to be resolvable experimentally. The EMC3-EIRENE fluid plasma and kinetic neutral transport code will be used to study neutral and impurity transport and exhaust in these topologies. This work was funded in part by the Department of Energy under grant DE-SC0012315 and by startup funds of the Department of Engineering Physics at the University of Wisconsin-Madison. [Preview Abstract] |
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GP12.00090: Heat flux and scrape-off layer width scaling in NSTX and NSTX-U Travis Gray, Joon-Wook Ahn, Kaifu Gan, Michael Jaworski, Rajesh Maingi, Adam McLean, Vlad Soukhanovskii While it has been shown experimentally that the inter-ELM scrape-off layer width, $\lambda_q$ in tokamaks scales as I$_p^{-1}$, the underlying physical mechanism for this scaling is not yet understood. Additionally, the physics behind the broadening of the heat flux profile during detachment, described by the S parameter in the so-called Eich fitting function [Eich NF 2013], is just beginning to be explored. During the final run campaigns of NSTX, it was shown experimentally that the addition of evaporative lithium wall coatings reduced $\lambda_q$ and S and correlated with a reduction in overall divertor pressure. Conversely during detachment experiments with CD$_4$ injection, while divertor total pressure increased during CD$_4$ injection, both $\lambda_q$ and S increased correspondingly. This is in qualitative agreement with measurements made on other tokamaks and shows a clear scaling of $\lambda_q$ and S with upstream density, which is used as a proxy for divertor density. Expected behavior and preliminary results from NSTX-U will also be presented. [Preview Abstract] |
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GP12.00091: Divertor spectroscopy for radiative divertor feedback control and snowflake divertor experiments in {NSTX~Upgrade} V.A. Soukhanovskii, F. Scotti, M.E. Weller, S.P. Gerhardt, R. Kaita, B. Stratton In the NSTX-U tokamak, steady-state peak divertor heat fluxes are projected to reach 10-30 MW/m$^2$ thus challenging plasma facing component thermal limits. The snowflake divertor magnetic configuration and radiative divertor with feedback-controlled D$_2$ or impurity seeding are presently envisioned for divertor power handling, based on NSTX experiments and modeling with edge transport code UEDGE. In addition to the existing NSTX divertor diagnostics, new spectroscopic diagnostics are installed to improve understanding of snowflake divertor transport and to measure divertor radiation and plasma temperature for impurity radiation feedback control. A radially viewing divertor Phantom camera will be used to elucidate on the null-region churning mode. An upgraded vacuum ultraviolet spectrometer SPRED and a multichannel ultraviolet spectrometer would provide estimates of divertor impurity radiated power and divertor $T_e \sim 0.5-10$ eV via the $\Delta n=0; 1; 2$ of C and N line intensity ratios, and deuterium Balmer B7-B11 line ratios, respectively. The measurements are calibrated using atomic physics models and the collisional-radiative code CRETIN. Using the upgraded divertor gas injectors, the characteristic radiative divertor control time is expected to be under 50 ms. [Preview Abstract] |
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GP12.00092: Spectroscopic diagnostics and experimental planning for plasma-surface interaction studies in NSTX-U F. Scotti, V.A. Soukhanovskii, J.P. Allain, F. Bedoya, R. Kaita, A.L. Roquemore, C.H. Skinner In the mixed-material environment of the NSTX-U first wall, visible imaging diagnostics will be used to study the evolution of the plasma facing component (PFC) surface conditions and the distribution of impurity influxes. Characterizing the dynamic material environment originating from wall conditioning techniques (boronization, lithium evaporation) on graphite PFCs requires simultaneous monitoring of emission from different atomic species. Full poloidal/toroidal coverage of impurity emission is achieved via a combination of bandpass-filtered fast cameras viewing upper and lower PFCs and line-scan cameras. Two image-intensified radiation-hardened cameras expand these capabilities with the ability to image weaker visible lines and a custom-built two-color system for the simultaneous imaging of different wavelengths. Intensified camera views include the lower divertor and a close-up of the surface analysis sample system Material Analysis and Particle Probe (MAPP). Redundant views via multiple cameras and two-color setups will enable a more accurate determination of impurity influxes (via line ratio techniques) and the simultaneous characterization of carbon (chemical/physical), lithium and oxygen influx evolution following lithium and boron wall conditioning. The imaging of MAPP samples will allow comparing the evolution of surface composition determined via surface analysis techniques to visible spectroscopy. Supported by U.S. DOE Contracts: DE-AC02-09CH11466, DE-AC52-07NA27344, DE-SC0010717 [Preview Abstract] |
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GP12.00093: Spectroscopic diagnostics for upper divertor and central stack on NSTX-U Kaifu Gan, Alessandro Bortolon, Travis Gray, Joon-Wook Ahn, Brian Wirth, Rajesh Maingi NSTX has demonstrated a number of discharge characteristics that improved with increasing lithium coatings, all with nominal thickness \textgreater \textgreater ion implantation depth [1]. The asymmetries in the lithium coating and erosion and re-deposition of lithium in other regions possibly explain this phenomenon. In order to investigate the role of these mechanisms, new high resolution UV-VIS-NIR spectroscopic diagnostics are installed in NSTX-U to monitor the previously uncovered upper divertor and central stack region. The diagnostics consist of a high speed ProEM-HS 512 camera, an IsoPlane SCT320 spectrometer and 32 sightlines: 16 sightlines on the upper divertor and 16 sightlines on the central stack. The ratio of lithium emission to carbon emission as a function of pre-discharge lithium deposition will be measured in these two region to evaluate the hypothesis. The diagnostics can be also used to measure impurity influx and ion temperature. First diagnostics results from the 2015 NSTX-U campaign will be presented.\\[4pt] [1] R. Maingi \textit{et al}., Phys. Rev. Lett. 107, 145004 (2011). [Preview Abstract] |
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GP12.00094: Initial Lithium Capabilities for NSTX-U and Plans for Mitigating Associated Risks R. Kaita, W. Blanchard, D. Cai, M. Jaworski, R. Lunsford, A.L. Roquemore, H. Schneider, L. Cadwallader The effect on plasma performance of lithium plasma-facing components (PFCs) is an important research focus on NSTX-U. Lithium evaporators (LITERs) are mounted at two upper dome locations to evaporate lithium onto the lower divertor region. The introduction of lithium into NSTX-U will also be conducted by the injection of small granules from a lithium granule injector (LGI). The main risk for the LITERs and the LGI is commensurate to the state of the lithium. The lithium used in the LGI is in the form of solid, micron-sized granules, and relatively few granules would enter NSTX-U should any possible failure modes occur. Each LITER, however, can operate with up to 80 grams of liquid lithium. The PFC water-cooling lines will thus be emptied during lithium operations, to prevent exposure of liquid lithium to water if there is a simultaneous LITER structural failure and a water leak. If there is a simultaneous LITER structural failure and a large air leak, the graphite PFCs could then be subject to high heat from rapid lithium oxidation. The likelihood that it would cause serious PFC damage is being evaluated. A mitigation scheme is a gas purge system that fills the vacuum vessel with argon should there be a significant pressure excursion when the lithium is at elevated temperature. [Preview Abstract] |
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GP12.00095: Utilizing solid impurity granules for ELM pacing in NSTX-U Robert Lunsford, L. Roquemore, M.A. Jaworski, R. Kaita, R. Maingi Periodic edge localized modes (ELMs) rapidly transport stored energy from the edge plasma to the divertor. These events result in abrupt heating of the plasma facing components (PFCs) which reduces their effective lifetime as well as generating a strong impurity influx. If the frequency of these ELMs can be increased through controlled triggering, also known as pacing, then the inverse relationship between the peak heat flux and the frequency of the ELMs can be utilized to prevent material damage that could result from otherwise unmitigated ELMs. At NSTX, the ability of small (300 -- 1000 micron) impurity granules to trigger and pace these ELMs is being explored. In these experiments, ELMs are triggered by seeding a density perturbation within the edge-pedestal region through low speed injection and ablation of impurity granules, thus generating a localized instability. Granules are dropped from a reservoir and transit a vertical flight tube at which point a rotating impeller imparts horizontal momentum into the falling granules. This drives them into the edge of the discharge at speeds ranging from 50-150 m/s and average injection frequencies of up to 200 Hz depending upon the settings of the injector. Results from the initial laboratory injection tests of lithium, boron carbide (B$_{4}$C) and vitreous carbon granules and their subsequent implementation in NSTX-U experiments will be discussed. [Preview Abstract] |
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GP12.00096: Design considerations for NSTX-U high-Z divertor upgrade and future liquid metal PFCs Michael Jaworski, K. Tresemer, A. Brooks, S. Gerhardt, R. Kaita, J. Menard, J. Nichols, M. Ono, R. Maingi, P. Rindt, N. Lopes-Cardozo, T. Gray A clear comparison between plasma-facing components composed of high-Z solid metals versus low-Z liquid lithium fusion device is sought to experimentally demonstrate integrated, high-performance scenarios in the spherical tokamak configuration. The NSTX-U is beginning a staged upgrade of the in-vessel PFCs to high-Z metal to accomplish this experimental program. The first high-Z divertor upgrade will consist of the installation of a continuous toroidal row of molybdenum tiles in the outboard divertor of the machine. The experimental program is designed to test the heat-flux handling capabilities of these bulk metal tiles and assess the impact of the high-Z tiles on plasma operations. Incremental upgrades toward flowing liquid lithium systems are also envisioned with one possible embodiment being pre-filled, liquid lithium, self-replenishing PFCs. Design and analysis of the base tiles and pre-filled target tiles will be presented. [Preview Abstract] |
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GP12.00097: Initial results and operation of the Materials Analysis Particle Probe (MAPP) on NSTX-U Felipe Bedoya, Jean Paul Allain, Robert Kaita, Mathew Lucia, Filippo Scotti, Charles Skinner Understanding the plasma-wall interaction remains a critical issue for magnetic fusion. The mechanisms controlling NSTX-U plasma performance by lithium conditioning of graphite are not completely understood. MAPP is the first in-vacuo surface-sensitive compositional/chemical analysis diagnostic to elucidate plasma-material interactions at a tokamak plasma edge. MAPP enables inter plasma-shot in-vacuo diagnosis of PFCs positioned in the outboard divertor far SOL of NSTX-U. The diagnostic provides surface composition during the time scale of plasma-induced modification in the critical region of incident hydrogen implantation. MAPP can expose four samples to plasma discharges and retract them to its chamber for analysis. MAPP's capabilities include XPS, TDS, LEISS and DRS. This work summarizes the commissioning of MAPP for the NSTX-U FY15 experimental campaign. NSTX-U will use boronization and lithiumization sequentially to prepare its graphite PFCs. MAPP will carry four samples; two ATJ graphite, one TZM and one gold sample that will be exposed to the same conditioning as the walls of the tokamak. XPS and TDS data collected during the procedures will document the progressive modification of the walls in NSTX-U. [Preview Abstract] |
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GP12.00098: Development of a prototype infrared imaging bolometer for NSTX-U G.G. van Eden, L.F. Delgado-Aparicio, T.K. Gray, M.A. Jaworski, T.W. Morgan, B.J. Peterson, M.L. Reinke, R. Sano, K. Mukai Measurements of the radiated power in fusion reactors are of high importance for studying detachment and the overall power balance. A prototype Infrared Video Bolometer (IRVB) is being developed for NSTX-U complementing resistive bolometer and AXUV diode diagnostics. The IRVB has proven to be a powerful tool on LHD and JT-60U for its 2D imaging quality and reactor environment compatibility. For NSTX-U, a poloidal view of the lower center stack and lower divertor are envisaged for the 2016 run campaign. The IRVB concept images radiation from the plasma onto a 2.5 $\mu $m thick 9 x 7 cm$^{\mathrm{2\thinspace }}$calibrated Pt foil and monitors its temperature evolution using an IR camera (SB focal plane, 2-12 $\mu $m, 128x128 pixels, 1.6 kHz). The power incident on the foil is calculated by solving the 2D$+$time heat diffusion equation. Benchtop characterization is presented, demonstrating a sensitivity of approximately 20 mK and a noise equivalent power density of 71.5 $\mu $W cm$^{\mathrm{-2}}$ for 4x20 bolometer super-pixels and a 50 Hz time response. The hardware design, optimization of camera and detector settings as well as first results of both synthetic and experimental origin are discussed. [Preview Abstract] |
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GP12.00099: Global mixed-material migration modeling of NSTX-U and a parameterized Li-C-O surface model J.H. Nichols, M.A. Jaworski, R. Kaita, K. Schmid NSTX-U will initially operate with graphite walls, periodically coated with thin lithium films to improve plasma performance. Prior experiments with Li evaporation on NSTX suggest that poloidally inhomogenous mixed-material C/Li/O surfaces will evolve over the course of the campaign due to wall material migration during plasma operation. Understanding the depletion and accumulation of Li in different parts of the machine is a key component of optimizing the Li conditioning process. To that end, the WallDYN global mixed-material surface evolution model [K. Schmid et al., J. Nucl. Mater. 415, S284-S288 (2011)] has been applied to the NSTX-U geometry. The WallDYN model couples local erosion and deposition processes with plasma impurity transport in a non-iterative, self-consistent manner that maintains overall material balance. For this work, a C/Li/O mixed-material erosion model has been generated by parameterizing dynamic sputter and reflection yield calculations from SDTrimSP. The sensitivity of global lithium migration rates to various surface model parameters will be examined. [Preview Abstract] |
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GP12.00100: Probing spherical tokamak plasmas using charged fusion products Werner U. Boeglin, Ramona V. Perez, Douglass S. Darrow, Marco Cecconello, Iwona Klimek, Scott Y. Allan, Rob J. Akers, Owen M. Jones, David L. Keeling, Ken G. McClements, Rory Scannell The detection of charged fusion products, such as protons and tritons resulting from D(d,p)t reactions, can be used to determine the fusion reaction rate profile in large spherical tokamak plasmas with neutral beam heating. The time resolution of a diagnostic of this type makes it possible to study the slowly-varying beam density profile, as well as rapid changes resulting from MHD instabilities. A 4-channel prototype proton detector (PD) was installed and operated on the MAST spherical tokamak in August/September 2013, and a new 6-channel system for the NSTX-U spherical tokamak is under construction. PD and neutron camera measurements obtained on MAST will be compared with TRANSP calculations, and the design of the new NSTX-U system will be presented, together with the first results from this diagnostic, if available. [Preview Abstract] |
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GP12.00101: Parametric studies of next-step spherical tokamaks using high-temperature superconductors Jonathan Menard, Tom Brown, Laila El-Guebaly, Yuhu Zhai High-temperature superconducting (HTS) magnets are potentially attractive for compact spherical tokamak (ST) applications due to higher operating temperature which could reduce thermal shielding requirements and reduce device size relative to configurations that utilize low-temperature superconductors (LTS). HTS conductors can also operate with very high current densities and high magnetic fields. Recent studies have shown that for elongation and kink stability dependencies on aspect ratio consistent with NSTX data, accounting for the engineering limits of HTS magnets, and having only a modest central solenoid, the optimal aspect ratio for an HTS tokamak pilot plant is between A $=$ 1.7 and 2.3 depending on inboard shielding thickness. These results point to the interesting finding that the optimal aspect ratio for a compact HTS pilot plant may be near A $=$ 2 which is an unexplored configuration in the present fusion program. The potential implications for required energy confinement, plasma formation and sustainment, power exhaust, and other performance parameters will be discussed. [Preview Abstract] |
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GP12.00102: Towards fully non-inductive operation in NSTX-U Francesca Poli, Robert Andre, Nicola Bertelli, Gary Taylor The start-up, ramp-up, and sustainment of a tokamak plasma utilizing little to no induction from a central solenoid is a major challenge in magnetic fusion. Because of the scientific and operational challenges, the problem is best solved by employing an iterative loop between experiment and simulation where validation plays a critical role in the improvement of modeling and in the projection to new experiments. On NSTX-U, high harmonic fast waves (HHFW) and NBI are combined to ramp the plasma current non-inductively. Current profile control is critical in order to attain the desired target and avoid the peaking of profiles at start-up and ramp-up that are conducive to ideal MHD instabilities. An interesting synergy is observed in the simulations between the HHFW and Electron Cyclotron (EC) waves. The addition of EC wave heating can in fact significantly increase the effectiveness of the RF power and relax the requirements on the total level of power that must be coupled to the start-up plasma. The feasibility of EC/EBW startup is discussed. This work describes and revisits these simulations in light of the first experimental campaign on NSTX-U, with emphasis on the limitations in our modeling capabilities. [Preview Abstract] |
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GP12.00103: Impurity Monitoring on NSTX-U with Three New Extreme Ultraviolet Spectrometers M.E. Weller, P. Beiersdorfer, V.A. Soukhanovskii, E.W. Magee, T.D. Rognlien, B.C. Stratton The National Spherical Torus Experiment -- Upgrade (NSTX-U) is a significant advancement from NSTX offering increased toroidal field, plasma current, and neutral beam injector input power. Due to these improvements generating more intense plasma conditions, impurities penetrating into the core could also be enhanced, despite efforts to improve wall conditioning (bake out, boronization, and lithium evaporation). To monitor and study these impurities, three extreme ultraviolet (EUV) spectrometers have been implemented on NSTX-U. All three are flat field grazing incident spectrometers capable of capturing time-resolved data to about 5.0 ms. Two of the spectrometers, the X-ray and Extreme Ultraviolet Spectrometer (XEUS, 5 -- 65 {\AA}) the Long-Wavelength and Extreme Ultraviolet Spectrometer (LoWEUS, 190 -- 440 {\AA}) were previously implemented on NSTX. The third has been dubbed the Metal Monitor and Lithium Spectrometer Assembly (MonaLisa, 50 -- 220 {\AA}). A new laser blow-off (LBO) system has also been developed in conjuncture with the spectrometers to introduce low and high-Z elements to study core impurity transport. The three spectrometers, along with the new LBO system, provide a unique opportunity to attain highly resolved spectra of impurities from 5 -- 440 {\AA} with time-resolution. [Preview Abstract] |
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GP12.00104: Assessment of NSTX-U pedestal control and disruption avoidance Fil Alexandre We report on the pedestal control and disruption avoidance strategies in NSTX-U. Edge localized modes (ELMs) represent a challenge to future fusion devices, due to the high heat fluxes on plasma facing surfaces. One aim of NSTX-U is to characterize the H-Mode pedestal structure at increased BT, Ip and NBI heating power and compare it to NSTX. We will assess the pedestal stability in both standard and snowflake configurations and identify the underlying mechanisms controlling the pedestal structure using the high spatial resolution edge diagnostics (e.g., BES, ME-SXR, bolometer). The new capabilities of NSTX-U will be used to effectively control the pedestal for optimum performance (e.g. LGI, molecular cluster injector). Control tools already deployed on EAST and DIII-D will be used. Disruptions also represent a major challenge for ITER and future devices due to the high heat fluxes on PFCs, electromagnetic forces on the structure and the generation of runaway electrons during the current quench. We will report on active resistive wall mode and plasma rotation control for disruption avoidance in NSTX-U. This work is supported by the US DOE under DE-AC02-09CH11466 [Preview Abstract] |
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GP12.00105: Initial Physics Operation of the National Spherical Torus Experiment- Upgrade Dennis Mueller The National Spherical Torus Experiment Upgrade (NSTX-U) is an experiment designed to study the physics of Spherical Torus (ST) at about twice the toroidal field and neutral beam injection (NBI) power as NSTX for 5 s. at full parameters. In its initial operational period NSTX-U will limit operation to B$_{\mathrm{T}} \le $ .75 T but the full complement of 6 neutral beam (NB) sources will be available. Three NB sources added during the upgrade inject more tangentially and will be essential to investigate the physics of neutral beam current drive. In NSTX-U, use of a digital real-time plasma control system and the application of wall conditioning techniques will be used to achieve routine operation with good confinement. The wall conditioning techniques include bakeout to over 300$^{\circ}$C, helium glow discharge cleaning, boronization of the plasma facing surfaces using deuterated trimethylboron gas in a helium glow discharge and lithium evaporation onto the walls. Auxiliary heating by up to 6 MW of High Harmonic Fast Waves will be available. The operational experience during the plasma commissioning phase will be discussed. [Preview Abstract] |
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GP12.00106: Simulations of kink-like modes on NSTX Mario Podesta, Ge Dong Kink-like instabilities which are commonly observed in NSTX plasmas can lead to deterioration of plasma confinement. Understanding their stability properties and dynamics is therefore important to improve the machine performance. In this study, we performed simulations of kink-like instabilities in NSTX equilibrium for a variety of scenarios using the gyrokinetic toroidal code (GTC). In fluid limit with experimental plasma density and temperature profiles, a scan of the q profile shows that the non- resonant kink mode (NRK) can be stabilized if q$_{min}$ is above 1.5. In simulations with kinetic thermal ions, the growth rate of NRK are slightly reduced, and the mode structure can be affected. In simulations with kinetic fast ions, the fishbone mode with finite real frequency and rotating mode structure can be destabilized. [Preview Abstract] |
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GP12.00107: The Lithium Tokamak eXperiment - Upgrade (LTX-U) R. Majeski, R. Bell, D. Boyle, A. Diallo, R. Kaita, T. Kozub, B. LeBlanc, M. Lucia, E. Merino, J.C. Schmitt, T.M. Biewer, T.K. Gray, S. Kubota, W.A. Peebles, C. Hansen, T. Jarboe, J. Bialek, B. Koel, P. Beiersdorfer, K. Widman, K. Tritz Results from the LTX program during the last 18 months have significantly advanced the concept of the liquid lithium-walled tokamak. These results include energy confinement times in an ohmic, wall-limited tokamak which exceed ITER ELMy H-mode scaling by a factor of 2-4, the development of very flat electron temperature profiles, and measurements of lithium concentrations in the core which are less than 0.5{\%}, with a full liquid lithium wall. Although considerable investigation of ohmically heated discharges remains, the results strongly support an extension to regimes with strong auxiliary heating, in order to better determine whether liquid lithium walls should be deployed in a large confinement device. A widened operational window, in both toroidal field and plasma current, is also advisable, as well as eventual operation in diverted geometry. An upgrade of LTX, imaginatively named LTX-U, has been proposed. The upgraded device will be described. The results which form the basis for this next step will be briefly summarized. [Preview Abstract] |
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GP12.00108: Low impurity concentrations and enhanced confinement in the Lithium Tokamak Experiment (LTX) D.P. Boyle, R.E. Bell, R. Kaita, R. Majeski, J.C. Schmitt, F. Scotti, T.M. Biewer, T.K. Gray Significant improvements in confinement and overall performance have been shown in many devices using lithium wall-coatings, though impurities have often been an issue. Previous results with partial coatings in LTX, a modest-sized, ohmically-heated spherical tokamak, demonstrated energy confinement times exceeding ITER ELMy H-mode scalings. Here we report the results of new experiments with fully lithium coated walls, including first-ever successful operation of a tokamak plasma with a full liquid lithium wall. Energy confinement estimates based on magnetic analysis exceed the ITER98P scaling by 2-4x, and can now be confirmed with electron temperature and density profiles from Thomson scattering. Past attempts at a full liquid Li coating in LTX were unsuccessful, with difficulty achieving breakdown and short, cold, impurity dominated plasmas. Now, spectroscopic measurements in discharges with full liquid coatings indicate low core core impurity concentrations of Li, C, and O. The implications for impurity transport will be discussed. The results for confinement and impurity behavior with solid and liquid lithium on stainless steel surfaces in LTX are relevant to future devices and upgrades with all-metal walls, including NSTX-U. [Preview Abstract] |
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GP12.00109: Linking PFC surface characteristics and plasma performance in the Lithium Tokamak Experiment M. Lucia, R. Kaita, R. Majeski, D.P. Boyle, M.A. Jaworski, J.C. Schmitt, F. Bedoya, J.P. Allain The Lithium Tokamak Experiment (LTX) is a spherical torus magnetic confinement device designed to accommodate lithium as the primary plasma-facing component (PFC). Results are presented from the implementation on LTX of the Materials Analysis and Particle Probe (MAPP), a compact \textit{in vacuo} surface science diagnostic. With MAPP, \textit{in situ} surface analysis techniques of x-ray photoelectron spectroscopy and thermal desorption spectroscopy are used to study evolution of the PFC surface chemistry in LTX as a function of varied lithium coating, hydrogen plasma exposure, and PFC surface temperature ($20-300\,^{\circ}C$). Surface analysis results are then correlated with various measures of LTX plasma performance, including toroidal plasma current, line-integrated plasma density, and density-normalized impurity emission. Lithium coatings are observed to convert within hours to $Li_2O$ by gettering oxygen from both the residual vacuum and the PFC substrate. However, plasma performance remains elevated even with discharges operating against $Li_2O$-coated PFCs. Hydrogen is retained by these $Li_2O$ coatings during a discharge, but it is almost completely desorbed as outgassed $H_2$ in the minutes following the discharge; no persistent $LiH$ formation is observed. [Preview Abstract] |
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GP12.00110: A Neutral Beam for the Lithium Tokamak eXperiment Upgrade (LTX-U) Enrique Merino, Richard Majeski, Robert Kaita, Thomas Kozub, Dennis Boyle, John Schmitt, Artem Smirnov Neutral beam injection into tokamaks is a proven method of plasma heating and fueling. In LTX, high confinement discharges have been achieved with low-recycling lithium walls. To further improve plasma performance, a neutral beam (NB) will be installed as part of an upgrade to LTX (LTX-U). The NB will provide core plasma fueling with up to 700 kW of injected power. Requirements for accommodating the NB include the addition of injection and beam-dump ports onto the vessel and enhancement of the vacuum vessel pumping capability. Because the NB can also serve as a source of neutrals for charge-exchange recombination spectroscopy, ``active'' spectroscopic diagnostics will also be developed. An overview of these plans and other improvements for upgrading LTX to LTX-U will be presented. [Preview Abstract] |
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GP12.00111: Equilibrium Reconstructions and Eddy Currents in LTX J.C. Schmitt, J. Bialek, C.H. Hansen, R. Majeski, J.E. Menard The Lithium Tokamak eXperiment (LTX) is a spherical tokamak with a close-fitting low-recycling wall of lithium deposited on a stainless steel-lined copper shell. The combination of low resistivity of the copper shell, toroidal and poloidal breaks in the shell and transient coil and plasma currents results in long-lived non-axisymmetric eddy currents in the shell which produce a non-axisymmetric magnetic field. Magnetic sensors measure a ``local'' magnetic field in the toroidal break region that differs from the toroidally-averaged field. To use these signals as constraints in 2-D axisymmetric equilibrium reconstructions requires compensation of the 3-D components present in the signals. The work will will discuss the results of the 3-D modeling of the eddy currents and magnetic fields with the VALEN code, along with the progress made with equilibrium reconstructions with PSI-TRI and LRDfit. Work supported by US DOE contract DE-AC02-09CH11466. [Preview Abstract] |
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GP12.00112: Electron Density Measurements on LTX Using Microwave and Millimeter-Wave Diagnostics S. Kubota, X.V. Nguyen, W.A. Peebles, D.P. Boyle, R. Kaita, T. Kozub, R. Majeski, E. Merino, J.C. Schmitt The dynamic evolution of the electron density profile is tracked using microwave and millimeter-wave diagnostics on LTX. The 296 GHz ($\lambda$$=$1 mm) interferometer provides a radial line density measurement at the midplane, while an FMCW (frequency-modulated continuous-wave) reflectometer (13.5$-$33 GHz, or O-mode $0.2$$-$$1.3$$\times$$10^{13}$ cm$^{-3}$) provides density profile measurements for the low-field side. Data taken during FY2015 will be compared with measurements from Thomson scattering and estimates of the plasma position from LRDFIT. Measurements of density fluctuations due to low-frequency ($<$100 kHz) MHD instabilities will also be shown. Future plans include the installation of a correlation reflectomter (Ka-band, 27-40 GHz) with dual tuneable sources and a frequency bandwidth of up to 5 MHz. This system will utilize the same antennas as the profile reflectometer to provide radial and/or toroidal/poloidal correlations. Further diagnostic details will be presented at the meeting. [Preview Abstract] |
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GP12.00113: Non-solenoidal Startup via Local Helicity Injection on Pegasus: Progress and Plans J.A. Reusch, J.L. Barr, G.M. Bodner, M.W. Bongard, M.G. Burke, R.J. Fonck, E.T. Hinson, B.T. Lewicki, J.M. Perry, D.J. Schlossberg Non-solenoidal plasma startup via local helicity injection (LHI) at the Pegasus toroidal experiment now provides routine operation at I$_{\mathrm{p}}\approx $ 0.17MA with I$_{\mathrm{inj}}\approx $ 5kA and V$_{\mathrm{inj}}\approx $ 1kV from four active arc injectors. Experiments in the past year have advanced the understanding of the governing physics of LHI and its supporting technology. Injector impedance scales as V$_{\mathrm{inj}}^{\mathrm{3/2}}$ and is governed by two effects: a quasineutrality constraint on electron beam propagation, related to the tokamak edge density, and double-layer sheath expansion, related to n$_{\mathrm{arc}}$. Injector design improvements permit operation at V$_{\mathrm{inj}} \quad \ge $ 1 kV without deleterious PMI or impurity generation. Discharges with varied shape, I$_{\mathrm{p}}$(t), and helicity input test a predictive 0D power-balance model for LHI startup. Anomalous, reconnection-driven T$_{\mathrm{i}}$ \textgreater 800 eV and strong MHD activity localized near the injectors are observed during LHI. Preliminary core Thomson scattering measurements indicate surprisingly high T$_{\mathrm{e}}$ \textgreater 300 eV, which if verified may indicate the dominance of high-energy electron fueling from the injector current streams. A new divertor injector system has been designed to substantially increase the available helicity input rate and support critical studies of confinement during LHI and reconnection activity at high I$_{\mathrm{p}}$. A proposed upgrade to the Pegasus experiment will extend these studies to NSTX-U relevant parameters. [Preview Abstract] |
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GP12.00114: The Pegasus-Upgrade Experiment R.J. Fonck, M.W. Bongard, J.L. Barr, H.G. Frerichs, B.T. Lewicki, J.A. Reusch, O. Schmitz, G.R. Winz Tokamak operation at near-unity aspect ratio provides access to advanced tokamak physics at modest parameters. High plasma current is accessible at very low toroidal field. This offers H-mode performance at T$_{\mathrm{e}}$ levels that allow use of electrostatic and magnetic probe arrays through the edge pedestal region into the plasma core. An upgrade to the Pegasus ST is planned to exploit these features and pursue unique studies in three areas: local measurements of pedestal and ELM dynamics at Alfv\textbf{\'{e}}nic timescales; direct measurement of the local plasma response to application of 3D magnetic perturbations with high spectral flexibility; and extension of Local Helicity Injection for nonsolenoidal startup to NSTX-U-relevant confinement and stability regimes. Significant but relatively low-cost upgrades to the facility are proposed: a new centerstack with larger solenoid and 2x the number of toroidal field conductors; a new TF power supply and conversion of the 200 MVA OH power supply to a cascaded multilevel inverter configuration; and installation of an extensive 3D-magnetic perturbation coil system for ELM mitigation and suppression studies. The upgraded facility will provide 0.3 MA plasmas with pulse lengths of 50-100 msec flattop, aspect ratio \textless 1.25, and toroidal field up to 0.4 T. These research activities will be integrated into related efforts on DIII-D and NSTX-U. [Preview Abstract] |
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GP12.00115: Effect of Aspect Ratio on H-mode and ELM Characteristics K.E. Thome, G.M. Bodner, M.W. Bongard, M.G. Burke, R.J. Fonck, D.M. Kriete The H-mode confinement regime is achieved at near-unity aspect ratio ($A<1.2)$ in the Pegasus Toroidal Experiment via high-field-side fueling and low edge recycling. Ohmic H-mode is attained in both limited and diverted magnetic topologies. This regime is characterized by: reduced D$_{\alpha}$ emissions; increased core rotation; increased central heating; formation of edge current and pressure pedestals; and measured energy confinement consistent with the ITER98pb(y,2) scaling. The H-mode power threshold, $P_{LH} $, behaves quite differently at low-$A$ when compared with high-$A$ operations. $P_{LH} /P_{LH\_ITPA08}$ increases sharply as $A$ is lowered and no difference in $P_{LH} $ for limited and diverted plasmas is observed at $A\sim 1.2$. No minimum in $P_{LH} $ with density is observed. Some of these results are consistent with the FM$^{3}$ model for the L-H transition.\footnote{Fundamenski \textit{et al}., Nucl. Fusion \textbf{52}, 062003 (2012.)} Two classes of ELMs have been observed. Small, Type III-like ELMs are present at low input power and have $n\le 4$. At $P_{OH} >>P_{LH}$, they transition to large, Type-I-like ELMs with intermediate $5 < n < 15$. The Type III ELM magnetic structures behave opposite that of high-$A$ plasmas, with $n$ much higher, presumably due to the naturally higher $J/B$ peeling mode drive at low-$A$. Long-sought measurements of the $J_{edge} (R,t)$ pedestal collapse during an ELM event show a complex, multimodal pedestal collapse and the subsequent ejection of a current-carrying filament. [Preview Abstract] |
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GP12.00116: Power Balance Modeling and Validation for ST Startup Using Local Helicity Injection J.L. Barr, G.M. Bodner, M.W. Bongard, M.G. Burke, R.J. Fonck, E.T. Hinson, J.M. Perry, J.A. Reusch, D.J. Schlossberg Local Helicity Injection (LHI) uses localized current injectors for routine $I_{p} $\textless 0.18 MA non-solenoidal startup on the Pegasus ST. A power-balance model is under development for predictive $I_{p} \left( t \right)$ using helicity-balance to quantify LHI's effective current drive, $V_{eff} $. Analytic formulas for low-A plasma inductance and vertical field are used to account for the inductive effects of dynamic shape evolution. These formulas are being validated against magnetic reconstructions of LHI discharges with varied shape evolutions. Initial results match experimental $I_{p} \left( t \right)$ within 20 kA with assumed shaping and average resistivity (Spitzer, $T_{e} =$60 eV). Geometric effects and inductive drive provide 2.0 V along with $V_{eff} =$0.3 V to balance 1.1 V of resistive losses and 1.2 V inductive reactance to ramping $I_{p} $. The model is especially sensitive to resistivity when $T_{e} $\textless 150 eV. Initial Thomson Scattering results give core $T_{e} =$72$\pm $22 eV, and at times suggest higher central electron energies. Spatial and temporal scans are underway to quantify LHI plasma resistivity and transport. MA-class startup in NSTX-U will require increased area ($A_{inj} \ge $40 cm$^{2})$ LHI systems that play a larger role in current drive than geometric effects, with $V_{eff} $ dropping from \textgreater 10 V to on-par with inductive effects. This regime is accessed in Pegasus at $I_{p} \cong $300 kA. [Preview Abstract] |
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GP12.00117: Physics of Plasma Cathode Current Injection During LHI E.T. Hinson, J. Barr, M. Bongard, M.G. Burke, R. Fonck, J. Perry Localized helicity injection (LHI) ST startup employs current sources at the tokamak edge. Max $I_{\mathrm{p}}$ in LHI scales with injection voltage $V_{\mathrm{inj}}$, requiring an understanding of injector impedance. For the arc-plasma cathode electron injectors in Pegasus, impedance is plasma-determined, and typically $V_{\mathrm{inj}}$\textgreater 1kV for $I_{\mathrm{inj}}=$2kA. At low $I_{\mathrm{inj}}$, $I_{\mathrm{inj}}\propto V_{\mathrm{inj}}^{\mathrm{3/2}}$, an indication of a double layer (DL) common to such devices. However, at $I_{\mathrm{inj}}$\textgreater $\sim $1kA, $I_{\mathrm{inj}}\propto V_{\mathrm{inj}}^{\mathrm{1/2}}$ occurs, a scaling expected for limited launched beam density, $n_{b} \equiv {I_{inj} } \mathord{\left/ {\vphantom {{I_{inj} } {(e\sqrt {{2eV_{inj} } \mathord{\left/ {\vphantom {{2eV_{inj} } {m_{e} }}} \right. \kern-\nulldelimiterspace} {m_{e} }} A_{inj} )}}} \right. \kern-\nulldelimiterspace} {(e\sqrt {{2eV_{inj} } \mathord{\left/ {\vphantom {{2eV_{inj} } {m_{e} }}} \right. \kern-\nulldelimiterspace} {m_{e} }} A_{inj} )}\sim I_{\mathrm{inj}}$/$V_{\mathrm{inj}}^{\mathrm{1/2}}$. An ohmic discharge injection target was created to test this hypothesis. Langmuir probe data showed $I_{\mathrm{inj}}$/$V_{\mathrm{inj}}^{\mathrm{1/2}}\propto n_{\mathrm{edge}}$ at low $n_{\mathrm{edge}}$, consistent with a limit ($n_{\mathrm{edge}}\ge n_{\mathrm{e,b}})$ imposed by quasineutrality. If edge fueling maintained $n_{\mathrm{edge}}\ge n_{\mathrm{e,b}}$, spectroscopic measurements of source density $n_{\mathrm{arc}}$ indicated $I_{\mathrm{inj}}$/$V_{\mathrm{inj}}^{\mathrm{1/2}}\propto n_{\mathrm{arc}}$, as expected from DL expansion. Thus $n_{\mathrm{b}}$ established by $n_{\mathrm{arc}}$ or $n_{\mathrm{edge}}$ determines $V_{\mathrm{inj}}$ up to the onset of cathode spot (CS) arcing. Technology development has increased obtainable V$_{\mathrm{inj}}$ and reduced CS damage using new ring shielding and a cathode design drawing CS's away from insulators. This involved a novel optimization of conical frustum geometry. Finally, consistent with NIMROD predictions of coherent streams in the edge during LHI, pairwise triangulation of outboard Mirnov data assuming beam m$=$1 motion has allowed an estimate of beam R(t), Z(t) location that is near the injector R, and consistent across the array. [Preview Abstract] |
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GP12.00118: New Electron Temperature Measurements During Local Helicity Injection and H-mode Plasmas at the Pegasus Toroidal Experiment D.J. Schlossberg, G.M. Bodner, R.J. Fonck, J.A. Reusch, G.R. Winz Extrapolation of non-solenoidal startup via local helicity injection (LHI) to larger devices depends critically on confinement during the injection process. To begin quantifying confinement regimes, the Thomson scattering diagnostic on the Pegasus ST was upgraded to include 12 radial positions and high temperature (0.1 \textless T$_{\mathrm{e}}$ \textless 1 keV) capability. Previous measurements during high-density, quiescent, Ohmic L-mode discharges yielded well-resolved thermal electron distributions, with central T$_{\mathrm{e}} \quad =$ 150 eV. In the low-density LHI startup plasmas shot-to-shot averaging of data improves background measurements and increases signal-to-noise ratio. Initial core measurements during the drive phase of LHI suggest average T$_{\mathrm{e}}$ of several hundred eV for plasmas with n$_{\mathrm{e}} \quad \approx $ 3x10$^{\mathrm{18}}$ m$^{\mathrm{-3}}$ and I$_{\mathrm{p\thinspace }}\approx $ 0.15 MA. Experiments are underway to verify these unexpectedly high electron temperatures. If verified, these temperatures may reflect the dominance of high-energy electrons via fueling with LHI current streams with average energy $\approx $1 keV. Further investigations will explore the dependence of the inferred electron distribution on fueling source, density, and electron injection potential. The upgraded Thomson scattering diagnostic will also be applied to Ohmic H-mode plasmas in Pegasus. [Preview Abstract] |
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GP12.00119: Spatial Expansion and Automation of the Pegasus Thomson Scattering Diagnostic System G.M. Bodner, M.W. Bongard, R.J. Fonck, J.A. Reusch, D.J. Schlossberg, G.R. Winz The Pegasus Thomson scattering diagnostic system has recently undergone modifications to increase the spatial range of the diagnostic and automate the Thomson data collection process. Two multichannel spectrometers have been added to the original configuration, providing a total of 24 data channels to view the plasma volume. The new system configuration allows for observation of three distinct regions of the plasma: the local helicity injection (LHI) source (R $\sim $ 67-73.8 cm), the plasma edge (R $\sim $ 51.5-57.6 cm), and the plasma core (R $\sim $ 35-41.1 cm). Each spectrometer utilizes a volume-phase holographic (VPH) grating and a gated-intensified CCD camera. The edge and the LHI spectrometers have been fitted with low-temperature VPH gratings to cover $T_{e}=10-100\thinspace \thinspace eV$, while the core spectrometer has been fitted with a high-temperature VPH grating to cover $T_{e}=0.1-1.0\thinspace \thinspace keV$. The additional spectrometers have been calibrated to account for detector flatness, detector linearity, and vignetting. Operation of the Thomson system has been overhauled to utilize LabVIEW software to synchronize the major components of the Thomson system with the Pegasus shot cycle and to provide intra-shot beam alignment. Multi-point Thomson scattering measurements will be obtained in the aforementioned regions of LHI and Ohmic discharges and will be compared to Langmuir probe measurements. [Preview Abstract] |
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GP12.00120: H-mode Edge Turbulence and Pedestal Measurements in Pegasus Plasmas using Langmuir Probes D.M. Kriete, G.M. Bodner, M.W. Bongard, R.J. Fonck, K.E. Thome, D.S. Thompson In Pegasus discharges, L-H mode transitions are induced using Ohmic heating and high-field-side fueling. H-mode plasmas have energy confinement consistent with the ITER98pb(y,2) scaling law, indications of increased electron and ion temperature, and an increase in core rotation compared to L-mode plasmas. Electron density and temperature profiles have been measured in the edge region using a scannable triple Langmuir probe on a shot-by-shot basis. In H-mode, a pressure pedestal that has a hyperbolic tangent shape and a $\sim $ 2 cm $\nabla p_{e} $ scale length is observed, in contrast to a linear shape in L-mode. Autopower spectra of the collected ion saturation current in H-mode discharges show a factor of $\sim $ 3 reduction in fluctuations in the 50--200 kHz band with respect to L-mode. Two Langmuir probes with 8 cm poloidal separation have been installed on Pegasus. The turbulence correlation length in the edge will be measured by radially scanning the probes. Knowledge of the correlation length will be used to inform the design of a future 8-channel radial multiprobe array. This system will simultaneously measure the dynamic $n_{e} (R,t)$, $T_{e} (R,t)$, and $\Phi (R,t)$ profiles and fluctuations across the L-H mode transition and be used to investigate nonlinear ELM dynamics. [Preview Abstract] |
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GP12.00121: Progress Toward a New Technique for Measuring Local Electric Field Fluctuations in High Temperature Plasmas M.R. Bakken, M.G. Burke, R.J. Fonck, B.T. Lewicki, M.M. Liben, D.S. Thompson, G.R. Winz A new diagnostic measuring local E$_{\mathrm{z}}$(r,t) fluctuations is being developed at the Pegasus Toroidal Experiment. A novel multiple volume phase holographic grating spectrometer, designed to have high resolution (0.25{\AA}) and high \'{e}tendue (U $=$ 0.01cm$^{2}$-ster), measures the line separation of the $\pi $ components of the H$_{\mathrm{\alpha}}$ motional Stark spectrum of emitted beam light. The spectra are recorded at high frequency ($f_{\mathrm{Ny}} \approx $ 500kHz) by a high speed CMOS imaging detector. The groove density of the objective grating is varied linearly along its surface to counter geometric Doppler broadening. A low divergence ($\Omega \approx $ 0.5$^{\mathrm{o}})$, 80kV, 2.5A H$^{0}$ diagnostic neutral beam is being deployed on Pegasus. The beam uses a washer-stack arc ion source to maximize full energy species fraction in the injected neutral beam. Laboratory tests of the ion source demonstrate stable, repeatable plasmas with T$_{\mathrm{e}} \le $ 20eV and n$_{\mathrm{e}} \approx $ 5x10$^{17}$m$^{-3}$, sufficient to sustain a 6mA/cm$^{2}$ current density at the focal plane for up to 20ms. A three phase resonant converter power supply, with low amplitude ($\delta $V/80kV $\approx $ 0.05{\%}), high frequency ($f_{\mathrm{rip}}\approx $ 280kHz) ripple, is in development to provide the 80kV accelerator power. [Preview Abstract] |
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GP12.00122: Ion Heating During Local Helicity Injection Plasma Startup in the Pegasus ST M.G. Burke, J.L. Barr, M.W. Bongard, R.J. Fonck, E.T. Hinson, J.M. Perry, J.A. Reusch Plasmas in the Pegasus ST are initiated either through standard, MHD stable, inductive current drive or non-solenoidal local helicity injection (LHI) current drive with strong reconnection activity, providing a rich environment to study ion dynamics. During LHI discharges, a large amount of impurity ion heating has been observed, with the passively measured impurity T$_{i}$ as high as 800 eV compared to T$_{i}\approx $60 eV and T$_{e}\approx $175 eV during standard inductive current drive discharges. In addition, non-thermal ion velocity distributions are observed and appear to be strongest near the helicity injectors. The ion heating is hypothesized to be a result of large-scale magnetic reconnection activity, as the amount of heating scales with increasing fluctuation amplitude of the dominant, edge localized, n$=$1 MHD mode. An approximate temporal scaling of the heating with the amplitude of higher frequency magnetic fluctuations has also been observed, with large amounts of power spectral density present at several impurity ion cyclotron frequencies. Recent experiments have focused on investigating the impurity ion heating scaling with the ion charge to mass ratio as well as the reconnecting field strength. The ion charge to mass ratio was modified by observing different impurity charge states in similar LHI plasmas while the reconnecting field strength was modified by changing the amount of injected edge current. [Preview Abstract] |
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GP12.00123: Two-fluid equilibrium transition during multi-pulsing CHI in spherical torus T. Kanki, M. Nagata Two-fluid dynamo current drive has been studied to achieve a quasi-steady sustainment and good confinement of spherical torus (ST) plasmas by multi-pulsing CHI (M-CHI) in the HIST device. The density gradient, poloidal flow shear, and radial electric shear enhanced by applying the second CHI pulse is observed around the separatrix in the high field side to cause not only the \textbf{\textit{E}}x\textbf{\textit{B}} drift but also the ion diamagnetic drift, leading the two-fluid dynamo. The two-fluid equilibrium transition during the M-CHI in the ST is investigated by modelling the M-CHI in the two-fluid equilibrium calculations. The toroidal magnetic field becomes from a diamagnetic to a paramagnetic profile in the closed flux region due to the increase of the poloidal electron flow velocity in the central open flux column (OFC) region, while the diamagnetic profile is kept in the OFC region. The toroidal ion flow velocity is increased from negative to positive values in the closed flux region due to the increase in the drift velocity and the Hall effect. As the ion diamagnetic drift velocity is changed in the same direction as the\textbf{\textit{ E}}x\textbf{\textit{B}} drift velocity around the separatrix in the high field side through the negative ion pressure gradient there, the poloidal ion flow velocity is increased in the OFC region, enhancing the flow shear. The radial electric field shear around the separatrix is enhanced due to the strong dependence on the magnetic force through the interaction of toroidal ion flow velocity and axial magnetic field. The density is decreased in the closed flux region according to the generalized Bernoulli law and its negative gradient around the separatrix steepens. [Preview Abstract] |
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GP12.00124: Design of a microwave back-scattering diagnostic for lower-hybrid waves on TST-2 N. Tsujii, Y. Takase, A. Ejiri, H. Furui, H. Homma, K. Nakamura, W. Takahashi, T. Takeuchi, H. Togashi, K. Toida, T. Shinya, M. Sonehara, S. Yajima, H. Yamazaki, Y. Yoshida Non-inductive plasma start-up with lower-hybrid waves has been investigated on the TST-2 spherical tokamak at the University of Tokyo. The current drive efficiency drops dramatically above the density that is roughly consistent with the wave accessibility limit. For a more quantitative analysis, wave measurement in the plasma core is important. A new microwave back-scattering diagnostic is being fabricated for this purpose. The design of the back-scattering system and the analysis of the expected signal level are presented. The actual wave measurements will also be presented if available. [Preview Abstract] |
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GP12.00125: Shaping of the plasma column in a small aspect ratio tokamak Julio Herrera, Ismael Arroyo, Esteban Chavez This is a follow-up to the work presented in last year's meeting, on the conceptual design of a small aspect ratio tokamak of variable configuration. The base parameters for this device would be similar to those in the START tokamak. The shaping of the plasma column is known to have important effects in the plasma performance, including the value of $\beta $, bootstrap currents, and intrinsic rotation. The main feature being explored here is the inclusion of independent control coils in the inboard and outboard sides; six in the first case, and up to seven in the latter. By varying the strength in their currents it is possible to achieve a wide variety of shapes: elliptical, conventional D-shape, inverse D-shape, and Bean-shape. As the control coils are activated, the strength of the toroidal magnetic field needs to he weakened, in order to keep reasonable values of the safety factor $q.$The study presented here is made by means of the 3D-MAPTOR code, which produces the Poincar\'{e} maps of the magnetic field lines, given the currents. For this purpose, a seed plasma current must be provided. All studies presented here assume equatorial symmetry, due to limitations in the code. [Preview Abstract] |
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GP12.00126: L-H threshold results in hydrogen plasmas in JET-ILW E. Delabie, C. Maggi, E. Solano, H. Meyer, E. Lerche, D. Keeling The ITPA scaling law for the H-mode power threshold, P(L-H), is strongly weighted to a dataset of carbon wall JET (JET-C) discharges. Identical discharges with the Be/W wall (JET-ILW) in deuterium have shown a 30{\%} reduction of P(L-H) and a minimum as function of density, not observed with the current divertor in JET-C [1]. A strong dependence of P(L-H) on the divertor configuration was found, linked to changes in the divertor recycling pattern [2]. Subsequently, an experiment was conducted in hydrogen to investigate the isotope effect on P(L-H) in JET-ILW. P(L-H) is increased by a factor 2 in the high density branch, as expected. Remarkably, ne,min is shifted to higher density. Comparison between the hydrogen and deuterium discharges show the transition occurs at similar values of stored energy and closely matched edge density and temperature profiles. \\[4pt] [1] C.F. Maggi et al., Nucl. Fus. 54 (2014) 023007\\[0pt] [2] E. Delabie et al., proceeding of the 24th IAEA conference, St. Petersburg, Russia (2014). [Preview Abstract] |
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GP12.00127: Beam ion acceleration by ICRH in JET discharges R.V. Budny, M. Gorelenkova, N. Bertelli The ion Monte-Carlo orbit integrator NUBEAM, used in TRANSP has been enhanced to include an ``RF-kick'' operator to simulate the interaction of RF fields and fast ions. The RF quasi-linear operator (localized in space) uses a second R-Z orbit integrator. We apply this to analysis of recent JET discharges using ICRH with the ITER-like first wall. An example of results for a high performance Hybrid discharge for which standard TRANSP analysis simulated the DD neutron emission rate below measurements, re-analysis using the RF-kick operator results in increased beam parallel and perpendicular energy densities ($\simeq$ 40{\%} and 15{\%} respectively), and increased beam-thermal neutron emission ($\simeq$ 35{\%}), making the total rate closer to the measurement. Checks of the numerics, comparisons with measurements, and ITER implications will be presented. Supported in part by the US DoE contract DE-AC02-09CH11466 and by EUROfusion No 633053. [Preview Abstract] |
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GP12.00128: Overview of the EUROfusion Medium Size Tokamak program Piero Martin, Marc Beurskens, Stefano Coda, Thomas Eich, Hendrik Meyer As a result of the new organization of the European fusion programme, now under the umbrella of the EUROfusion Consortium, the MST (Medium Size Tokamaks) task force is in charge of executing the European science programme in the ASDEX Upgrade, TCV and MAST-U tokamaks. This paper will present an overview of the main results obtained in the 2014 campaign--where only ASDEX upgrade was operating--and the preliminary achievements of the recently started 2015/16 campaign, where also TCV will contribute. The main subjects of the experimental campaigns are (i) the development of scenarios relevant for the ITER Q=10 goal, in an all metal wall device (ii) the understanding of ELM mitigation/suppression with pellets and resonant magnetic perturbations, and in particular the effect of density versus collisionality, (iii) the understanding and optimization of methods for disruption mitigation or avoidance and runaway electrons control and (iv) the exploration of ITER and DEMO relevant scenarios with high normalized separatrix power flux, $P_{sep}/R$, ($P_{sep}$ is the power through the separatrix, $R$ the major radius) and tolerable target heat loads. The overview of the future programs in MST will be given. [Preview Abstract] |
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GP12.00129: Characterization of edge fluctuations on JET during the LH transition studies Gianluca De Masi, Silvia Spagnolo, Hugo Arnichand, Jon Hillesheim, Luis Meneses, Hendrik Meyer, Ephrem Delabie, Costanza Maggi In this contribution we present an experimental characterization of ELM-related edge fluctuations observed during the LH transition experimental campaign on JET. These fluctuations have been detected in both the fast density measurements obtained by the radial correlation reflectometer and the external magnetic measurements: their typical frequency range (40-100 kHz) and their radial position (pedestal top) have been assessed. Moreover, we investigated the relation of the fluctuations amplitude with the relevant pedestal quantities, such as the temperature gradient. A preliminary attempt to reconstruct their toroidal and poloidal structure is also given. Their physical interpretation is finally discussed: they are found to share some features with the pedestal fluctuations observed in different machines such as Alcator C-mod, DIII-D [1] and EAST [2] and interpreted in terms of kinetic-ballooning modes; however, recent observations [3] on MAST of inter-ELM fluctuations, suggest a possible interpretation in terms of microinstabilities. \\[4pt] [1] A. Diallo et. al., Phys. Rev. Lett., 112 (2014) 115001\\[0pt] [2] H.Q. Wang et al., Nucl. Fusion 54 (2014) 043014\\[0pt] [3] J. C. Hillesheim et al, submitted to PPCF [Preview Abstract] |
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GP12.00130: A Heavy Ion Beam Probe for ASDEX Upgrade D.R. Demers, T.P. Crowley, P.J. Fimognari, G. Birkenmeier, A. Herrmann, U. Stroth A feasibility study of a heavy ion beam probe (HIBP) on the ASDEX Upgrade tokamak is being conducted. The diagnostic can target multiple critical issues associated with the H-mode pedestal region, including the mechanisms underlying the L-H transition, the formation of zonal flows, and the onset and evolution of ELMs. The HIBP is uniquely capable of measuring the radial electric field simultaneously with fluctuations of electron density and electric potential. Results of the feasibility study indicate that $\rho _{pol}$~\textgreater ~~0.8 can be probed using a 500 keV Cs$^{+}$ beam. The high electron density and temperature that develop during H-mode in the outer region of the plasma are adequate to induce ionization of the beam and, consequently, produce sufficient signal levels which enable atypical operation as an edge diagnostic. Unique characteristics of the system include a relatively small beam Larmor radius and electrostatic deflection plates within AUG for redirecting secondary beam ions to the detector. Beam ion trajectories, the secondary deflection plate model, and measurement sensitivities for $\phi $, $\tilde{{n}}/n$, and $\tilde{{\phi }}$ will be presented. [Preview Abstract] |
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GP12.00131: High normalized beta plasmas exceeding the ideal stability limit and projected RWM active stabilization performance using newly installed feedback sensors in KSTAR Y.S. Park, S.A. Sabbagh, J.W. Berkery, J.M. Bialek, S.W. Yoon, Y.M. Jeon, J.G. Bak, W.H. Ko, S.H. Hahn, C. Bae, Y.S. Bae, Y.K. In, J. Kim, S.G. Lee, J.G. Kwak, Y.K. Oh, H.K. Park, M.J. Choi, G.S. Yun H-mode plasma operation of KSTAR has been expanded to significantly surpass the ideal MHD no-wall beta limit by achieving normalized beta up to 4.3 while reducing plasma internal inductance to near 0.7 exceeding the computed $n$~$=$~1 ideal no-wall limit by a factor of 1.6. These high normalized beta values have been achieved in discharges having B$_{T}$ in the range 0.9-1.1 T after the plasma reached flattop current of 0.35-0.4 MA, with the highest neutral beam heating power of 4 MW. A significant conclusion of the analysis of these plasmas is that low-$n$ global kink/ballooning or RWMs were not detected, and therefore were not the cause of the plasma termination. Advances from the 2015 run campaign aiming to achieve prolonged pulse duration at maximum normalized beta and to subsequently investigate the MHD stability of these plasmas will be reported. As KSTAR H-mode operation can now routinely surpass the ideal no-wall stability limit, $n$~$=$~1 RWM active control is planned for the device. RWM active feedback using a newly installed set of poloidal magnetic field sensors mounted on the passive stabilizer plates and designed for optimal performance is analyzed using the VALEN-3D code. The advantages of the new sensors over other device sensors for RWM active control are discussed. [Preview Abstract] |
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GP12.00132: Versatile controllability of non-axisymmetric magnetic perturbations in KSTAR experiments Hyunsun Han, Y.M Jeon, Y. In, J. Kim, S.W. Yoon, S.H. Hahn, H.S. Ahn, M.H. Woo, B.H. Park, J.G. Bak A newly upgraded IVCC (In-Vessel Control Coil) system equipped with four broadband power supplies, along with current connection patch panel, will be presented and discussed in terms of its capability on various KSTAR experiments. Until the last run-campaign, there were impressive experimental results on ELM(Edge Localized Mode) control experiments using the 3D magnetic field, but the non-axisymmetric field configuration could not be changed in a shot, let alone the limited number of accessible configurations. Introducing the new power supplies, such restrictions have been greatly reduced. Based on the preliminary commissioning results for 2015 KSTAR run-campaign, this new system has been confirmed to easily cope with various dynamic demands for toroidal and poloidal phases of 3D magnetic field in a shot. This enables us to diagnose the plasma response in more detail and to address the 3-D field impacts on the ELM behaviors better than ever. [Preview Abstract] |
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GP12.00133: Comparative ELM study between the observation by ECEI and linear/nonlinear simulation in the KSTAR plasmas Minwoo Kim, Hyeon K. Park, Gunsu Yun, Jaehyun Lee, Jieun Lee, Woochang Lee, Stephen Jardin, X. Q. Xu The modeling of the Edge-localized-mode (ELM) should be rigorously pursued for reliable and robust ELM control for steady-state long-pulse H-mode operation in ITER as well as DEMO. In the KSTAR discharge {\#}7328, a linear stability of the ELMs is investigated using M3D-C1 and BOUT$++$ codes. This is achieved by linear simulation for the $n=$ 8 mode structure of the ELM observed by the KSTAR electron cyclotron emission imaging (ECEI) systems. In the process of analysis, variations due to the plasma equilibrium profiles and transport coefficients on the ELM growth rate are investigated and simulation results with the two codes are compared. The numerical simulations are extended to nonlinear phase of the ELM dynamics, which includes saturation and crash of the modes. Preliminary results of the nonlinear simulations are compared with the measured images especially from the saturation to the crash. [Preview Abstract] |
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GP12.00134: Progress of long pulse operation with high performance plasma in KSTAR Young Bae Recent KSTAR experiments showed the sustained H-mode operation up to the pulse duration of 46 s at the plasma current of 600 kA. The long-pulse H-mode operation has been supported by long-pulse capable neutral beam injection (NBI) system with high NB current drive efficiency attributed by highly tangential injections of three beam sources. In next phase, aiming to demonstrate the long pulse stationary high performance plasma operation, we are attempting the long pulse inductive operation at the higher performance (MA plasma current, high normalized beta, and low q95) for the final goal of demonstration of ITER-like baseline scenario in KSTAR with progressive improvement of the plasma shape control and higher neutral beam injection power. This paper presents the progress of long pulse operation and the analysis of energy confinement time and non-inductive current drive in KSTAR. [Preview Abstract] |
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GP12.00135: Numerical study of plasma response for ELM crash suppression by mixed non-axisymmetric fields in KSTAR G.Y. Park, J. Kim, B.H. Park, S.W. Yoon, T.E. Evans, N.M. Ferraro Control of the edge localized modes (ELMs) is one of the most critical issues for a successful operation of ITER. Recently, It was revealed that strong n=1 non-resonant magnetic perturbation (NRMP) fields could degrade the ELM suppressed state obtained by n=2 resonant magnetic perturbation (RMP) in KSTAR. This result is completely opposite to the prediction based on the vacuum island overlap physics. Thus, it suggests that plasma response to the RMP with strong fraction of non-resonant components may play an important role in actual ELM suppression process. In this presentation, we report on the numerical results of response of a plasma to applied non-axisymmetric magnetic perturbation field due to the n=2 middle and n=1 top/bottom RMP coils. Simulations for several KSTAR discharges are implemented using a two-fluid code, M3D-C1 [1]. We try to explain the basic experimental characteristics of ELM suppression under mixed RMPs configuration in KSTAR on the basis of the calculated plasma response results. Detailed study of the effect of the perpendicular electron rotation on the penetration of n=2 RMP and its variation with the addition of n=1 NRMP will be presented.\\[4pt] [1] N.M.Ferraro, Phys. Plasmas 19(5), 056105 (2012) [Preview Abstract] |
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GP12.00136: Effects of electrode biasing in STOR-M Tokamak Debjyoti Basu, Masaru Nakajima, Akbar Rohollahi, David McColl, Joseph Adegun, Chijin Xiao, Akira Hirose STOR-M is an iron-core, limiter based tokamak with major and minor radii of 46cm and 12 cm, respectively. Recently, electrode biasing experiments have been carried to study the improved confinement. For this purpose we have developed a DC power supply which can be gated by a high power SCR. The rectangular SS electrode has a height of 10 cm, a width of 2 cm and a thickness of 0.2 cm. The radial position of the electrode throughout the experiments is kept around 4mm inside the limiter in the plasma edge region. After application of positive bias with voltages between $+$90 V to $+$110 V during the plasma discharge current flat top with slightly higher edge-q$_{\mathrm{a}}$ (nearly 5 to 6), noticeable increment of average plasma density and soft x-ray intensity along the central chord have been observed. No distinguishable change in H$\alpha $ emission has been measured. These phenomena may be attributed to improved confinement formed at the inner region but not at the edge. In the upcoming experimental campaign, Ion Doppler spectroscopy will be used to measure possible velocity shear inside the inner plasma region. Edge plasma pressure gradient will also be measured using Langmuir probes. Detailed experimental results will be presented. [Preview Abstract] |
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GP12.00137: Indication of GAM and electrode biasing effect on GAM in STOR-M Tokamak Masaru Nakajima, Debjyoti Basu, Akbar Rohollahi, David McColl, Joseph Adegun, Chijin Xiao, Akira Hirose STOR-M is a small, iron-core, limiter based tokamak with major and minor radii of 46 cm and 12 cm, respectively. Recent experimental studies have been carried out to detect GAM in this machine. Four Langmuir probe sets have been inserted into the plasma. The first three Langmuir probe sets are located in the same toroidal plane, inserted from top, bottom and outboard of the mid-plane. The fourth set is inserted from the outboard of the mid-plane, but toroidally separated from the others by 90$^{\circ}$. Each probe set consists of three Langmuir probe tips for I$_{\mathrm{sat}}$, floating potential and I-V curve measurements. Preliminary experimental results with slightly higher edge-q$_{\mathrm{a}}$ (within 5 to 6) clearly indicate a 180$^{\circ}$ phase difference between the up and down density fluctuation signals near 20 kHz. The floating potential fluctuation signals from the same locations at the same frequency showed no observable phase shift. Preliminary data indicate the presence of conventional GAM in STOR-M. In the near future, magnetic fluctuation properties of GAM oscillations in STOR-M as well as the responses of the GAM properties to electrode biasing will be studied. Detailed experimental results will be presented. [Preview Abstract] |
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GP12.00138: Development of the STPX Spheromak System R.L. Williams, J. Clark, C.A. Weatherford The progress made in starting up the STPX Spheromak system, which is now installed at the Florida A{\&}M University, is reviewed. Experimental, computational and theoretical activities are underway. The control system for firing the magnetized coaxial plasma gun and for collecting data from the diagnostic probes, based on LabView, is being tested and adapted. Preliminary results of testing the installed magnetic field probes, Langmuir triple probes, cylindrical ion probes, and optical diagnostics will be discussed. Progress in modeling this spheromak using simulation codes, such as NIMROD, will be discussed. Progress in investigating the use of algebraic topology to describe this spheromak will be reported. [Preview Abstract] |
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GP12.00139: The STPX Spheromak System: A Topological Study Roselyn Williams, Ronald Williams We present results of studies that have recently started to further understand the topology of the STPX Spheromak system that is installed at the Florida A{\&}M University. We investigate the use of algebraic topology to understand the properties of the magnetic helicity and to describe and analyze the magnetic field topology. We investigate the application of braid theory and knot theory. Furthermore we investigate the possibility of applying homotopy theory as a way to model and describe magnetic helicity as an algebraic system. [Preview Abstract] |
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GP12.00140: Plasma shape and position controller design for advance plasma configurations in TCV Himank Anand, Stefano Coda, Federico Felici, Jean Marc Moret, Hoang Bao Le The performance and stability of tokamak plasma configurations depend strongly on its shape and position. They play a particularly important role in the stability of global magneto-hydrodynamics (MHD) modes and in heat and particle transport. We report on the controller design of a new generalised plasma shape and position controller for advance plasma configurations, using the linearised plasma model RZIP. The controller design is based on an isoflux control scheme and utilises singular value decomposition (SVD), which provides a natural framework for limiting the controlled parameters to the set with the largest singular values, while respecting the combined poloidal field coil current (PF) limits. It also includes the option of weighting the various observers based on the level of importance for a given plasma configuration. The generalised plasma shape and position control algorithm has been successfully tested off-line for limiter and diverted plasma (single null and snowflake configuration) shapes. The testing and commissioning of the controller will commence in the next TCV experimental campaign. [Preview Abstract] |
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