Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session TP11: Poster Session VII:
Poster
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Room: Hall A |
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TP11.00001: FUNDAMENTAL PLASAMA PHYSICS: WAVES, INSTABILITIES, AND SHOCKS; TURBULENCE AND TRANSPORT; SINGLE-COMPONENT PLASMAS
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TP11.00002: Enhanced Particle Slowing from 1D Long-Range Like-Sign Collisions Francois Anderegg, Charles F Driscoll Recent theory [1] predicts enhanced collisional slowing rates for like-sign particle collisions in strong magnetic fields, in density and temperature regimes where binary Boltzmann collisions dominate over statistical Fokker-Planck collisions. Here, the enhancement is from "long-range" collisions, with impact parameters greater than the cyclotron radius. For protons (or anti-protons) at n~10^8/cm3 and B~3T, the enhancement is large for T<1.eV; and for Mg+ ions at n~10^7/cm3 studied here, the enhancement is large for T<10.meV. Prior experiments have indirectly measured Mg+ and MgH+ collisions causing damping of plasma waves well into the enhancement regime [2], obtaining damping rates consistent with enhanced inter-species collisional drag . Two recent experimental campaigns have utilized LIF to directly measure Mg+ test-particle distributions f(v) colliding with warmer equilibria, with test-particle energies T>100.meV. Here, the measurements are consistent with the predicted factor-of-two enhancement, but the (increasing) rates have not yet been measured into the strong enhancement regime. [1] D.H.E. Dubin, Phys Plasmas 21, 052108 (2014) [2] M.Affolter et al, Phys. Rev. Lett 117, 155001 (2016) |
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TP11.00003: e-/H- Plasmas: Enhanced Centrifugal Separation and Other Disparate Mass Effects Charles F Driscoll, Andrey Kabantsev Recent experiments quantify the strong centrifugal separation effects in e-/H- plasmas, here cylindrical columns with ne~10^7/cm3, Bz~10.kG, T~25.meV, and H- fraction 1% to 20%. Most striking is the outward transport of H- on the sub-second timescale, substantially faster than the 10^4 sec predicted for collisional drag between species. [1] Here, the H- ions couple to the collective diocotron mode, causing algebraic damping of the mode at a rate proportional to the H- creation and outward transport. The thermalization of axially hot H- ions onto cold electrons is observed to be 20-40 times slower than expected for radially-overlapping species. In contrast, H- ions perp-heated by ICRH couple energy rapidly to parallel e- motion, suggesting a collective process. Similarly, the "inter-species drag" damping of excited TG waves depends strongly on their radial mode number. The recently developed "plasma modes thermometer" comparing diocotron and T-G mode frequencies provides quantitative non-destrictive information on the plasma temperature and H- fraction evolutions; but quantitative analysis of collisional and collective species couplings necessitates a MCP dump diagnostic imaging H-. [1] A.A. Kabantsev et al, AIP Conf. Proc. 1928, 020008 (2018). |
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TP11.00004: Diocotron mode instability for an electron plasma confined in the magnetic field of a long, straight, current carrying wire Thomas M O'Neil As a large aspect ratio approximation to a pure electron plasma confined by the magnetic field of a current hoop [1], consider a pure electron plasma that is confined by the magnetic field of a long, straight, current carrying wire. This limit has the theoretical advantage that equilibrium states are cylindrically symmetric about the wire, facilitating stability analysis of diocotron modes. For the case where the radial density profile multiplied by the radius squared [i.e.,n0(r)r2 ] is a top hat extending from r1 to r2 and where only E×B drifts are retained, analytic theory yields a mode growth rate of order (4π e2n0)/(I kz) when kz r1is substantially larger than unity and kz(r2-r1) is about one. Here, I is the current strength and kz is the axial wave number of the diocotron mode. Numerical results for similarly hollow, but rounded, density profiles show similar growth rates. The modes cannot be stabilized by giving the wire a uniform negative charge since the drift velocity vd = c Er /Bθ from the charged wire is independent of r, and simply adds a shift to the real part of the mode frequency. Numerical solutions show that curvature and grad-B drifts become important and do stabilize the mode when kz λD approaches unity, where λD is the Debye length. |
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TP11.00005: Plasma mode degeneracy and avoided crossings from plasma column curved ends Andrey Kabantsev, Charles F Driscoll We observed parametric coupling between the axial center-of-mass mode (mr =1,mθ =1,mz =1) and the (2,0,3) mode as their frequencies become degenerate and they form an avoided crossing; the mode frequencies never overlap. In infinitely long (or periodic boundary) theory all the modes are independent (have orthogonal eigenfunctions) and therefore there would be no such coupling between the modes. However, in real experiments, the standing TG waves of the plasma column are supported by the (generally) curved end reflections, and hence some TG modes may become degenerate in the case of fTG(mr,mθ,mz) ≈ f*TG(mr*,mθ*,mz*) if they have the same mθ and nominal kz. The TG mode frequencies are set mostly by the (kz/k) ratio, with temperature Te(t) as a fine tuning knob. There are two relatively simple but interesting cases: fTG(1,0,1) ≈ f*TG(1,±1,2) and fTG(1,0,1) ≈ f*TG(2,0,3). The former case shows no degeneracy and no modes coupling since those two modes have different θz-parity. In the latter case, the mode degeneracy does not allow for those two modes to have exactly same frequency, fTG(1,0,1) ≠ f*TG(2,0,3). When these modes become strongly coupled during the temperature evolution, they form an avoided crossing behavior as their frequencies become heavily degenerate. |
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TP11.00006: Recent advances in understanding neoclassical transport processes in nonneutral plasmas Daniel H Dubin Slow radial expansion of a long nonneutral plasma column of length L, caused by θ-asymmetric E or B fields, has for decades posed challenges to theory. This poster will review recent experimental and theoretical work that has begun to unravel the neoclassical processes involved. For large magnetic fields the radial transport is generally dominated by bounce-averaged dynamics, enhanced by phase-space boundary layers at separatrices caused by weak potential or magnetic ripples (“super-banana” transport). At lower magnetic fields, the transport is dominated by bounce-rotation resonances. A plateau-regime theory of this resonance transport mechanism reproduces “Driscoll curve” (L/B)2 scaling1, assuming random localized asymmetries associated with patch potentials on the conducting walls. The plateau theory also reproduces the stronger 1/B2.7 scaling observed for the asymmetry associated with an overall magnetic tilt2. Finally, the plateau theory also predicts a reduction in transport when the external axial potential well is near - harmonic, as observed in some experiments3.1C.F. Driscoll and J.H. Malmberg, Phys. Rev. Lett. 50, 167 (1983); 2D.H.E. Dubin, A.A. Kabantsev and C.F. Driscoll, Phys. Plasmas 19, 056102 (2012); 3A. Mohri et al., Jpn. J. Appl. Phys. 37, 664 (1998). |
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TP11.00007: Inviscid damping of the m=2 mode on a two-dimensional vortex subject to a strain flow Pakorn Wongwaitayakornkul, James R Danielson, Noah C Hurst, Daniel H Dubin, Clifford M Surko Inviscid damping is an important process in the dynamics of rotating vortices that leads to axisymmetry in isolation, or to an elliptical steady state in the presence of an external strain field. Here, relaxation of the m=2 mode of a two-dimensional vortex under external strain is studied using an electron plasma in Penning-Malmberg trap. The ExB motion of the plasma is analogous to the dynamics of a rotating vortex in a 2D inviscid, incompressible fluid. Eight-sector electrodes are used to apply an external ExB strain flow. Without strain, a perturbed vortex with a smoothly decreasing profile exterior to the core can undergo inviscid spatial Landau damping. A novel technique to control the smoothness of the vorticity profile is described, and the effect of angularly asymmetric initial condition on mode damping is studied. Trapping oscillations and the formation of cat's eye patterns in phase space are observed. These dynamics are studied in the presence of strain. The effect of external strain on the mode decay rate, rotation frequency of the mode about equilibrium, and trapping oscillations will be discussed. These results are compared to particle-in-cell simulations and available theoretical results. |
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TP11.00008: Comparison of linear and nonlinear planar mode spectra in an ultracold ion Penning trap plasmas Wes Johnson, Chen Tang, Athreya Shankar, John J Bollinger, Scott E Parker Penning trap plasmas with a rotating wall potential and Doppler laser cooling provide a platform for quantum simulation and quantum sensing experiments [J. G. Bohnet, et al.,Sci. 352, 1297 (2016), K. Gilmore, et al. Phys. Rev. Lett. 118, 263602 (2017)]. Such ultracold plasmas form a stable circular two-dimensional ion crystal with a temperature of approximately one milli-Kelvin. Axial modes (|| B/out-of-plane motions) of the crystal are well diagnosed and widely utilized for quantum information protocols. Less experimental information is known about the planar modes (⊥ B/in-plane motions) which split into two branches - magnetron (or E x B) modes and cyclotron modes. Direct numerical simulation provides insights into the planar dynamics. For example, simulation has shown thermal displacements from the crystal equilibrium corresponding to 10mk in the planar direction broaden the axial mode spectrum even when these axial modes are cooled below 1mk [Athreya Shankar et al. Phys. Rev. A, 10 102 (2020)]. Here, we linearize the Coulomb force in our existing molecular-dynamics-type Penning trap code [C Tang, et al., Phys. Plasmas, 26 073504, (2019)]. Linearization provides the capability to isolate the planar laser cooling model without the effects of mode coupling both within and between the two planar mode branches. |
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TP11.00009: Emission characteristics for a diode with various cathode geometries Madison R Howard, Joshua E Coleman, Steve M Lidia The cathode test stand at LANL consists of a diode capable of voltages up to 500kV driven by a PFN capable of providing a 2.6us pulse. The test stand is used to evaluate field emission over a range in pulse length. Using diagnostic tools such as E-dots, B-dots, and a scintillator coupled with a pepperpot, we are able to measure the voltage, current, emittance, and distribution of the beam pulse. In order to optimize the current and emittance of the electron beam, Xenos is used to create different diode set-ups and different cathode geometries. The extracted current, emittance, current density, and various other parameters are then compared for different cathode geometries, recesses, and sizes. Experimental data collected on the test stand under similar circumstances is then compared to the simulated results. |
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TP11.00010: Parametric decay of Alfvenic wave packets in nonperiodic low-beta plasmas Feiyu Li, Xiangrong Fu, Seth E Dorfman Parametric decay instability of Alfven waves is an important magnetohydrodynamic process that is relevant to the development of turbulence and energization of charged particles in laboratory, space, and astrophysical plasmas. While the instability has received extensive theoretical and numerical studies primarily in a periodic system, actual Alfvenic fluctuations are often finite in time and associated interactions nonperiodic. Here, we study the parametric decay of finite Alfvenic waves in nonperiodic low-beta plasmas using one-dimensional hybrid simulations. Compared with the periodic system, a wave packet under the absorption boundary condition shows very different decay dynamics, including reduced energy transfer, and localized density cavitation and ion heating. The resulting Alfvenic propagation is influenced by the growth rate, central frequency, and unstable mode bandwidth of the instability. A stable final state of the wave can be achieved provided the instability is insufficiently developed within the packet envelope. Under proper conditions, much enhanced backscatter cascades can also be excited with an amplitude even higher than that of the pump wave. These results may help interpret laboratory and spacecraft observation of Alfvenic dynamics, and refine our understanding of associated energy transport and ion heating. |
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TP11.00011: Searching for Whistler Waves in the Madison Symmetric Torus (MST) Tokamak Plasma* Abdulgader Almagri, Mark A Thomas, Brett E Chapman, Luis F Delgado-Aparicio, Noah C Hurst, Steven P Oliva, Alex S Squitieri, Paul Wilhite, Cary B Forest Experiments and simulations show Whistler waves can be driven by runaway electrons. High frequency spectra measured in MST show multiple coherent lines. These lines maybe due to discrete toroidal modes. Multipole single-turn coils are used to measure magnetic fluctuation and wave number spectra. Whistler-like magnetic fluctuation up to 400MHz have been observed. Magnetic fluctuation and x-ray intensity show a strong correlation. Both signals show alternating bands of high and low activities. The initial insertable probe design had several resonances that caused difficulty interpreting k spectra. A new prototype high frequency probe has been designed. With improved shielding and termination, the measured frequency response is free of resonance up to 400MHz. Using two-point correlation method, k⊥ and k‖ are measured. Signalers are digitized at 5 GHz. The target Plasma has BT=1.38kG, Ip~64kA, 0.007x1013< ne (cm-3)< 0.4x1013, q(a)~1.8, and fce ~ 2.5. The fluctuation amplitude of these frequency lines decrease with increased density and are absent at 0.4x1013 cm-3. X-ray pulses show band-like structure similar to the magnetic fluctuations. Magnetic fluctuation and x-ray spectra variation with density and magnetic field will be discussed. |
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TP11.00012: Status of and plans for the Basic Plasma Science Facility Troy A Carter, Walter N Gekelman, George J Morales, Christoph Niemann, Patrick Pribyl, Shreekrishna Tripathi, Stephen T Vincena The Basic Plasma Science Facility (BaPSF) at UCLA is a collaborative research facility for studies of fundamental processes in magnetized plasmas, supported by US DOE and NSF. The centerpiece of the facility is the Large Plasma Device (LAPD), a 20m long, magnetized linear plasma device. The LAPD is used to study a number of fundamental processes, including: collisionless shocks; dispersion and damping of kinetic and inertial Alfvén waves; turbulence and transport; flux ropes and 3D reconnection; and interactions of energetic ions and electrons with plasma waves. A major upgrade to the plasma source of the LAPD was completed over the last year, replacing the former BaO hot cathode source with a new LaB6 plasma source along with a new magnet section capable of producing up to 0.8T fields in the source region. This new plasma source provides a significant increase in the discharge power density and allows access to higher density and temperature operating regimes; the source hardware and plasma conditions achieved during operation will be discussed. An overview of recent research using the facility will be given along with a discussion of future plans including upcoming solicitation for experimental runtime on LAPD. |
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TP11.00013: Thee-wave coupling between shear Alfvén waves and kink waves in the Large Plasma Device Stephen T Vincena, Shreekrishna Tripathi, Walter N Gekelman, Patrick Pribyl Both kink waves and shear Alfvén waves occur simultaneously in plasmas |
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TP11.00014: Driving large-amplitude multiphase ion acoustic waves Vadim R Munirov, Lazar Friedland, Jonathan S Wurtele It is demonstrated how to excite and control large-amplitude ion acoustic waves by nonlinear phase locking to the corresponding driving small amplitude traveling waves. The Lagrangian formulation and the Whitham's averaged variational principle are used to analytically derive the equations describing the evolution of the slowly varying amplitudes and the phases of the wave. The conditions for double autoresonance are studied. Finally, the fully nonlinear numerical simulations are used to corroborate analytical results. |
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TP11.00015: Investigating Reconnection Driven Inductive Electric Field Leading to Particle Acceleration in a Laboratory Plasma Jet Yi Zhou, Paul M Bellan Transient 6 keV x-ray bursts are detected in a laboratory current-carrying plasma jet with 2 eV electron temperature. Preliminary evidence suggests that x-rays are generated when the plasma jet is choked and broken by Rayleigh-Taylor ripples. Breaking of the plasma jet should interrupt or redistribute the electric current and alter the topology of the magnetic field. We suspect this change in magnetic field causes an inductive electric field capable of accelerating charged particles to 6 keV. The collision of these high-energy particles should produce bremsstrahlung which is presumably the observed x-rays. Through numerical modeling, we are investigating the dynamics of this inductive electric field and the particle acceleration associated with it. |
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TP11.00016: Applying Kelvin's method to plasma equilibrium and stability in the presence of shear-driven waves: Modal vs. Nonmodal evolution Serdar A Bilgili, Mark E Koepke After decades of allowing the “non-normal” effects, introduced by gradients and shear, to evade the lens of generic time-asymptotic normal-mode (and, consequently, exponential time-dependence) stability analysis; the plasma community is adopting a new “nonmodal stability theory” paradigm for describing transience and inhomogeneity in fluid-like flows, following the hydrodynamical communities. Applying this new paradigm to DIII-D observations is the focus of the poster. |
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TP11.00017: Study of Drift-KH type Fluctuations in PHASMA Prabhakar Srivastav, Peiyun Shi, Earl Scime
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TP11.00018: Simulations of the Kink Instability of Flux Ropes in the WVU PHASMA Experiment Regis John, Paul A Cassak, Milton Arencibia, Peiyun Shi, Prabhakar Srivastav, Earl Scime Magnetic flux ropes are columns of plasma with axial and azimuthal magnetic fields. The kink instability of a magnetic flux rope is a fundamental process observed in many laboratory, astrophysical, and space plasmas such as pinches, relativistic jets, and eruptive phenomena in the solar corona. Here, we present results from the 3D magnetohydrodynamics code F3D of a flux rope motivated by the West Virginia University PHAse Space MApping (PHASMA) experiment. The evolution of an m=1 kink instability as a function of flux rope axial current, axial magnetic field, and boundary conditions (line-tied and non-line-tied) is investigated and compared to laboratory observations. The experiments show evidence of the excitation of an Alfvenic mode that develops after the m=1 kink nonlinearly saturates. The simulation results are compared to laboratory measurements of kink mode frequency, Alfvenic mode frequency, and kink threshold current. |
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TP11.00019: Explosion Debris-Driven Electromagnetic Ion-Ion Beam Instabilities Ari Le, Brett Keenan, Dan Winske, Adam J Stanier, Blake A Wetherton, Fan Guo, Misa Cowee We use a hybrid (fluid electrons + kinetic ions) version of the particle-in-cell code VPIC to analyze electromagnetic ion-ion beam instabilities driven by super-Alfvenic debris streaming parallel to the magnetic field from a localized explosion. The explosion of ionized debris into a background magnetized plasma occurs in toastrophysical systems such as supernovae, space contexts including magnetospheric chemical release experiments, and laboratory experiments driven by laser ablation [see 1 and references therein]. In collisionless systems, debris streaming parallel to the magnetic field couples to the background through resonant and non-resonant ion-ion beam instabilities [2]. Based on the instability growth rates and 1D & 2D hybrid simulations of nonlinear saturation, we estimate the amount of debris required to generate a quasi-parallel shock in the laboratory [3]. |
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TP11.00020: Benchmarking magnetized coupling coefficients: Comparing particle-in-cell simulations with analytic solutions of three-wave equations in backscattering geometry Yuan Shi In the presence of a strong oblique magnetic field, three-wave coupling coefficients noticeably differ from their unmagnetized values. Instead of the familiar Raman and Brillouin couplings, three-wave interactions are now mediated by magnetized plasma waves, and the general formula for magnetized coupling coefficients remains to be benchmarked against numerical simulations. This study focuses on backscattering, where pump and seed lasers are counter propagating at an oblique angle with the background magnetic field. To benchmark the formula, particle-in-cell (PIC) simulations are carried out in the linear regime, where pump depletion is negligible. To extract the coupling coefficients and wave damping rates from PIC simulations, the seed intensity is scanned, and numerical results are fitted to analytic solutions of the linearized three-wave equations. The general solution to the linearized problem is derived systematically using Laplace transform, to allow for arbitrary initial and boundary conditions in a semi-infinite plasma domain, which matches the setup of PIC simulations. The numerically extracted coefficients agree reasonably well with analytical results for a wide range of magnetic field strengths and propagation angles in regimes where the analytic formulae are valid. |
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TP11.00021: Topological waves and bulk-edge correspondence in magnetized plasmas Yichen Fu, Hong Qin Topological phases of electronic and photonic systems have become a rapidly emerging field of research. Studies have shown that these topological ideas can also be applied to continuous media, such as plasmas and fluids. In a recent study [Nat. Commun. 12, 3924 (2021)], we found that there are ten topological phases in the parameter space of density $n$, magnetic field $B$, and parallel wavenumber $k_z$, and topologically protected edge modes can be excited at the interface between topologically different plasmas, as required by the bulk-edge correspondence. A necessary and sufficient condition for the existence of these edges modes was given. We will also report the recent progress in topological phase transitions and edge modes at general 2D plasma interfaces and the energy and momentum transport due to these topologically protected excitations. |
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TP11.00022: Nonmodal growth of MHD shear flows with stabilizing magnetic fields Adrian E Fraser, Jeff S Oishi, Alexis K Kaminski Shear flows are common and important in astrophysical plasmas, where they often serve as free energy sources able to drive fluctuations and turbulence. In turn, these motions enhance momentum, energy, and particle transport, thus crucially affecting the evolution of the system. The parameter boundaries delineating shear-driven fluctuations are often assumed to be provided by normal-mode linear stability analyses. However, in many important systems, including pipe flow and stratified shear flows in neutral fluids, such analyses are known to be misleading. Perturbations can grow significantly through nonmodal or non-normal growth, potentially driving turbulence and mixing even at parameters where linear stability analyses predict no growth. Here, we explore the degree of nonmodal growth when equilibrium magnetic fields aligned with the flow nearly or entirely stabilize shear-flow instabilities. Pseudospectra indicate significant nonmodal amplification in linearly stable regimes for 2D systems. We also present progress towards comparisons of 2D and 3D optimal linear perturbations that maximize fluctuation growth. |
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TP11.00023: Attenuation of shear Alfven waves due to scattering off quasi-static random magnetic fluctuations Rupak Mukherjee, Abhijit Sen, Brahmananda Dasgupta The attenuation of shear-Alfven waves while propagating through an ideal incompressible plasma medium that contains stationary random magnetic field fluctuations is calculated both analytically and through numerical simulations. Such an attenuation can have an impact on Alfven wave heating in tokamaks or other fusion devices, as well as in solar coronal heating. Our analytic calculations, based on the method discussed in Howe [1], provides estimates of the wave decay rate in two limits, namely, for scattering off short scale and long scale length fluctuations. The decay rate is found to vary linearly with the magnitude as well as the correlation length of the stochastic magnetic field and quadratically with the mode number of the propagating wave. We compare our analytical results with numerical solutions of the shear-Alfven wave propagation equation in the presence of magnetic fluctuations and find good agreement between the two. Finally, we compare the decay rates due to the collisional damping and the stochastic damping of shear-Alfven waves and provide a consolidated parametric view of their combined effect. [1] M.S. Howe, J. Fluid Mech. 45 (1971) 769 |
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TP11.00024: Magnetogenesis by Ion-Acoustic Waves Ian E Ochs, Nathaniel J Fisch For a planar electrostatic wave interacting with a single species in a collisionless plasma, momentum conservation implies current conservation. However, when multiple species interact with the wave, they can exchange momentum, leading to current drive. A simple, general formula for this driven current is derived [1-3]. For ion-acoustic waves, the wave results in momentum exchange between resonant electrons and nonresonant ions. The resulting force on the electrons can have a curl, and thus, as for the Biermann battery, give rise to compensating electric fields with curl on magnetohydrodynamic timescales. As a result, a magnetic field can be generated [4]. Surprisingly, in some astrophysical settings, this mechanism can seed magnetic fields with growth rates even larger than through the traditional Biermann battery, and thus could give rise to seed fields. |
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TP11.00025: Spontaneous and explicit parity-time-symmetry breaking in drift wave instabilities of magnetized plasmas Hong Qin, Yichen Fu, Alexander S Glasser, Asher Yahalom The drift wave in magnetized plasmas typically depends on 7 or 8 dimensionless parameters, resulting in many different paths to instabilities. We introduce a method of Parity-Time (PT)-symmetry analysis to study the high dimensional, complex parameter space of drift wave instabilities. The main results include: (1) It is shown that spontaneous PT-symmetry breaking leads to the Ion Temperature Gradient (ITG) instability, whereas the collisional instability and the universal mode are the results of explicit PT-symmetry breaking. (2) Drift wave instabilities by spontaneous PT-symmetry breaking have instability thresholds due to topological constraints on the spectrum, but those by explicit PT-symmetry breaking do not. (3) When the ITG mode is stabilized, the relative phase between density perturbation and ion flow in the inhomogeneous direction needs to be locked at π/2, which can be used as a measurable criterion in experiments. (4) A new unstable drift wave induced by finite collisionality is identified. (5) It is also found that gradients of ion temperature and density can destabilize the ion cyclotron waves when PT symmetry is explicitly broken by a finite collisionality. |
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TP11.00026: Geometric Electrostatic Particle-In-Cell Simulations on Unstructured Meshes Zhenyu Wang, Hong Qin, CS Chang A geometric electrostatic Particle-In-Cell (PIC) algorithm on unstructured meshes has been developed [1]. Present algorithm treats ions as fully kinetic 6D particles with adiabatic electron response. The algorithm is derived from a discrete variational principle on an unstructured triangular or tetrahedral mesh, or a partially unstructured prism mesh similar to that used in the XGC code. The discrete variational principle mandates that Whitney 0-forms are used for charge deposition and Whitney 1-forms for field interpolation, which implies that the ‘shape functions’ for charge deposition and field interpolation are different. The Whitney 1-forms on the prism mesh are self-consistently derived. The algorithm has been applied to simulate the Ion Bernstein Wave (IBW) in a 2D circular region with the fixed boundary condition. The spectrum and eigenmode structures of the IBW are obtained from the simulation. We plan to implement an equilibrium of a magnetized plasma with density and temperature gradient in a slab geometry [2] using the unstructured prism mesh and simulate the excitation and growth of the ITG instability. |
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TP11.00027: Radio and Plasma Wave Generation by an Electron Beam in a Laboratory Plasma Seth E Dorfman, Vadim S Roytershteyn, Gian Luca Delzanno, Cynthia Cattell, Quinn Marksteiner, Christopher Colpitts, Haoran Xu, Jesus A Perez Interaction between relativistic electron beams and magnetized plasma is a fundamental and practical problem relevant to many challenging issues in space physics and astrophysics. We present results from a 20 keV beam experiment on the Large Plasma Device (LAPD) at UCLA motivated by the problem of how naturally occurring electron beams may produce type II/III solar radio emissions as well as recent proposals to place compact high-energy electron beam sources on future spacecraft. These spacecraft-borne beams may be used to map magnetic field lines in the Earth's magnetosphere or to generate waves for radiation belt remediation. In the LAPD experiments, electromagnetic emission between the plasma and upper hybrid frequencies is observed by both in-situ probes and by an antenna outside of the plasma. The parallel phase speed of the excited waves is measured to be consistent with generation via a resonance process at the point on the dispersion relation where the beam mode and Z-mode couple. Kinetic modeling suggests that the apparent absence of strong instabilities is due to velocity dispersion imposed by the beam injection conditions. Consistent with this, A 15-45% perturbation of the beam density is estimated from the parallel electric field measurements. |
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TP11.00028: Improving upon Landau and van Kampen-Case: Proper Asymptotics and Disappearance of Decaying Discrete Modes Frank M Lee, Bradley A Shadwick Landau's solution (generalized by Jackson) of the initial value problem for the one-dimensional linear Vlasov-Poisson system, shifts and deforms the Bromwich contour around the poles of the analytically-continued dielectric function. For an unstable equilibrium, this results in the growing, but not the decaying, discrete modes contributing. However, in the van Kampen-Case construction, both growing and decaying discrete modes have non-zero amplitudes, thus a contradiction seems to arise. We present a more general, yet more transparent approach and show that the decaying modes do not actually contribute; a part of the continuum always exactly cancels the decaying discrete modes. We evaluate the Bromwich integral using properties of Cauchy-type integrals instead of deforming the contour and therefore avoid difficulties arising from the Landau-Jackson analytic continuation. The latter can result in divergences from incorrect asymptotic assumptions, where the initial condition plays an important role that we properly take into account. We avoid complicated principal value integrals and singular eigenfunctions of van Kampen-Case; a straightforward Laurent series expansion is used instead. We show specific examples using equilibria and initial conditions with distinct properties. |
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TP11.00029: Excitation of nonlinearly coupled harmonics by linearly independent plasma waves Min Uk Lee, Jeong-Young Ji, Gunsu S Yun Time-dependent flows in a plasma produce electromagnetic (EM) waves. The EM waves perturbed by the flows and the intrinsic waves in the plasma can excite the nonlinearly coupled harmonics.1 The nonlinear modes observed from a plasma contain spatiotemporal information about the plasma flow, so understanding the wave-wave nonlinearity is essential to interpret the plasma behavior. The Vlasov-Maxwell equations are solved to describe the nonlinear coupling between the linearly independent plasma waves propagating parallel to the magnetic field or in an unmagnetized plasma. Extended dispersion relations obtained from the kinetic analysis show the excitation of coupled harmonics with the frequency selection rule. Additionally, the wavenumber selection rule is suggested for the harmonics. Both the selection rules are verified by the fully kinetic particle-in-cell simulation2. The nonlinear coupling between the flow-induced perturbation and the background plasma wave may explain observed harmonics in various plasmas and the wave propagating perpendicular to the magnetic field with the whistler wave frequency3. |
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TP11.00030: Collisional damping of surface ion-acoustic mode in the semi-bounded and collisional plasmas Myoung-Jae Lee, Young-Dae Jung The dissipation of ion-acoustic surface waves in a semi-bounded and collisional plasma is evaluated from the dispersion integral formulated by employing the specular reflection boundary condition. The distinction between the collisionless Landau damping and collisional damping is addressed here. The collisional damping of the surface wave is investigated for two ionization cases: weakly ionized and completely ionized plasmas. In the case of weakly ionized plasma, the damping is found to be independent of the ion temperature, but it has been shown that the ion temperature can play an important role in the fully ionized plasma. Particularly, in the case of weakly ionized plasma, the effect of collisional damping hardly appears in the small wave number region. However, the collisional damping is important for the fully ionized plasma in the small wave number region. For both weakly ionized and fully ionized plasmas, the collisional damping dominates over the collisionless Landau damping in the large wave number region. |
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TP11.00031: Numerical study of ion acoustic instability Zhuo Liu, Ryan White, Lucio M Milanese, Nuno F Loureiro We studied the linear saturation and nonlinear evolution of a collisionless ion acoustic unstable plasma using a Vlasov-Poisson code. We apply a weak constant external electric field that accelerates electrons and ions and causes ion acoustic instability. The preliminary result shows ion acoustic instability saturates through quasi-linear relaxation of electron distribution. Ion acoustic turbulence (IAT) creates a sharp peak of anomalous resistivity, which is close to that predicted by quasi-linear theory rather than Sagdeev resistivity, when the instability saturates. The increase of current does not come to a stop through IAT itself as anomalous resistivity gradually decreases back to zero after the saturation. During this process, the ion acoustic instability further relaxes through a combination of a reduction of the temperature ratio and a quasi-linear relaxation of the background distribution functions to a stable non-Maxwellian configuration. Run-away electrons are not observed in the simulation. We investigate whether saturation of the system will ultimately occur through the transition to other streaming instabilities. |
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TP11.00032: Kinetic modeling of solitary waves and surface waves in a neutralizing beam Nakul Nuwal, Deborah A Levin, Igor Kaganovich Ion beams are used in various engineering applications such as particle accelerators, ion-thrusters, and ion-implantations. The rate of neutralization of beams is affected by the energy and the location of the electron source relative to the ion beam. In recent numerical works by Lan and Kaganovich, electrostatic-solitary-waves (ESWs) were observed when the electrons were injected in a 2D planar ion beam. We will present the beam neutralization in 3D and show the formation of solitons and their movement along the beam axis using the Particle-in-Cell (PIC) method. Prediction of electron solitons is important as they may cause the heating of electrons and slow down the process of neutralization in the beam. A hybrid MPI-CUDA code, CHAOS, is used for this work, in which an argon beam is neutralized by the electrons emitted from a circular source along the beam axis. We will present our findings on the formation of solitons and long wavelength surface waves in cylindrical and planar beams for different beam widths. Prediction of the surface waves and their frequencies is important in experimental diagnostics and passive measurements in plasma beams. By comparing our results with the theory of planar surface waves, we will show how the long-wavelength surface waves get excited differently in a 2D planar and 3D cylindrical beam. |
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TP11.00033: Evidence for neutrals carrying ion-acoustic wave momentum in a partially ionized plasma Meenakshee Sharma, A. D. Patel, ZUBIN A SHAIKH, Narayanan Ramasubramanian, Rajaraman Ganesh, P.K. Chattopadhyay, Y. C. Saxena An experimental study of Ion Acoustic (IA) wave propagation is performed to investigate the effect of neutral density for Argon plasma in an unmagnetized linear plasma device1,2. The neutral density is varied by changing the neutral pressure, which in turn allows the change in ion-neutral, and electron-neutral collision mean free path. The collisions of plasma species with neutrals are found to modify the IA wave characteristics such as the wave amplitude, velocity, and propagation length. Unlike the earlier reported work where neutrals tend to heavily damp IA wave in the frequency regime ω < vin (where ω is ion-acoustic mode frequency and vin is ion-neutral collision frequency)3, the experimental study of IA wave presented here suggests that the collisions support the wave to propagate for longer distances as the neutral pressure increases4. A simple analytical model is shown to qualitatively support the experimental findings. |
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TP11.00034: Characterisation of a helicon source for non-linear microwave coupling experiments in a magnetised plasma Kieran J Wilson, Liam Selman, Colin G Whyte, Philip MacInnes, Alan R Young, Alan R Phelps, Adrian W Cross, Liang Zhang, Bengt Eliasson, David C Speirs, Craig W Robertson, Kevin Ronald, Robert Alan Cairns, Robert Bingham, Ruth Bamford, Mark E Koepke As a non-linear medium, parametric instabilities arise when powerful EM waves propagate in plasma. Such effects are seen in laser-plasma, RF–ionospheric and tokamak heating scenarios. Fusion plasmas in a spherical aspect tokamak are difficult to heat due to their high densities, making the lower cyclotron resonances unreachable via direct means. Beat-wave interactions involving multiple EM waves can be used to excite such resonances. Laser, fusion and ionospheric environments pose diagnostic challenges to fully investigate these processes and measure their impact on macroscopic properties such as density, temperature and energy distributions. For this reason a helicon source driven between 3 & 30 MHz has been commissioned. The 1 m diameter, 3 m long stainless-steel vessel is immersed in a static B-field of up to 90 mT formed by 6 electromagnets. The resulting relatively tenuous (1018 m-3), cool (<10 eV) plasma is ideal for parametric wave coupling experiments with powerful microwave beams. Microwave interferometry, frequency compensated Langmuir and RF pickup probes will diagnose how these processes impact the plasma. This paper presents initial experiments characterising the apparatus, with comparison to numerical simulations. |
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TP11.00035: Qubit Lattice Algorithm for an Electromagnetic Pulse Interacting with a Two-Dimensional (2D) Dielectric Obstacle Linda Vahala, George Vahala, Min Soe, Abhay K Ram A qubit lattice algorithm (QLA) is developed for the solution of Maxwell equations in a scalar dielectric 2D medium. QLA consists of an interleaved sequence of collide-stream operators appropriately chosen so that Maxwell equations in an arbitrary scalar medium are recovered to second order. Using the Riemann-Silberstein-Weber representation, the qubit evolution equations will have both Hermitian and antiHermitian operators. 1D QLA is extended to 2D by tensor products, requiring just 16 qubits/lattice site. Here we consider the propagation of a 1D electromagnetic pulse past a 2D dielectric cylinder whose diameter is several times larger than the pulse width. As in the 1D QLA simulations we find transmission and reflection of the pulse. There is a phase change in the electric field component on reflection off the dielectric cylinder while an additional component of the magnetic field is created due to the 2D dielectric so as to maintain div B = 0. The refraction of the incident pulse at the dielectric boundary and multiple reflections within the dielectric lead to circular wavefronts propagating in the surrounding medium. If desired, the antiHermitian operators can be readily coaxed into Hermitian form by doubling the number of qubits/lattice site. |
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TP11.00036: Spacetime structure of weak magnetohydrodynamic turbulence Augustus Azelis, Jean C Perez, Sofiane Bourouaine The two time energy spectrum of weak magnetohydrodynamic turbulence is found via application of a weak turbulence closure to the cumulant hierarchy constructed from the dynamical equation. Solutions are facilitated via asymptotic expansions in terms of the small parameter $\epsilon$, describing the ratio of timescales corresponding to Alfvénic propagation and nonlinear interactions between counter-propagating Alfvén waves. The strength of non-linearity at a given spatial scale is further quantified by an integration over all possible delta function correlated modes compliant in a given set of 3-wave interactions that are associated with energy flux through said scale. The weak turbulence closure for the two time spectrum uncovers a secularity occurring on timescale $\epsilon^{-2}$ and the asymptotic expansion for the spectrum is reordered in a manner comparable to the one-time case. It is shown that the two-time energy spectrum will oscillate in accordance with Alfvénic propagation on the linear timescale and exponentially decay on timescale $\epsilon^{-2}$ in proportion to the strength of the associated nonlinear interactions. |
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TP11.00037: Gyrokinetic Simulations of Global Tearing Modes and Multi-Scale Interaction with Micro-Turbulence Taweesak Jitsuk, Alessandro Di Siena, MJ Pueschel, Paul W Terry The existence of macro-scale tearing modes during plasma operations on the Madison Symmetric Torus, a reversed-field pinch (RFP), even in improved-confinement regimes has significant impacts on micro-scale instabilities and turbulent transport. This observation has motivated the modeling of the interplay of fluctuations at different scales using the gyrokinetic code GENE. Previous work modeled tearing-mode activity by implementing current-gradient drive as part of the fluctuating distribution function, coupled nonlinearly with turbulence fluctuations and zonal flows. To be able to more realistically model tearing modes, a shifted-Maxwellian distribution has been implemented in GENE for global simulations, consistent with the q-profile. The implementation has been benchmarked against linear flux-tube simulations through scans over plasma parameters. Additionally, comparisons are shown with global tearing results from other gyrokinetic and gyrofluid codes. Tearing modes in the RFP are then simulated, analyzed, and used to study multi-scale interactions with gyroradius-scale instabilities. |
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TP11.00038: Stable-mode-mediated nonlinear saturation, transport, and small-scale dissipation in MHD Kelvin-Helmholtz turbulence Bindesh Tripathi, Adrian E Fraser, Paul W Terry, Ellen G Zweibel, MJ Pueschel Shear-flow plasmas often lead to instability and turbulence. Reduced models of this turbulence commonly employ unstable modes from linear stability analyses and allow nonlinear interaction amongst them to modify the mean flow (via turbulent stresses). This simplified approximation generally overestimates turbulent fluxes. To reliably model these fluxes and better under nonlinear interactions, here we simulate MHD Kelvin-Helmholtz turbulence using Dedalus. We consider a flow-aligned magnetic field, and the flow is forced to retain its shear profile, unlike decaying in the earlier work1. We find the modes with frequencies complex-conjugate to the unstable ones (called stable modes herein) to be nonlinearly excited to an almost equal level as the unstable modes. These stable modes transfer energy to the mean flow from the large-scale fluctuations, thus reducing the energy cascade to smaller scales. We report that the stable and unstable eigenmode pairs reconstruct the turbulent flow largely and rectify the aforementioned overestimation of turbulent fluxes by capturing the counter-gradient momentum transport via stable modes. We detail the impacts of magnetic fields and forcing on energy transfer channels. 1Fraser et al. 2021, PoP 28 (2), 022309 |
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TP11.00039: Instability excitation and modification due to compressional magnetic fluctuation in global gyrokinetics Shu-Wei Tsao, MJ Pueschel, Tobias Görler, Tilman Dannert, Anna Tenerani, David R Hatch The compressional magnetic field fluctuation B// is commonly ignored in gyrokinetic simulations, and only the electric potential Φ and the parallel magnetic potential A// are considered. B// tends to have a small effect on turbulence in core plasmas but can strongly affect electromagnetic modes in the tokamak pedestal, reconnection turbulence in the solar corona, and drift-wave turbulence in LAPD high-β experiments [Pueschel et al., PoP 22, 062105 (2015)]. |
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TP11.00040: Investigating the role of stable eigenmodes in the nonlinear dynamics of resistive tearing instabilities Zachary R Williams, Julie C Timperman, Matthew S Dickerson, Adrian E Fraser The tearing instability and its subsequent nonlinear dynamics are ubiquitous to plasmas, seen across a wide range of parameter space from the sawtooth crashes in fusion plasmas to coronal mass ejections in solar plasmas. This work explores the role that stable eigenmodes play in the nonlinear dynamics that result from the resistive tearing instability. Prior work has demonstrated the significance of linearly stable fluctuations to turbulence driven by a number of different plasma instabilities. Importantly, these stable modes act as an energy sink existing at the same spatial scales as the linear instability, preventing energy from cascading to smaller scales. An analysis of a Harris sheet equilibrium reveals the presence of a large number of stable modes present concurrently with a single unstable tearing mode. The contributions of each of these modes to the nonlinear state of the tearing mode dynamics is evaluated over a range of different magnetic Prandtl numbers. Simulations show that a small number of stable modes contribute significantly to the dynamics alongside the unstable mode. The effectiveness of a truncated eigenmode expansion as a means of describing the fully nonlinear system is quantified and discussed. |
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TP11.00041: Extension of Regimes of Applicability of Collisional Transport in Magnetized Multi-Ion Plasma Mikhail Mlodik, Elijah J Kolmes, Ian E Ochs, Nathaniel J Fisch Collisional cross-field transport in magnetized multi-ion plasma exhibits curious effects such as charge incompressibility, ion stratification, and a novel heat pump.[1,2] We have extended cross-field transport response of plasma to a wider range with respect to collisional magnetization, which can be determined as the ratio of the light ion gyrofrequency to the collision frequency of light and heavy ion species. We have recovered previously known results that the heavy ions tend to concentrate in the low-temperature region of collisionally magnetized plasma and in the high-temperature region of collisionally unmagnetized plasma, respectively. Moreover, we have found the behavior of this effect in the intermediate regime of partially magnetized plasma.[3] The expansion of the range of validity of multi-ion collisional transport models makes them applicable to a wider range of laboratory plasma conditions. In particular, ion density profiles evolve sufficiently fast for radial impurity transport to be observable around stagnation on MagLIF, leading to expulsion of heavy ion impurities from the hotspot as long as plasma becomes sufficiently collisionally magnetized during the implosion.[3] In order to further expand the range of applicability of the corresponding transport models, we also identify the effects of collisional cross-field transport in partially ionized plasma, where ions are not fully ionized, as well as discuss their experimental implications. |
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TP11.00042: Generalized pinch relation in rotating, sheared plasma Tal Rubin, Elijah J Kolmes, Ian E Ochs, Mikhail Mlodik, Nathaniel J Fisch In magnetized plasma, in equilibrium, different ionic species tend to accumulate differently in what is sometimes called an impurity pinch effect. The disparities in the ion density profiles of different species are of interest in a variety of fusion devices, from magnetized Z pinches to tokamaks, and in mass filters. Recent work has generalized the pinch relation by taking the temperature and species-dependent potential into account. In addition, rotation introduces the centrifugal force, and might introduce shear stress, which modify the generalized pinch condition. Taking these effects into account, we investigate the new pinch condition analytically, and provide numerical validation by generalizing a multiple-fluid code to cylindrical geometry. |
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TP11.00043: Wave kinetics of inhomogeneous incompressible 2-D MHD turbulence Suying Jin, Ilya Y Dodin Traditional wave-kinetic approaches to MHD turbulence have been limited to the weak-turbulence regime and rely on scale separation between the turbulent Alfvenic fluctuations and background inhomogeneities. Such approaches are inapplicable to describing strongly nonlinear coherent structures, which are commonly found in MHD turbulence. Here, we present an alternative, Wigner-Moyal approach that is free from this limitation. MHD turbulence is treated within the Wigner-Moyal approach approach as plasma of effective quantumlike particles (Alfven waves) that have non-negligible wavelengths and interact via mean fields. We analytically explore phase-space dynamics and structure formation (modulational instabilities) in such "plasmas" using incompressible 2-D MHD equations for the Elsasser vorticities as the base model. |
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TP11.00044: Intermittent turbulence in Multi-Ion Plasmas in the LAPD Thomas R Look, Stephen T Vincena, Troy A Carter Intermittent turbulence and associated density-enhancement events ("blobs") are observed in the edge of a wide range of magnetic confinement devices. In the edge of tokamak plasmas, convective transport associated with blob propagation can dominate particle transport. Most studies of intermittency and blob transport have been performed in single ion species plasmas even though fusion plasmas will need to be mixed ion (DT). We are carrying out a study of blobs in controlled mixtures of hydrogen and deuterium in the Large Plasma Device (LAPD). We will present data and analysis of the properties of blobs (size, velocity, amplitude, transport) as the D-H mix is varied. |
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TP11.00045: Study of the Bryn Mawr Experiment's coaxial plasma gun and magnetic shaping coils via COMSOL Multiphysics modeling and simulation Joshua M Carlson, David A Schaffner, Carlos A Cartagena-Sanchez The Bryn Mawr Experiment (BMX) investigates the magnetic turbulence of plasma structures formed within a flux conserving cylindrical chamber. The BMX generates these self-contained structures using an internal coaxial plasma gun source, with an internal coil providing a radial magnetic stuffing flux. Two external coaxial coils provide additional magnetic flux for stuffing or shaping the plasma structures. In an effort to increase bulk plasma speed, the external coils are modeled as a magnetic nozzle in COMSOL Multiphysics as a basis for experiment on BMX. Results from the magnetic nozzle COMSOL simulation and preliminary experimental results are presented and discussed. |
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TP11.00046: Magnetic Correlation Studies: the Bryn Mawr Experiment and Simulaitons Carlos A Cartagena-Sanchez, David A Schaffner, Joshua M Carlson We present spatial and temporal correlations of magnetic fluctuations to characterize the outer and inner scales of the magnetized plasma turbulence generated through the injection of helicity with a magnetized coaxial gun source into a flux conserving cylindrical wind-tunnel. Experimental results are shown and compared to resistive MHD simulations of an evolving magnetized plasma plume within a cylindrical wind-tunnel. The simulations run on the Bridges2 supercomputer using the Dedalus framework; an open-source and MPI-parallelized environment written in Python. |
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TP11.00047: High-order finite-element simulations of forced and decaying Hasegawa-Mima turbulence Alejandro Campos, Ben Zhu, Ilon Joseph, Milan Holec, Chris J Vogl, Andris M Dimits, Ben S Southworth Simulations of neutral-fluid, two-dimensional turbulence in which localized forcing is balanced by linear damping have provided valuable insight into the physical mechanisms that underpin the dual spectral cascades [1]. We extend these forcing and damping terms to the regime of plasmas characterized by the viscous Hasegawa-Mima (HM) equation. The high-order MFEM finite-element framework is used to solve the governing equations. The plasma length scale and the background plasma gradient are varied in the HM model so as to sufficiently differentiate the turbulence dynamics from the neutral-fluid case. Emphasis is placed on understanding deviations of the HM turbulence from relatively recent neutral-fluid results. As such, we investigate the multiscale strain and vorticity interactions that drive the inverse cascade, the energy condensation state of large scales, logarithmic corrections and scalings for the forward cascade, and the extent of co-existing inertial ranges. [1] G. Boffetta & R. E. Ecke, Annu. Rev. of Fluid Mech., Vol. 44:427-451, 2012. |
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TP11.00048: Prediction of divertor heat flux width for ITER scenarios using BOUT++ transport code He Xiaoxue, Xueqiao Xu, Zeyu Li, Ben Zhu, Yue Liu Prediction of divertor heat flux width is performed for the first and the second Pre-Fusion Power Operation (PFPO) phases specified in the new ITER Research Plan and ITER baseline L-mode scenario using BOUT++ transport code. The initial plasma profiles inside the separatrix are taken from CORSICA scenario studies. Transport coefficients in transport code are calculated by inverting the plasma profiles inside the separatrix and are assumed to be constants in scrape-off-layer (SOL). An anomalous thermal diffusivity scan is performed with E×B and magnetic drifts. The results in both scenarios identify two distinct regimes: a drift dominant regime when diffusivity is smaller than the respective critical diffusivity and a turbulence dominant regime when diffusivity is larger than it. The critical diffusivity is 0.5 m$^{2}$/s in 5MA PFPO-I scenario, 0.3 m$^{2}$/s in 7.5MA PFPO-II scenario and 0.005 m$^{2}$/s in 15MA L-mode scenario. The ITPA multi-machine experimental scaling yields a lower limit of the width. Separatrix temperature and collisionality also have a significant impact on the heat flux width in the drift dominant regime. By fixing the safety factor , the separatrix temperature , the critical diffusivity is \chi$_{c}$ \propto A$^{\(1/2}$/(Z(Z+1)$^{\(1/2}$Bp$^{2}$). |
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TP11.00049: Multi-Ion Plasma Shock Formation From Colliding Supersonic Jets on the PLX Brett Keenan, Samuel J Langendorf, Feng Chu, Andrew L LaJoie, William T Taitano The LANL Plasma Liner Experiment (PLX) facility has began a study of multi-ion shock fronts in a connected vacuum chamber. Using the state-of-the-art, multi-ion, Vlasov-Fokker-Planck (VFP) code, iFP,1 we explore shock-driven multi-ion stratification effects in the collision of two PLX-like plasma jets. We compare these simulation results to PLX diagnostic data: including spatially-resolved spectroscopy of the shock structure, and ion temperature profiles via Doppler broadening. Finally, supplemental fluid and multi-physics modeling was done to assess the role of missing physics (e.g., ionization and multi-dimensional effects; which are absent from iFP). |
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TP11.00050: Ion shock layer characterization in multi-ion-species plasma jet experiments Ameer I Mohammed, Colin S Adams We report measurements of density, electron temperature, and ionization state across shock layers formed during collisions between multi-ion-species plasma jets and quasi-stagnant plasma. Previous experiments suggest that these shocks are collisional, with a post-shock mean-free-path substantially smaller than the centimeter scale shock layer. Present efforts focus on characterizing the axial and radial distribution of ion species on scales between the expected ion shock and electron preheat layer thicknesses using spatially resolved spectroscopy and high-speed photography. These jet experiments provide a platform to study the role of non-LTE effects in plasma shocks while also benchmarking existing theoretical and computational models relevant to astrophysical scenarios and high-energy-density physics. |
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TP11.00051: Decay Regime Transitions in Plasma Shock-Turbulence Interaction Michael F Zhang, Seth Davidovits, Nathaniel J Fisch Plasma viscosity scales strongly with increasing temperature. Prior work has shown that turbulence in a compressing box of plasma can undergo a sudden viscous dissipation effecta. For equivalent compression, a strong shock can achieve far higher temperature increases than a metric compression. In this work, we investigate how turbulence in a plasma could be dissipated after passage of a shock. Statistical analysis of plasma turbulence parameters shows the dissipation scales can increase across a shock, in contrast to the shock-turbulence interaction for neutral gases. A corresponding transition to different decay regimes may result in rapid dissipation of turbulence with potential for use as a novel method for inferring viscosity in plasmas. |
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TP11.00052: MFE: HIGH FIELD & LONG PULSE TOKAMAKS; PINCH, MIRRORS, SPHEROMAK, AND OTHER MAGNETIC RELAXATION
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TP11.00053: Zap Energy Shear-Flow-Stabilized Z-Pinch Reactor Results and Concept Benjamin J Levitt, Matthew C Thompson, Brian A Nelson, Uri Shumlak The sheared-flow-stabilized (SFS) Z-pinch concept, developed at the University of Washington with LLNL collaborators, is now on a path to commercialization at Zap Energy Inc. Recent experiments corroborate expected thermonuclear fusion reaction rates, as the discharge current is scaled towards compact reactor conditions. The Fusion Z-pinch Experiment (FuZE) employs high power-handling electrodes, flexible gas injection, and independently-switched capacitor bank modules to tailor the discharge current and gas distribution to establish stabilizing sheared flow and pinch current. An extensive set of diagnostics provide key measurements, with collaborators from LANL, PPPL, LLNL, UCB, CalTech, UCSD, among others. Experimental campaigns are underway to increase the pinch current, stability duration, and DD fusion neutron production. These efforts aim to scale the pinch current, plasma density, and plasma temperature to reach scientific breakeven equivalent conditions by early 2023 in the next generation device FuZE-Q, which is currently being commissioned. The design for a compact, scalable fusion reactor is also presented. |
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TP11.00054: Bayesian Optimization for Sheared-Flow Stabilized Z-Pinches Aria Johansen, Anton D Stepanov, Uri Shumlak The search for optimal operation settings for a fusion device requires searching a high dimensional parameter space. To complicate matters, the nonlinearity of plasma dynamics causes some shot-to-shot variation, despite macroscopically invariant parameters. Bayesian optimization is a reinforcement learning technique for searching a high dimensional, continuous parameter space efficiently for the optimal value, while taking into consideration that the samples taken on the manifold are generated by a distribution. This optimization technique is more efficient than other search techniques such as gradient descent, or grid search, and is designed for continuous parameter space making it better suited than Bandit algorithms. For a sheared-flow-stabilized Z pinch the search space includes gas puff timings and pressures which create the initial distribution of gas in the vacuum chamber, and capacitor bank voltages, and discharge timings which sets how much and how the energy is put into the system. This framework allows for the optimization of various objective functions such as neutron yield, ion temperature, plasma density, or quiescent period duration. Here the results of initial deployment on the FuZE device are presented. |
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TP11.00055: Towards an integrated plasma perception system for stabilized Z-pinch experiments at ZEI Anton D Stepanov Modern plasma experiments can generate copious amounts of data, but the problem of how to leverage data quantity to improve inference quality remains unsolved. The inference task can be generalized to finding an unknown vector-valued function of space and time ΠΛ(x,t) = [ρ, v, T, B,…](x,t) that carries all the relevant information about the plasma (density, velocity, etc.). Most often, ΠΛ has to be inferred from indirect diagnostic measurements like chord integrated spectroscopy, resulting in ill-posed inverse problems in the absence of regularization constraints. Most notably, these constraints must include the laws of physics in PDE form. The ZEI integrated plasma perception system uses deep neural networks to model ΠΛ(x,t). By comparing synthetic diagnostic signals computed from ΠΛ to actual diagnostic data, the ΠΛ network can be trained to become the solution to the inverse problem. In addition, the error-free differentiability of neural nets allows for straightforward application of PDE constraints. Experiments with phantoms, bench setups, and initial plasma data at ZEI are presented. |
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TP11.00056: Insights from two-fluid modeling of linear growth and nonlinear saturation of kink and sausage modes in the Z pinch Eric T Meier, Uri Shumlak Five-moment two-fluid modeling is used to explore linear and nonlinear behavior of Z-pinch instabilities. Key features of PIC results are reproduced, opening the possibility of whole device modeling with physical fidelity comparable to kinetic simulations. Initial conditions, based on Bennett equilibria, represent experimental observed plasma conditions in the FuZE experiment. Radially sheared axial flows of various strengths are included. Growth rates of m=0 sausage instabilities are benchmarked against results from prior Hall MHD and PIC kinetic calculations. Agreement with Hall MHD results is excellent, and growth rates decrease for normalized axial wavenumbers kza>10, as observed in PIC modeling. A Braginskii-based transport model is used to capture the effects of viscosity and heat flux, including gyroviscosity and diamagnetic heat flows. Nonlinear saturation of the m=0 instability is also studied. Mode mixing due to moderate sheared flow yields a quasi-steady state after pinch ion inventory and thermal energy losses of approximately 30% and 10%, respectively. In addition, 3D modeling of linear and nonlinear behavior of m=1 kink instabilities will be presented. All modeling is done in the high-order discontinuous Galerkin WARPXM framework. |
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TP11.00057: Summary of Power Handling Systems Development at Zap Energy Matthew C Thompson, Ben Levitt, Brian A Nelson, Uri Shumlak Zap Energy Inc. studies the evolution of z-pinch plasma discharges stabilized via sheared flow. The Z-pinch configuration offers the promise of a compact fusion device owing to its simple geometry, unity beta, and absence of external magnetic field coils. In addition to a robust experimental program pushing plasma performance towards breakeven conditions [1], Zap Energy has parallel programs developing power handling systems suitable for future power plants. Technologies under development include high-average-power repetitive pulsed power, high-duty-cycle cathodes, and liquid metal wall systems. |
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TP11.00058: Improved Optical Diagnostic System for the FuZE-Q Experiment Andrew Taylor, Anton D Stepanov Spectral data from non-invasive diagnostics provide a straightforward method of determining relevant plasma parameters (ne, Ti, vi, B, etc.). Previous Ion Doppler Spectroscopy on the ZaP and FuZE experiments utilized a radial and oblique telecentric telescope to calculate Ti and vi through measuring the C III 229.7nm or O V 287.1nm lines. However, the system could not encompass the entire pinch plasma, and the telescopes were limited to a preset configuration which restricted the system's ability to measure arbitrary impurity emission lines. A new spectroscopic system that addresses the previous shortcomings will be employed for FuZE-Q. Simultaneous measurements of the z-pinch will increase from two to four with two sets of radial and oblique telescopes measuring in the x- and y-direction, respectively. Each telescope will view 15 chords to image the entire pinch plasma. In addition, a rail mount will allow the system to measure along the z-direction to provide 3D spectroscopic tomography of Ti and vi. These data will be compared to solutions from 3D reconstruction algorithms. A low-cost, swappable lens design will provide access to more emission lines, thereby increasing our understanding of impurity and ionization migration. Initial spectra and plasma parameters are presented. |
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TP11.00059: Graphite Electrode Characterization on the ZaP-HD Sheared-Flow-Stabilized Z-Pinch Device Amierul Aqil b Khairi, Bennett H Diamond, Michelle M Graebner, Aria Johansen, Timothy J Lloyd, Elliot L Claveau, Eleanor G Forbes, Hannah M Meek, Brian A Nelson, Anton D Stepanov, Tobin R Weber, Uri Shumlak Applying sheared velocity flow to the Z-pinch successfully mitigates MHD instabilities, enabling the concept to scale to high energy densities on the ZaP-HD device. This provides a unique platform for studying the plasma material interactions (PMI) of the coaxial configuration in a high temperature environment for a prolonged duration. The inner electrode is exposed to the plasma while forming a part of the discharge current path, resulting in significant erosion of the tungsten-sprayed copper nose cone and contamination of the plasma. A graphite nose cone was installed to investigate its material behavior and effect on pinch performance. Plasma self-emission spectroscopy and magnetic field probes were used to identify impurities, measure ion temperature, and determine stability of the pinch. This work lays the foundation for continued study of PMI through design of a procedure for pinch characterization, an apparatus for studying candidate materials, and extensive ex-situ surface topography measurements of targets embedded in the nose cone. |
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TP11.00060: A Suite of Diagnostics for the FuZE Stabilized Z-pinch Experiment Glen A Wurden, Thomas E Weber, Bruno S Bauer, Stephan R Fuelling, Aidan W Klemmer, Aqil Khairi, Ben Levitt, Brian A Nelson, Anton D Stepanov, Tobin R Weber We report on our ARPA-E diagnostic capability team results from measurements on the Zap Energy FuZE experiment. The experiment has recently been moved from the U of Washington to a new facility in Everett, Washington. Our diagnostics include fast gated visible imaging, fast soft x-ray imaging, multichord visible survey snapshot spectroscopy, an EUV spectrometer, visible spectral line monitors (time resolved), a very sensitive arsenic neutron activation detector (total DD neutron yields), a seven channel foil-filtered soft x-ray diode set, and a four-channel phosphor/PMT soft x-ray system. These diagnostics supplement other measurements on FuZE (particularly magnetics and doppler broadening ion measurements). Our goals are to characterize the pinch column stability, look for hot spots or thermal features, estimate the electron temperature, and study the impurity evolution, for optimized shear-flow pinch conditions. The neutron yield provides an absolute integral measurement to enable better shot-by-shot feedback to the physics operator during plasma parameter scans. |
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TP11.00061: Portable and Adaptable Neutron Diagnostics for ARPA-E Capabilities and Deployments Amanda Youmans, James Mitrani, Matt McMahon, Josh Brown, Thibault A Laplace, Bethany L Goldblum, Drew P Higginson The Portable and Adaptable Neutron Diagnostic for ARPA-E (PANDA) was developed to verify the neutron yield and presence of thermonuclear fusion. An array of plastic scintillators at different angles and LaBr3 activation detectors record neutron data from fusion experiments with yields above 5E6 neutrons. Because of the flexibility in experimental setup and analysis methods, PANDA can be used at a variety of facilities pursuing fusion in the magnetic, inertial, and magneto-inertial fusion regimes. For emission times < 100 ns, time of flight method is used to determine the neutron energy. For emission times > 1 µs, plastic scintillators are operated in pulse counting mode and the shape of a pulse integral spectrum is used to determine neutron energy. The first uses of PANDA are at a staged Z-pinch device operated by Magneto-Inertial Fusion Technologies, Inc at the UC San Diego and a sheared-flow stabilized Z-pinch device operated by Zap Energy, Inc. These experiments will demonstrate the ability of PANDA to measure neutron yields and energy-angle distributions for fusion sources to determine the presence of beam versus thermonuclear fusion. |
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TP11.00062: Overview of the Centrifugal Mirror Fusion Experiment (CMFX) Carlos A Romero-Talamás, Ian G Abel, John L Ball, Debjyoti Basu, Brian Beaudoin, Leah Dorsey, Nathan Eschbach, Adil B Hassam, Timothy W Koeth, Zachary D Short, Nicholas Schwartz We present progress on the design and construction of the Centrifugal Mirror Fusion Experiment (CMFX). The goal of this experiment is to azimuthally rotate plasmas in a mirror configuration at supersonic speeds to confine plasmas with n=1018 m-3 and Te = Ti = 0.5 keV, for at least 15 ms. A pair of superconducting magnets will enable 3 T fields at the mirror throat and up to 0.5 T at midplane. A cylindrical chamber with a length of 6.7 m and diameter of almost 0.8 m will contain a center electrode with bucket-shaped insulators to allow for applied voltages of up to 100 kV. The applied voltage will yield an azimuthal E × B drift that has been shown to create stabilizing velocity shear in prior centrifugal mirror experiments. RF plasma initiation will allow for finer control of density than that achieved by simply meeting Paschen breakdown conditions between electrodes. Ion Doppler spectroscopy and interferometric measurements of hydrogen plasmas will be initially deployed to validate analytical and numerical modeling and help design discharges with deuterium that are planned for later in the program. |
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TP11.00063: Modelling of Equilibria and Confinement for Centrifugal Mirror Machines Ian G Abel, Adil B Hassam, Timothy W Koeth, Carlos A Romero-Talamás, Nick Schwartz Due to the success of the Maryland Centrifugal Experiment (MCX) [R. F. Ellis et. al. Phys. Plasmas 8, 2057 |
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TP11.00064: Designing a Noble Gas Excimer Neutron Diagnostic for CMFX with GEANT4 John L Ball, Michael A Coplan, Carlos A Romero-Talamás, Jao-Jang Su, Timothy W Koeth The Centrifugal Mirror Fusion Experiment (CMFX) at the University of Maryland, College Park is slated to run with a deuterium plasma during the last stage of its operating plan. It is expected to produce approximately 107 - 108 D-D (2.45 MeV) fusion neutrons per discharge. The coaxial solenoidal magnets of the experiment produce a substantial stray magnetic field over the area surrounding the experiment, making traditional neutron detectors like Helium-3 tubes impractical. With input from the NASA PICASSO supported DOWSER project, we explore the use of a noble-gas-excimer-based neutron detector technology for measuring D-D fusion neutrons and building spatial neutron diagnostics. GEANT4 was used to simulate a series of detector and moderator configurations and evaluate their angular sensitivity to incident neutrons, as well as a source with the geometry of CMFX. The performance of an axial array of modular square detectors with 7.5cm thick polyethylene block moderators is evaluated. |
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TP11.00065: Ion Rotational Velocity Diagnostic for the Centrifugal Mirror Fusion eXperiment Zachary D Short, Nathan Eschbach, Brian Beaudoin, Carlos A Romero-Talamás A multi-chord ion Doppler spectroscopy (IDS) diagnostic has been designed for the Centrifugal Mirror Fusion eXperiment (CMFX), under construction at the University of Maryland, College Park. The cylindrical, mostly hydrogen CMFX plasma will be set into azimuthal E x B rotation via a radial electric field, generated by a coaxial inner electrode, and a solenoidal magnetic field. The resulting rotational velocity shear can suppress magnetohydrodynamic instabilities. Therefore, the measurement of ion rotational velocity profiles is critical to the characterization of the CMFX plasma. A fiber-optic array consisting of ten tangential viewing chords will be set up to take line-integrated spectroscopic measurements of the intensity, Doppler broadening, and Doppler shift of impurity helium emission (He II, 468.6 nm) at the midplane. The diagnostic can be moved to different positions along the axis of symmetry. Radial profiles of emissivity and rotational velocity can be obtained via an Abel-like matrix inversion of the spectra. We present the design of the IDS system and initial testing carried out on the Dusty Plasma Laboratory eXperiment (DPLX) at the University of Maryland, Baltimore County. |
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TP11.00066: Planned Electron Density Measurements with a Multiple-Beam Interferometer for the Centrifugal Mirror Fusion Experiment Nathan Eschbach, Zachary D Short, Brian Beaudoin, Carlos A Romero-Talamás Plans, designs, and methods for interferometric density measurements at the Centrifugal Mirror Fusion Experiment (CMFX) are presented. We expect axially symmetric and reproducible plasmas; a single, and eventually multiple beam, 1310 nm NIR fiber-optic interferometer will be used at CMFX. The main interferometer components will be off-the-shelf, speeding up its design and implementation, reducing overall cost, and providing modularity to all measurement beams. Utilizing a rigid 2D mechanical alignment system, and CMFX’s many near-midplane ports, line integrated density measurements will be possible at multiple axial and radial locations inside the mirror. Using Abel inversion, radial density profiles will be calculated at midplane and near the mirror throat. Density measurements at midplane and near the mirror throat will help track cone losses and validate theoretical models for confinement and stability. Methods and considerations regarding testing, installation, data interpretation, and vibration mitigation will also be discussed. |
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TP11.00067: Physics considerations in design of the Wisconsin HTS Axisymmetric Mirror Jay K Anderson, Douglass A Endrizzi, Cary B Forest, Jonathan D Pizzo, Kunal Sanwalka, Piotr Bagryansky, Alexei Beklemishev, Alexander A Ivanov, Vadim V Prikhodko, Dmitriy V Yakovlev A new magnetic mirror (WHAM) is under construction at UW-Madison with the primary mission of achieving MHD- and kinetically- stable plasmas in a low-collisionality regime, where the particle confinement increases rapidly with average ion energy. Several factors of the design benefit from expertise, experience and experimental data through collaboration with the GDT team at Budker Institute. The vessel diameter and pumping scheme (in both central and expander regions) are chosen to allow low-neutral pressure operation and minimize charge exchange losses of fast ions. Axisymmetric MHD stability is achieved via biasing end rings with respect to a central limiter (the vortex confinement scheme) and will allow modest plasmas in initial experiments, and electron temperature approaching 1 keV following the boost of the central magnetic field in the second experimental phase. Electrical and geometrical design of the electrodes follow development of similar systems in the GDT device. Scenarios have been developed for fast ion deposition via neutral beam injection and electron cyclotron resonant startup in the strong field device. |
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TP11.00068: Engineering challenges and construction status of the Wisconsin HTS Axisymmetric Mirror (WHAM) John P Wallace, Jay K Anderson, Mike Clark, Cary B Forest, Jeremiah Kirch, Grant Kristofek, Roderick E McNeill, Daniel Nash, Jonathan D Pizzo, Steve P Oliva, Alexey Radovinsky, Oliver Schmitz The Wisconsin HTS Axisymmetric Mirror (WHAM) is in the middle stages of construction at UW-Madison. The backbone of the magnetic confinement scheme is a pair of record-setting REBCO mirror coils with anticipated 17 T field at the center of 5.5 cm warm bore. Computer modelling optimizes these HTS coils spaced 1.96 meters apart. With additional pulsed copper coils (from the decommissioned W7A stellarator), 0.3 to 0.9 T fields can be produced within the central cell. At maximum field, the central cell vacuum vessel must react 35 tons of compressional force, with additional loading in off-normal events such as quenching of one superconducting coil. This work highlights the engineering challenges involved in the machine design and construction, including additional ancillary systems such as pumping, neutral beam operation and pulsed power in a high magnetic field environment. Design of auxiliary RF (26 MHz) and EC (110 GHz) heating systems are at final-review stage. A large, repurposed vessel from the Los Alamos CTX device provides expansion volumes beyond the mirror throats, required for plasma stability and control systems. Included is a report on the current status of construction, with several major components scheduled to arrive before November, 2021. |
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TP11.00069: Diagnostic Development for the WHAM Device Douglass A Endrizzi, Jay K Anderson, Cary B Forest The Wisconsin HTS Axisymmetric Mirror (WHAM) device anticipates fusion relevant densities and temperatures enabled by the combination of new high-field magnet technology, high harmonic fast wave (HHFW) heating of neutral beam fast ions, ECRH heated electrons confined by magnetic expanders, and vortex shear stabilization of MHD instabilities. An overview of the plasma diagnostics in development and how they are incorporated in the vessel will be presented, including: 1. Flux loop and magnetic pick-up coils, 2. mm-Wave interferometry, 3. Soft X-ray spectroscopy, 4. Fusion neutron detectors, 5. D-$\alpha$ arrays, 6. Fast neutral particle and RGA measurements, 7. Visible and IR imaging, and 8. Fusion proton detectors. Diagnostics provided by collaborators (Thomson scattering and Zeeman measurements from ORNL), will also be included, as well as future diagnostics under development. |
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TP11.00070: Electron Cyclotron Heating Systems and Modeling for the WHAM Device Jonathan D Pizzo, Cary B Forest, Jay K Anderson, John Lohr, Steve P Oliva, Mike Clark, Roderick E McNeill, John P Wallace The Wisconsin HTS Axisymmetric Mirror (WHAM) device will use 110 GHz Electron Cyclotron Heating (ECH) waves to generate the plasma through ECH breakdown as well as control and maintain the desired electron temperature profile. An overview of the ECH system in WHAM will be presented including the gyrotron system, the waveguide run and the launching structure. The current state of computational modeling for the ECH waves using the Genray ray tracing code and a custom particle and power balance program will also be discussed, with particular emphasis on how this combines with other heating systems to generate the desired plasma parameters and profiles. The focus will be on a high field side, fundamental X mode launch for which modeling shows >95% power absorption, however alternative launch scenarios which have shown up to 90% absorption under high density conditions are also examined. Lastly, initial results from gyrotron testing into burn paper will be shown. |
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TP11.00071: HHFW Heating in the WHAM Device Kunal Sanwalka, Cary B Forest, Jay K Anderson, Robert W Harvey, Yuri V Petrov Ion heating in the Wisconsin HTS Axisymmetric Machine (WHAM) will be done via the injection of High Harmonic Fast Waves (HHFW) designed to damp at the turning points of fast ions injected via neutral beams. Preferential damping on the beam injected ions is due to their larger Larmor orbits when compared to thermal ions. This process gives rise to a sloshing ion distribution which helps electrostatically confine a warm plasma at the center of the device. The warm plasma in the center helps with kinetic stability. This approach is expected to yield a reaction rate of 5x10^{11}n/s using 100% D. We present the latest simulation results done via Genray/CQL3D on the WHAM device as well as a potential next step volumetric neutron source (WHAM-VNS). |
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TP11.00072: Particle-in-cell Modeling of Magnetic Mirror Confinement Devices with VPIC Blake A Wetherton, Ari Le, Adam J Stanier, Jan Egedal, Cary B Forest Magnetic mirrors have recently been the subject of renewed interest, both as a neutron source and a fusion concept, in large part due to the advent of high-temperature superconductors making much stronger magnetic fields feasible to maintain and some promising results from the GDT experiment in Novosibirsk [1]. A new high-field mirror experiment is under construction at U. Wisc.-Madison. As such, computational modeling capabilities are necessary to supplement and guide ongoing experimental research. We present simulations of magnetic mirror systems in the fully-kinetic particle-in-cell (PIC) VPIC code and in the fluid electron, PIC ion Hybrid-VPIC. Fully kinetic PIC simulations of the expander region are in excellent agreement with collisionless guiding center theory [2]. Simplified two-dimensional simulations of a full device include the self-consistent ambipolar potential, a population of injected fast sloshing ions, Coulomb collisions, and DT fusion burn. Loss rates and confinement are analyzed. |
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TP11.00073: Neutron Transport Modeling for the Design of WHAM VNS Mason Yu, Kunal Sanwalka, Tim D Bohm, Cary B Forest, Jay K Anderson A compact, high flux volumetric neutron source (WHAM VNS) is being designed as a successor to the Wisconsin HTS Axisymmetric Mirror (WHAM) experiment. Because initial analysis revealed that a pair of shielded REBCO coils with radius of 45 cm would be damaged by fast neutrons within days of full-power operation, neutron shielding requirements for the HTS magnets is expected to constrain minimum device size. Therefore, CQL3D simulations were utilized to generate realistic VNS models with a target of 5 × 1017 n/s DT source rate expected from the experiment. This model was then used for the optimization of shielding geometry and material for a range of magnet sizes with a parameterized model built in OpenMC. Additional factors such as nuclear heating, long-term material damage, fast neutron activation of structural materials and access to locations of peak flux for fusion material testing were considered. Scenarios allowing for operation for more than 10 full-power years have since been identified with peak fast neutron flux at the first wall reaching 5 × 1013 n/cm2s. A preliminary design for a tritium-breeding blanket is also being developed in conjunction with the neutron reflecting shield. |
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TP11.00074: Verification of Magnetic Mirror Fusion concepts in ARPA-E BETHE using Gkeyll simulations Liang Wang, James L Juno, Manaure Francisquez, Ammar Hakim, Bhuvana Srinivasan In a Mirror Fusion device, magnetic field lines pinched to the ends of the device act like mirrors to reflect charged particles and confine the plasma towards the midplane. The ARPA-E BETHE program presently supports two Mirror Fusion concept projects in search of a low-cost solution for fusion confinement: the UMBC Centrifugal Mirror Fusion Experiment (CMFX), and the Wisconsin High-field Axisymmetric Mirror (WHAM). In CMFX, a supersonic azimuthal flow is set up by a strong radial E field to effectively stabilize, heat, and confine the plasma. WHAM, instead, focuses on using innovative plasma heating and high-field superconducting magnets to increase mirror field strength. |
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TP11.00075: The Helicity Drive Magneto-Inertial Fusion Concept Setthivoine You, Seth Pree, Paul M Bellan, Natalija Marin, Grace A Warznak, Carlos A Romero-Talamás, Shengtai Li, Hui Li Helicity Drive is a novel fusion concept [1] based on peristaltic magnetic compression [2] of a plectonemic plasma [3] pre-heated by magnetic reconnection [4,5]. A plectoneme is a double-helical Taylor state [3,4] that combines the properties of a spheromak and a shear-flow-stabilized pinch. The principle is to merge a number N>>2 plectonemes to high ion temperature and then have the merged plasma transported and compressed by an externally imposed traveling peristaltic magnetic field. The triple product scales as N^(3/2) which gives an additional adjustable parameter for achieving fusion conditions and so reduces plasma gun constraints. Our development program consists of the ECLAIR experiment currently under construction in-house, and collaborations with Caltech [6] and UMBC [7] on the peristaltic compressor, with LANL on simulations, and with PPPL on high-power solid state switch technology. |
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TP11.00076: Engineering Design and Testing of the HelicitySpace Novel Rocket Concept Natalija Marin, Grace A Warznak, Setthivoine You, Paul M Bellan, Seth Pree, Carlos A Romero-Talamás Engineering design, hardware, and test results for a novel fusion propulsion experiment proposed by HelicitySpace in collaboration with the Caltech and UMBC research teams are presented. The Helicity Drive [1] is a novel magneto-inertial fusion propulsion concept for deep space travel. Theoretical design of a peristaltic compressor [2] to adiabatically compress plasma preheated by magnetic reconnection is used to determine coil parameters for this design. The nozzle consists of twenty Bitter-type magnets that are modeled as a transmission line and are ultimately meant to compress merging plasma jets by forming a double-peaked traveling pulse. The tabletop experiment version that is being constructed at UMBC will be operated first without plasma to test and compare with theoretical predictions of the transmission line performance including system inductance, impedance, and magnetic field generation. A computer program as a signal generator, connected to an amplifier, is used to generate current waveforms in the transmission line similar to those that will be used in future experiments with plasma. |
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TP11.00077: High-β, large current amplification sustained spheromak equilibria with Mercier stable pressure profiles and plasma shaping Derek A Sutherland, Christopher J Hansen, Aaron C Hossack, Kyle D Morgan High-β sustained spheromak configurations are of interest for fusion energy applications due to the potential for high fusion power densities with modest magnetic flux densities generated primarily by internal plasma currents instead of external coil sets. For a sustained spheromak making use of magnetic helicity injection current drive (HICD), both high current amplification and efficient power coupling are needed for attractive fusion operating points with low recirculating power and reasonable engineering requirements. Driven resonant current amplification is studied in sustained spheromak equilibria with both hollow and uniform λ-profiles and Mercier stable pressure profiles. Both current profile and plasma shaping are used in a variety of flux conserver geometries to optimize sustained spheromak equilibria with high-β and large current amplification. Additionally, preliminary stability analyses of optimized equilibria will be provided. Lastly, an example use case of this framework to optimize the design of a next-generation Proof-Of-Concept (POC) sustained spheromak experiment will be presented. |
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TP11.00078: First density profile measurements of high-performance plasmas in HIT-SI3 and progress on HIT-SIU Aaron C Hossack, Kyle D Morgan, Joshua B Perry, Christopher J Hansen, Derek A Sutherland The final run campaign of the HIT-SI3 experiment at the University of Washington produced discharges with record performance, including toroidal plasma current exceeding 100 kA and the first experimental observations of peaked plasma density profiles during sustainment as measured by a new multi-chord interferometer. Beginning in 2014, the HIT-SI3 program studied application of steady inductive helicity injection, using three independent injectors driven with frequencies in the 10’s of kHz, to form and sustain spheromak plasmas [1]. The recent results were enabled by switching power amplifier (SPA) and capacitor bank upgrades and improved control of the six driver circuits (two per injector), provided by a GPU-based feedback system [2]. Initial results from the new HIT-SIU experiment, which provides additional control over the applied mode spectrum and plasma density, will also be presented. |
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TP11.00079: The role of non-axisymmetric perturbations on operations of the HIT-SI3 device Kyle D Morgan, Aaron C Hossack, Derek A Sutherland, Christopher J Hansen The HIT-SI3 device at the University of Washington uses three steady inductive helicity injectors to form and sustain spheromak plasma equilibria. Each injector is a semi-toroid connected to the main confinement volume and is operated by two sets of coils oscillating in phase to inductively drive helical magnetic structures that inject magnetic helicity which forms and sustains a toroidally symmetric spheromak equilibrium. The toroidal spectrum of the imposed perturbation has been found to be primarily dependent on the temporal phase of the helicity injection waveforms. A recently implemented control system [1] has allowed the operation of a variety of spectra involving significant amounts of n=1, n=2, and n=3 perturbations. These results are compared with several linear and nonlinear models, including extended MHD simulations using the NIMROD code and composite Taylor state equilibria computed using the PSI-Tet code [2]. |
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TP11.00080: Characterization of neutral transport in the HIT-SI3 and HIT-SIU spheromak devices Joshua B Perry, Aaron C Hossack, Derek A Sutherland, Christopher J Hansen, Kyle D Morgan Neutral gas is an important energy loss mechanism in quasi steady-state plasma confinement devices, making its control an important consideration in fusion devices. The HIT-SI3 and HIT-SIU devices study spheromak plasmas created and sustained through steady inductive helicity injection. Over the short duration of the discharge, neutral density inside the confinement volume is controlled primarily by passive pumping volumes which relieve neutral pressure at the edge. A langmuir probe is used to characterize plasma parameters inside these volumes, which are compared to a multifluid transport model to estimate their pumping rate. The HIT-SIU experiment improves on control of neutral and plasma density by using helicon sources to pre-ionize gas fueling the helicity injectors. Plasma parameters in the helicon plumes will be characterized with a langmuir probe and ion doppler spectroscopy diagnostics [1]. Additionally, neutral density will be measured with a fast ionization gauge and optical imaging of the plume shape. This will support optimization of helicon operating parameters to provide sufficient density for efficient operation of the helicity injectors while controlling plasma and neutral density in the confinement volume. |
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TP11.00081: Development of a plasma-coupled circuit model for MHD simulations of inductive helicity injection Christopher J Hansen, Aaron C Hossack, Kyle D Morgan, Derek A Sutherland Due to the complexity of current drive mechanisms in inductive helicity injection, numerical simulations are vital to understanding present results and predicting future devices. Numerical simulations of the HIT-SI(3) experiments using Hall-MHD models in the NIMROD [1] and PSI-Tet [2] codes have produced good agreement with experimental observations. However, differences remain in several important quantities (ex. mean current and magnetic profiles). Previously, experimental comparisons directly imposed the observed injector flux and current waveforms. However, such definitions imply a “high-impedance” injector current circuit in contrast to reality. Additionally, observations show significant feedback between plasma dynamics and driven circuit waveforms. Improvements to the injector boundary conditions have been developed to: 1) support “low-impedance” for both the injector flux and current and 2) enable coupling a self-consistent external circuit to the MHD model to provide a complete plug-to-plasma model. Development of these models and results from simulations will be presented, focusing on experimental comparison and validation. |
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TP11.00082: Compact, Portable Ultrashort Pulse Reflectometry (USPR) Diagnostic Calvin W Domier, R J Pereira, Jon Dannenberg, Jordan Steer-Furderer, Yilun Zhu, N C Luhmann Ultrashort Pulse Reflectometry (USPR) is a plasma diagnostic technique involving the propagation of ultrashort duration (~few nsec) chirps which contain frequency components spanning large portions of the plasma density profile. Upon reflection, each frequency component reflects from a distinct density layer. The reflected wave packet is down-converted and passed through a multi-channel filter bank, with time-of-flight (TOF) measurements made on each of the filtered wave packets. A highly portable version of this diagnostic is being fabricated for electron density profile measurements on compact, short duration devices such as spheromaks and FRCs. At the heart of the 48 channel system spanning 26.5 to 75 GHz is a field programmable gate array (FPGA) that acquires and processes data collected on each pulsed discharge. Details will be provided about the USPR diagnostic which will be installed and commissioned on the HIT-SIU device in Fall, 2021. |
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TP11.00083: Integration study of the Portable Diagnostic Package on the Helicity Injected Torus with Steady Inductance spheromak Nischal Kafle, Drew B Elliott, Zichen He, Kyle D Morgan, Aaron C Hossack, Christopher J Hansen, Zhili Zhang, Theodore M Biewer An Advanced Research Projects Agency-Energy (ARPA-E) funded Portable Diagnostic Package (PDP) with high-portability has a planned deployment to the Helicity Injected Torus - Steady Inductive Upgrade (HIT-SIU) device, located at the University of Washington, Seattle. The PDP consisting of a Thomson Scattering (TS) system and an Optical Emission Spectroscopy (OES) system has been designed at the Oak Ridge National Laboratory to travel to various plasma devices relevant to fusion along with a team of researchers. The TS and OES diagnostics are used to measure ne, Te, ni, and Ti at multiple spatial positions with varying temporal resolution. The PDP will focus on measuring these key plasma parameters on HIT-SIU after the device is reconfigured from HIT-SI3. Electron temperatures of 5-15 eV and electron densities of 1-5 x 1019 m-3 have been measured previously in the HIT-SI3 device. The implementation of the portable system along with a sensitivity analysis of the diagnostics on the HIT-SIU device will be discussed. |
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TP11.00084: A data analysis and visualization routine for a portable Thomson scattering diagnostic and optical emission spectroscopy system Zichen He, Drew B Elliot, Nischal Kafle, Holly Flynn, Zhili Zhang, Theodore M Biewer A portable diagnostic package (PDP) equipped for laser Thomson scattering (TS) and optical emission spectroscopy (OES) has been designed and constructed at Oak Ridge National Laboratory, aiming to simultaneously measure temperature and number density of electrons and ions in fusion-relevant plasma devices. The PDP receives TS and plasma emission light via a 11-by-3 optical fiber array, which provides 11 lines-of-sight for TS and OES each. An automated data analysis and visualization routine has been developed to process PDP's 11 lines-of-sight of TS and OES measurements. The analytic procedure that calculates temperature and number density of electrons includes intensity calibration for the spectrometers and detectors, different background subtraction methods for TS and OES spectra, baseline correction for OES spectra and a curve fitting method for OES spectra. Example data from recent experiments will be shown. |
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TP11.00085: Integration of a portable spectroscopy system on the PFRC-2 device Drew B Elliott, Theodore M Biewer, Nischal Kafle, Zhili Zhang, Zichen He, Holly B Flynn, Samuel A Cohen A portable spectroscopic system has been developed to measure both the ion and electron properties of multiple innovative fusion concepts. The system is designed to maximize versatility and portability because it will be deployed on multiple devices which have significantly different parameters. There are 2 spectrometers with 6 total tunable gratings. The multiple tunable gratings allow for a wide variety of spectral resolution and spectral range to be assessed depending upon the plasma conditions. Both Thomson scattering and optical emission spectroscopy are measured simultaneously on the separate spectrometers. Ion temperatures are determined via Doppler broadening of impurities while electron density and temperature are determined from Thomson scattering as well as line ratio comparisons. This system was recently deployed on the Princeton Field Reversed Configuration-2. The specifics of that deployment are described here including calibration, spatial and temporal resolution achieved, as well as initial results. |
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TP11.00086: Inferring electron temperature in warm hydrogen plasmas from Balmer series spectral line ratios using a collisional radiative model Sangeeta Vinoth, Eric Palmerduca, Drew B Elliott, Nischal Kafle, Theodore M Biewer, Zichen He, Holly B Flynn, Eugene Evans, Arthur Dogariu, Samuel A Cohen The electron temperature of warm (10-400 eV), moderate density (1e11-1e13/cc) hydrogen plasmas has been investigated using a collisional radiative model (CRM) updated from earlier work [1]. Time-resolved H Balmer series spectral line radiation from pulsed hydrogen plasmas in the PFRC-2 device were measured with two spectrometers, both calibrated with an absolute-intensity standard source. The electron temperatures were derived by comparing the measured line ratios to a CRM. In conjunction, the electron density was independently measured by an interferometer and assumptions about the plasma diameter. The CRM-inferred electron temperature was shown to strongly depend on the method for determining the strength of the emitted light, the estimated ratio of atomic to molecular hydrogen densities, and the assumed profile of the electron density. The causes of these dependences and methods to reduce the uncertainty in this technique are described. The potential applicability of this CRM to other plasma physics experiments are discussed. |
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TP11.00087: Plans for MTF demonstration facility Michel Laberge General Fusion is developing Magnetized Target Fusion (MTF). After years of developing the various subcomponents of our system, we are set to build a large integrated prototype at the UKAEA Culham Campus. The precompression plasma will be a spherical tokamak with a major radius of 0.77 m and a minor radius of 0.72 m. The initial density will be ~5x1019 m-3. Compression will be provided by a liquid lithium rotating core that opens the initial 3 m diameter cavity by centrifugal forces. An array of pneumatic cylinders surrounding the rotating core will compress the fluid to reduce the dimension of the cavity by a volume factor of 1000 in 5 ms, compressing the plasma to a density of 5x1022 m-3 and a temperature of 10 keV, achieving around 10% of the Lawson criteria. |
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TP11.00088: Design Study for the MTF Fusion Demonstration Plant Michel Laberge General Fusion is developing a novel MTF reactor concept where a spherical tokamak target is compressed by a collapsing liquid metal liner. The liquid metal serves multiple purposes as a flux conserver, plasma facing surface, neutron blanket and reactor working fluid for heat extraction. The center column of the liquid metal wall can carry enough current to allow the vessel to act as single turn toroidal field coil. The liquid liner collapse is driven by a high-pressure pneumatic array, thus using compressed gas to apply hundreds of gigawatts of heating power to the plasma target, at significantly lower cost than conventional auxiliary heating sources. |
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TP11.00089: Planned diagnostic suite of the MTF Fusion Demonstration Plant Stephen J Howard, Curtis Gutjahr, Reto Corfu, Patrick Carle, Sandra Barski, Myles Hildebrand, William Young, Akbar Rohollahi, Joshua Hawke, Ryan Zindler, Daymon Krotez, Filiberto Braglia, Matt Herunter, Alex Mossman General Fusion is developing an integrated spherical tokamak MTF prototype called the Fusion Demonstration Plant (FDP) at the UKAEA Culham Campus. Diagnostic observation of key plasma properties during compression will allow General Fusion to advance physics predictions involved in this fusion concept in a path toward a power plant. |
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TP11.00090: Low Dimensional Integrated Model for Simulating Magnetized Target Fusion Systems at General Fusion Ivan Khalzov, Daymon Krotez, Raphael Segas Integrated numerical model being developed by General Fusion (GF) simulates magnetized target fusion (MTF) systems, in which liquid metal liner compresses spherical tokamak plasma to fusion conditions. The goal of this model is to guide the current design of Fusion Demonstration Plant (FDP) and the future design of an MTF reactor. The basic approach is to find the shape of the comression trajectory of the rotating liquid metal liner depending on the applied pressure pulses and the geometrical constraints. The code solves the reduced form of axisymmetric 2D Navier-Stokes equation, similar to the shallow water approximation where the fluid free surface is determined as function of poloidal coordinates and time. On the outer side, the code is coupled with self-consistent model of driving system, which produces the pressure pulses for liner compression. On the inner side, it is coupled to a target, which is a vacuum toroidal field diffusing into the liner, or a resistively evolving Grad-Shafranov plasma equilibrium. The main advantage of the code is its speed: the simple case takes several minutes of wall-clock time, allowing for fast optimization of FDP design. Comparison of simulations with ongoing GF compression experiments and results of FDP design optimization will be presented. |
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TP11.00091: Design of a Peristaltic Compressor to Adiabatically Compress Plasma Preheated by Magnetic Reconnection Seth Pree, Setthivoine You, Natalija Marin, Grace A Warznak, Carlos A Romero-Talamás, Paul M Bellan Experiments [1] have shown that almost all of the magnetic energy dissipated during reconnection can be deposited in ions during the merger of two magnetized plasmas. A new fusion concept [2] exploits this result by proposing to merge (more than two) magnetized plasmas to preheat the ions before adiabatic compression toward fusion conditions. The magnetic compression method uses an external double-peaked, traveling magnetic field that propagates via a tapered array of magnetic coils [3]. We have designed an array of capacitors and tapered magnetic coils that together act as a delay line propagating a constant energy magnetic field. As the field travels along this delay line, the decreasing radius and separation of the coils force the field to occupy a decreasing volume V and so the magnetic field strength increases as V-1/2. We present design considerations and numerical results relevant toward the construction of a practical compressor. |
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TP11.00092: Magnetic Relaxation and Energy Transport in Reversed-Field Pinch (RFP) Computations Urvashi Gupta, Carl R Sovinec The role of pressure-gradient driven dynamics in magnetic relaxation and global thermal transport of RFPs is studied with nonlinear 3D MHD computations. It is well known that average magnetic curvature in RFPs is unfavorable, so pressure can contribute to both tearing and interchange dynamics. Several non-linear computations have been performed with the NIMROD code to span the parameter space from low to high current density. High current, low beta cases produce a tearing dominant reversed final state that resembles RFPs. Linear stability analysis of equilibrium profiles extracted from the saturated states of these computations is being applied to understand the balance of drives for different modes. Initial results show that the highest energy, low-n tearing modes may be both pressure-driven and current-driven. Also, thermal transport from second-order correlations of parallel heat flux density with magnetic fluctuations is dominated by thermal conduction, as expected. However, higher-order correlation terms significantly reduce the contribution of parallel thermal conduction in the net global thermal transport. Further analysis of linear stability and thermal transport constitutes ongoing work to better characterize the effects of pressure in RFP dynamics. |
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TP11.00093: The Lundquist number scaling of nonlinear MHD fluctuations and transition from quasi-continuous to discrete reconnection activity in MST RFP plasmas Steph Z Kubala, Daniel J Den Hartog, Craig M Jacobson, Karsten J McCollam, John S Sarff Nonlinear MHD fluctuations appear in both natural and magnetic confinement settings, such as the solar wind, self-organization dynamics in the RFP and spheromak, and current disruptions in tokamak plasmas. Here we describe parameter scaling experiments oriented toward nonlinear MHD dynamics in RFP plasmas. Experimental data have been gathered spanning a wide range of parameter space characterized by Lundquist number, S ∼ 104 −107, and density, ne/nG, where nG is the empirical density limit. A new programmable power supply allows low-current, low-S operation, which overlaps with parameters available in numerical modeling. Experimental S scalings of magnetic fluctuation amplitude agree well with those from the nonlinear MHD codes DEBS and NIMROD. A transition from quasi-continuous activity to bursty relaxation having discrete sawtooth events is observed in going from low to high S, with a threshold at around S ∼ 105. The spectral properties of the magnetic fluctuations change at this transition, including a reduction in fluctuation phase velocity that suggests plasma flow and/or flow profile changes. Momentum transport and flattening of the flow profile are known features associated with sawtooth relaxation in RFP plasmas. |
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TP11.00094: Overview of SPARC Chris Chrobak, Dan Brunner, Valeria Riccardo, Robert T Mumgaard, Matthew L Reinke, Alexander J Creely, Martin J Greenwald, James H Irby, Adam Q Kuang, Earl S Marmar, Dennis G Whyte The SPARC device is a D-T burning tokamak designed to produce net fusion power and accelerate the path to practical commercial fusion energy. The device uses novel high field, high temperature superconductors (HTS) that enable a high gain plasma (Qfus > 10) in a compact form (parameters R0 = 1.85 m, a = 0.57 m, B0 = 12.2 T, IP = 8.7 MA). The SPARC device recently achieved a major program milestone to mark the start of the construction phase for the project. A site has been secured in Devens, Massachusetts, where construction of the SPARC tokamak building and HTS magnet manufacturing facility are underway. Extensive engineering design progress has been made for all the device systems in support of achieving first plasma in 2025. Tungsten has been chosen over carbon as the plasma facing material for the divertor and limiter surfaces, and high heat flux testing of materials and tile designs are underway. Disruption physics analysis has been performed to bound the device operating space. A 2.5m tall D-shaped toroidal field model coil has been constructed and is undergoing commissioning tests at the time of this writing. In this work we will present updates to the project status and physics/engineering challenges being addressed in the design. |
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TP11.00095: Prospects for I-mode Operation in SPARC Jerry W Hughes, Amanda E Hubbard, Nathan T Howard, Pablo Rodriguez-Fernandez, Theresa M Wilks, Alexander J Creely While the SPARC tokamak is designed to meet performance objectives using conventional H-mode operation, the device is flexible enough to attempt access to the I-mode regime. On existing tokamaks I-mode has demonstrated satisfactory energy confinement coupled with low core impurity accumulation and an absence of Type I Edge Localized Modes (ELMs), making it a desirable scenario for burning plasmas. The high field path in particular stands poised to benefit, since the power requirement to access I-mode has a weaker dependence on toroidal field BT than does that for H-mode, thereby opening a significant window for I-mode operation at BT of several Tesla and above. For the specific case of SPARC, nominally at BT=12.2T, the best empirical information available suggests access to I-mode is feasible for single null equilibria with B×▽B directed away from the X-point. Like most ELM-suppressed regimes, I-mode exhibits a reduced pedestal pressure relative to conventional H-mode. We assess the impacts of this by imposing a penalty on the pedestal assumed in discharge simulations used to project SPARC performance. Finally we discuss potential for transient divertor heat loading relative to the case of Type I ELMs. |
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TP11.00096: SPARC Divertor Mission, Physics and Engineering Basis Matthew L Reinke, Adam Q Kuang, Dina Yuryev, Michael Lagieski, Trey Henderson, Valeria Riccardo, Alexander J Creely, John Canik, Jeremy D Lore, Jerry W Hughes, Martin J Greenwald A subset of the physics and engineering basis for the plasma facing components (PFCs) in the SPARC tokamak are presented. Designs are shown that support weakly dissipative commissioning scenarios, moderately dissipative, PRAD,DIV/PSOL ~ 0.5, high power, Pα+Pext = 41 MW, Qfus > 2 plasmas, and a range of highly dissipative configurations that can be used to accomplish SPARC’s advanced divertor mission. Metrics for selection of plasma facing material are summarized, describing SPARC’s decision to operate with all tungsten-based PFCs. Heat load specifications for quasi steady-state heat flux, ELMs and disruptions are presented, along with relevant results of high heat flux tungsten exposure tests and finite-element simulations of tile concepts. Disruption electromagnetic load specifications for tiles are also discussed. The goals of the SPARC advanced divertor mission are summarized, which take the form of seven open questions necessary to answer to inform ARC design. The diagnostics that support this mission are highlighted, emphasizing minimum viable requirements. SOLPS-ITER simulations of fully-detached, Ne-seeded SPARC plasmas are used to help scope the dynamic ranges needed for diagnostic design. |
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TP11.00097: Equilibrium studies of the SPARC X-Point Target divertor Josiah T Wai, Matthew L Reinke, Adam Q Kuang, Pablo Rodriguez-Fernandez, Darren T Garnier, Egemen Kolemen An objective for the SPARC advanced divertor mission is to experimentally investigate the X-Point Target (XPT) divertor concept at reactor-relevant heat fluxes, This scenario may prove to be generally useful in fusion pilot plants for mitigating the damaging power and particle exhaust to the divertor, and is under consideration for ARC. The XPT features a radially extended divertor leg with a secondary x-point placed near the target plate, within one SOL heat flux width in the common flux. In this work we perform equilibrium design studies for identifying methods for entering the XPT configuration on SPARC. Lower null XPT and double null XPT scenarios are considered. Accounting for SPARC’s superconducting coil ramp rates and current limits, we present coil current trajectories and placement limits of the secondary x-point. We discuss the heat fluxes introduced on the divertor surfaces through the transition as the secondary x-point crosses the limiter into the main chamber. The flux separation between the primary and secondary x-points—which may be critical for power balance—is found to be sensitive to the coil currents, and control strategies are discussed. |
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TP11.00098: Design of a high-resolution x-ray crystal spectrometer for SPARC Conor J Perks, John E Rice We present a scoping study conducted to characterize the performance of a high resolution, x-ray crystal spectrometer intended to measure ion temperature and toroidal rotation in the SPARC tokamak. Given the expected electron temperature and density profiles, we have chosen some candidate transitions in seeded Xe, Kr, and intrinsic W (tungsten walls). Multiple lines need to be used to provide complete coverage which would require changing the crystal and Bragg angles. Emissivity profiles for the selected line radiation were modelled using the Aurora code and benchmarked against FLYCHK. Spectrometer geometry and choice of diffraction crystal were also considered to estimate the expected signal at the detector. Beryllium windows needed to be placed along the beamline due to D/T compatibility, so number and thickness was scoped to avoid significant loss of signal. Dose at the crystal had to also be evaluated due to the high neutron flux environment of a burning plasma. This diagnostic is intended to be installed and ready for SPARC operations day one |
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TP11.00099: Inferring neutral density profiles in opaque plasmas at high magnetic field on Alcator C-Mod Marco A Miller, Jerry W Hughes, Aaron M Rosenthal, Francesco Sciortino, Saskia Mordijck, Richard M Reksoatmodjo, Tomas Odstrcil, Matthew L Reinke An analysis workflow for inferring neutral density profiles from Alcator C-Mod deuterium Lyman-alpha (Ly-a) measurements has been developed for comparison with edge modeling codes. Ly-a data present a unique opportunity to investigate how deuterium neutrals affect the formation of the edge pedestal. High densities made possible by high fields on C-Mod allow study of edge neutral opacity on neutral penetration, important for next-generation devices. We present inferences of neutral density and ionization source profiles spanning a region of ~6cm about the last closed flux surface (LCFS). A tomographic inversion technique is used to calculate local neutral emissivity from line-integrated Ly-a brightness measurements at the outer midplane. This is combined with measurements of electron density and temperature from the edge Thomson scattering system, as well as scanning Langmuir probes when available. The two-point model is used to identify the position of the LCFS and shift the kinetic profiles. Uncertainty quantification techniques propagate errors from measurements to inferred profiles. An extensive database of these profiles, along with kinetic and neutral emissivity profiles, enables comparisons in different confinement regimes with edge modeling codes like KN1D and SOLPS-ITER. |
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TP11.00100: Deep collisional-radiative modelling of edge turbulent electron density and temperature fluctuations using gas puff imaging on Alcator C-Mod Abhilash Mathews, James L Terry, Jerry W Hughes, Seung Gyou Baek, Daren P Stotler, Detlev Reiter, Wladimir Zholobenko, Motoshi Goto, Stewart J Zweben, Adam Q Kuang Measuring turbulent ne and Te fluctuations in the edge of magnetic confinement fusion devices is essential towards better diagnosing blobby transport and the overall applicability of edge plasma turbulence modelling. For this task, the gas puff imaging (GPI) diagnostic on Alcator C-Mod can capture visible light (587.6 nm) arising from the dynamic interaction of edge plasma turbulence with neutral helium. The Phantom camera used for GPI resolves HeI emission on a field-aligned 2-dimensional (R,Z)-grid with a spatial resolution of approximately 1-2 mm and temporal resolution of 2.5 µs. But these GPI measurements are only indirectly connected to ne and Te. For deeper analysis of the turbulent fluctuations contributing to the observed single line emission, the tenets of collisional radiative (CR) theory for atomic helium in the edge of high field tokamaks are reviewed, and we invert the CR model in a novel physics-informed numerical optimization framework to convert GPI signals into consistent experimental measurements of the turbulent ne and Te. This poster will present results from this computational analysis along with various time-dependent perturbations and spatial constraints on HeI (e.g. ionization, drifts) to evaluate their impact on interpreting plasma fluctuations. |
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TP11.00101: Electron cyclotron emission diagnostics for the Compass Upgrade tokamak: preliminary quasi-optical design Saeid Houshmandyar, William L Rowan, Jaromir Zajac, Viktor Veselovsky, Vladimir Weinzettl Compass Upgrade (Compass-U) is a medium size (R = 0.894 m, a = 0.27 m) and high field (BT ? 5 Tesla, Ip ? 2 MA) tokamak that is capable of addressing key challenges in magnetic fusion including power exhaust and advanced confinement regimes. Compass-U has recently entered the final design phase and is expected to have its first plasma in 2023. Electron cyclotron emission (ECE) will be available among first diagnostics and will provide measurements of high spatial and temporal resolution of electron temperature profile evolution through a radial view; a separate oblique view at 12° from normal will be installed to study non-thermal electrons. Both the radial and oblique views will be located in the wide-angle port which has dimensions that enable simultaneous hosting of the front-end of the quasi-optical (QO) paths for the two views. Each QO design will have an in-situ hot calibration source in the front-end to provide a stand-alone and calibrated Te (R,t) measurements. Details of the diagnostic channels and beam resolution within the plasma will be presented. |
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TP11.00102: Can an Underdog Pulsed Power FNSF Compete with the Steady State Version? Jeffrey P Freidberg, Luca Guazzotto A fusion nuclear science facility (FNSF) is an important component on the path to a fusion reactor. It must establish the materials and component data base needed for a power reactor in a fully integrated facility that can be directly extrapolated to a reactor. The FNSF is not required to produce net electricity, but it must be considerably smaller (i.e. less expensive) than a full power producing reactor. Serious studies by the USA FNSF team based on conservative physics and engineering principles resulted in a detailed design for a steady state device operating over month long pulses. The cost and technological complication of the current drive system has motivated us to reexamine the desirability of a no-current-drive, short pulse (e.g. 1 hour) FNSF. Recent development of (a) high field, high temperature REBCO superconducting tapes, (b) advanced blanket design, (c) demountable joints in the TF coil for ease of component replacement, and (d) 3-D advanced manufacturing techniques open the possibility for a technologically attractive pulsed design. High field in particular should allow a smaller bore OH transformer to be used, leading to relative improvements in pulsed systems. Results of a high level comparative analysis between a pulsed and steady state FNSF will be presented. |
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TP11.00103: Development of Compact Reactor Use Cases to Inform Transport Studies Christopher G Holland, Xiang Jian, Eric M Bass, Dmitriy M Orlov, Joseph Mcclenaghan, Brendan C Lyons, Nathan T Howard, Pablo Rodriguez-Fernandez The OMFIT STEP integrated modeling workflow has been used to develop self-consistent core plasma use case scenarios for compact inductive and steady-state tokamak fusion reactors. The initial use cases target an up-down symmetric reactor able to produce 200 MW or more net electric power with B0 = 8 T, R = 4 m, a = 1.4 m, elongation κ = 2, triangularity δ = 0.5, and Zeff ~ 2. For this starting work, only the core plasma is modeled without inclusion of divertor or wall geometry, and only a simple model of electron heating and current drive sources is utilized. Starting from pedestal pressure predictions made with EPED, the workflow iterates between the CHEASE equilibrium solver, ONETWO transport code, and TGYRO transport solver to develop self-consistent core plasma scenarios. Both an inductive scenario with Ip = 16 MA, Paux = 50 MW, q95 ~ 4.9, fbs ~ 0.3, βN ~ 2.3, Qfus ~ 25, H98,y2 ~ 0.9 and steady-state scenario with Ip = 12 MA, Paux = 95 MW, q95 ~ 6.5, fbs ~ 0.65, βN ~ 3.2, Qfus ~ 13, H98,y2 ~ 1.3 are capable of meeting the power production goal for nped/nGW ~ 0.8. Details of dominant transport mechanisms, as well as benchmarking of TGLF and CGYRO predictions for these parameters, will be presented. |
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TP11.00104: Initial Boron Powder Injection Experiments on WEST Grant Bodner, Alessandro Bortolon, Clarisse Bourdelle, Ahmed Diallo, Alberto Gallo, Christophe Guillemaut, Jamie P Gunn, C.Christopher Klepper, Robert A Lunsford, Philippe Moreau, A. Nagy, Francis-Pierre Pellissier, Emmanuelle Tsitrone, E.A. Unterberg, Laure Vermare Boron powder (< 150 mm) was injected at various drop rates into lower single null (LSN) L-mode discharges in WEST, using a recently installed impurity powder dropper (IPD) developed at PPPL. IPDs provide real-time wall conditioning of plasma-facing components without the use of diborane gas. The long-pulse capabilities of WEST and the full-W environment make it an excellent testbed for evaluating the viability of the IPD at reactor relevant timescales. The discharges presented featured Ip ~ 0.5 MA, tpulse = 12-30 s, ne,0 ~ 4x1019 m-3, and PLHCD ~ 4.5 MW. During powder injection, a clear reduction in SOL D I and low-Z intrinsic impurity line intensities was observed, suggesting a reduction in recycling and possible screening of low-Z impurities. Concurrently, the plasma stored energy increased up to 25% depending upon the powder drop rate. The largest increases in stored energy were achieved at the highest drop rates (9-17 mg/s), possibly due to the large reduction in divertor recycling. Initial analysis suggests these increases in stored energy may be the result of stabilized ion temperature gradient (ITG) modes due to fuel dilution similar to gaseous impurity seeding experiments. |
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TP11.00105: Design and synthetic diagnostic of a multi-energy hard x-ray camera for profile measurements at WEST tokamak Tullio Barbui, Luis F Delgado-Aparicio, Yves Peysson, Brentley C Stratton, Gregory M Wallace, Oulfa Chellai, Kenneth W Hill, Novimir A Pablant The WEST tokamak is currently being prepared for long pulse operation with a water-cooled full tungsten divertor. Heating will be provided by radiofrequency systems, including lower hybrid current drive (LHCD). In this context the Princeton Plasma Physics Laboratory has built a compact multi-energy hard x-ray camera (ME-HXR) for space and energy-resolved measurements of the electron temperature, the fast electron tail density produced by LHCD and runaway electrons, and the beam-target emission of tungsten at the edge due to fast electron losses interacting with the target. |
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TP11.00106: A refined spectroscopic assessment of in situ net erosion at WEST Davis Easley, David C Donovan, Alex GROSJEAN, Christophe Guillemaut, Jamie P Gunn, Curtis A Johnson, Chris Klepper, Ezekial A Unterberg As part of the ongoing validation of tungsten (W) as a high-Z plasma facing component (PFC), in situ observation of net W erosion into the scrape-off layer is critical to ensuring the longevity of tokamak fusion reactors. Net erosion may be simply defined as the difference between gross erosion and prompt re-deposition. Under this reduced paradigm, W-I (400.87 nm) and W-II (434.81 nm) spectroscopic emission lines can be converted to particle fluxes of gross erosion and prompt re-deposition with the aid of ionization per photon (or S/XB) ratios. Here, we present preliminary in situ calculations of net erosion at the limiter and divertor of the W Environment in Steady-state Tokamak (WEST), an all-W-PFC device that allows for poloidally resolved study of net W erosion in the absence of all but intrinsic impurities. Central to this study is a spectroscopic fitting routine that is shown to reduce uncertainty in the fitting function parameters (necessary to extract W radiances) by time-averaging the spectral data before fitting compared to previously reported approaches at WEST, which time-average the fitted data. The uncertainty refinement is particularly important for off-divertor PFCs where small line emission over continuum is typical. |
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TP11.00107: Assessment of tungsten impurity concentration in the all-metallic environment of the WEST during experiments based on integrated multi-diagnostic analysis. Alex GROSJEAN, David C Donovan, C.Christopher Klepper, E.A. Unterberg, Davis Easley, Curtis A Johnson, James P Gunn, Christophe Guillemaut, Mehdi Firdaouss, Tennessee Radenac WEST is an actively cooled, long-pulse tokamak with nearly all plasma-facing components (PFC) made of tungsten (W). One of the aims of WEST is to study plasma operations with W PFCs in preparation for long-pulse operations on W divertor devices like ITER. For long-pulse operation, the W impurity content and transport to the core plasma are critical concerns that require further measurement and interpretation in order to improve plasma performance and PFC durability. This work details the overall impurity influxes in WEST by characterizing sources at the antennas and at the divertor during a series of discharges in which the Lower Hybrid (LH) injected power was incrementally increased during the C4 (2018) and C5 (2019) experimental campaigns. Visible spectroscopy was utilized to measure the spectral radiances generated by fuel particles (deuterium) and impurities (e.g., W, oxygen (O), boron (B)) at both locations. The needed SOL plasma conditions (Ne, Te) to evaluate the S/XB coefficients required to estimate the impurity fluxes obtained with a collisional-radiative model (ColRadPy) are measured at the divertor target with Flush Langmuir probes and near the OMP using reciprocating Langmuir probes. The array of edge diagnostics have identified an increase of O and W impurities during the C5 campaign, which is likely induced by the introduction of boron nitride (BN) limiters. The intensity of neutral W emission (W-I) near the divertor relative to the antenna region for the two campaigns will also be discussed with regard to identifying the dominant sources of impurity production. |
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TP11.00108: Interpretive Modeling of the Tungsten Source Distribution at the Divertor and Baffle Regions of the WEST Tokamak Jake Maeker, Jake H Nichols, David C Donovan, David C Easley, Alex GROSJEAN, Jamie P Gunn, Christophe Guillemaut, C.Christopher Klepper, E.A. Unterberg Preliminary simulations of tungsten (W) impurity transport in the WEST tokamak suggest that the baffle structure may be a significant source of W impurities in the plasma due to lower plasma density causing reduced prompt redeposition. A lower hybrid (LH) power scan is investigated, during which the injected power was increased from approximately 0.5 MW to 4.8MW. A grid is constructed extending to flux surfaces terminating at the lower divertor surface. The OSM-EIRENE code is used with divertor Langmuir probe data to create a background plasma model for these discharges. The impurity transport code DIVIMP is used to simulate transport of W impurities, verified against W-I and W-II spectroscopic data. W leakage probabilities out of the divertor and into the core are calculated for both divertor sources. Concurrently, an extended grid is constructed including flux surfaces extending to the baffle structure and upper divertor. A new DIVIMP model is used to simulate transport of W impurities on this extended-grid background and the results are compared to the traditional grid case. The comparison illustrates the relative importance of W transport near the baffle structure and its effects on global W impurity transport in WEST. |
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TP11.00109: Edge-Localized Mode Detection and Correlation with Rotating MHD modes for Disruption Event Characterization and Forecasting Jalal Butt, Steven A Sabbagh, John Berkery, Young-Seok Park, Juan D Riquezes, Veronika Klevarova, Yanzheng Jiang, Jun-Gyo Bak, J.W. Lee, H.S. Han, Jayhyun Kim, K. D Lee, Si-Woo Yoon, Juhyeok Jang, Mark D Boyer, Keith Erickson Edge-Localized Modes (ELMs) are transient instabilities that eject heat and particles from the edge of a tokamak plasma onto its walls. While typically not directly disruptive, ELMs can trigger more detrimental plasma instabilities that can disrupt plasma confinement. ELM identification is hence an important capability to determine the threat ELMs pose to plasma termination. The Disruption Event Characterization and Forecasting (DECAF) code works to resolve, characterize, and forecast event-chains that lead to disruptions, including disruptive event “seeds”. A newly developed DECAF capability to robustly and reliably identify ELMs using several plasma signals as input is presented. The detection algorithm uses D⍺ light to find D⍺ emission transients and electron temperature profiles to critically distinguish edge-localized events from global ones by processing the profile evolution through the mode dynamics. The presented ELM detection capability was validated on a database of ELMing and non-ELMing KSTAR and NSTX plasmas. Further, using the DECAF event chain analysis framework and its existing rotating MHD event module, we apply the novel ELM identification capability to preliminarily study the extent of correlation between ELM-events and rotating MHD-events. |
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TP11.00110: Predicting plasma pressure profiles with Gaussian process and a neural network in KSTAR based on magnetic signals MINSEOK KIM, Semin Joung, W.H. Ko, J.H. Lee, Young-chul Ghim As an equilibrium plasma is a force-balanced state, i.e., J×B = ∇p, in a tokamak, it is conceivable that magnetic signals, perhaps, can be used to infer the pressure profile. If such an inference can be reliably performed in real time, then this will greatly advance tokamak operations, especially for future fusion power plants where minimal number of diagnostics are only available. Thus, we have performed feasibility tests on predicting pressure profiles solely based on control magnetic signals by using a neural network. The neural network is trained with KSTAR pressure profiles which are obtained from Thomson Scattering (TS) and Charge Exchange System (CES) diagnostics where the Gaussian process is applied to the measured data. The neural network takes in-vessel coil currents, poloidal field current and plasma current as inputs and outputs the pressure profile. We present our preliminary results on our proposed scheme of predicting pressure profiles and discuss possibility of using the scheme for tokamak operations. |
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TP11.00111: Kinetic Equilibrium Reconstruction of KSTAR and the Impact on Stability Analysis of High Performance Plasmas Yanzheng Jiang, Steven A Sabbagh, Young-Seok Park, John Berkery, Jae Heon Ahn, J Riqueze, Jun-Gyo Bak, Wonha Ko, Jinseok Ko, Jongha Lee, Si-Woo Yoon, Alan H Glasser, Zhirui Wang High fidelity equilibrium reconstructions are an essential analysis for the accurate determination of physics-based models of plasma operational limits (e.g. density limits, power balance, stability) for disruption event characterization and forecasting analysis (DECAF). The safety factor is a key plasma profile used to optimize the confinement and determine the stability of a tokamak plasma. Kinetic equilibrium reconstructions include constraints from Thomson scattering and ion temperature from charge exchange spectroscopy diagnostics, as well as magnetic field pitch angle profile constraints diagnosed by motional Stark effect (MSE) to produce a reliable computation of the safety factor, q, profile. Low q shear near the pedestal and slightly negative shear in the plasma core are found by both polynomial and spline basis function models applied to represent the toroidal plasma current profile for high non-inductive plasmas in KSTAR. MHD stability analyses using the DCON and resistive DCON codes utilize these kinetic equilibrium reconstructions to compare to the experimental plasma stability. The stability analyses using kinetic equilibria from various models show sensitivity to the local q low shear, qmin, and pressure pedestal profile |
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TP11.00112: Refinement of Control-Oriented Model via TRANSP-Based Transport Analysis to Enable Systematic Model-based Scenario Planning in EAST Zibo Wang, Eugenio Schuster, Tariq Rafiq, Yao Huang, Zhengping Luo, Qiping Yuan, Bingjia Xiao, David A Humphreys The development of advanced plasma scenarios in EAST can benefit from a more systematic model-based approach to scenario planning. As originally proposed in [1], in this scenario-planning approach a plasma-response model is combined with an optimization algorithm to determine the feedforward control inputs (actuator waveforms) that are needed to achieve a desired scenario. Both plasma-state and actuator constraints are taken into account when solving the optimization problem. However, the quality of the solution of the optimization problem is highly correlated to the quality of the plasma-response model used by the optimizer to predict the behavior of the plasma. However, since all the computations are carried out off-line before the experiment, this optimization approach to scenario planning is capable of dealing with plasma-response models of arbitrary complexity. In this work, a refined plasma-response model combining the Magnetic Diffusion Equation with either empirical scalings or transport equations for the electron temperature and density is developed and used for optimal feedforward-control design in EAST. The refinement of this physics-driven model is enabled by TRANSP-based transport analysis of dedicated plasma-response characterization experiments where the equilibrium reconstruction has been constrained by measurements from the POlarimeter-INTerferometer (POINT) system. |
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