Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session BO03: Stellarators and Helical Systems: W7-X, LHD, HSX, CTH, and OthersLive Streamed
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Chair: Andrew Ware, University of Montana Room: Ballroom 100 C |
Monday, October 17, 2022 9:30AM - 9:42AM |
BO03.00001: Kinetic Ballooning Modes in Wendelstein 7-X high-performance experiments Ksenia Aleynikova, Christian Brandt, Alessandro Zocco, Carolin Nuehrenberg, Adrian von Stechow, Joachim Geiger, Golo Fuchert, Novimir A Pablant, Kian Rahbarnia, Henning Thomsen, Paul Mulholland, M.J. Pueschel, Josefine Proll Wendelstein 7-X stellarator (W7-X) aims to demonstrate steady state operation at high β (ratio of kinetic to magnetic pressure) values. This implies high plasma densities up to 2.1020 m-3 since the confinement scales beneficially with the density. In recent W7-X experiments, injection of hydrogen pellets was successfully applied for core fuelling [1]. The realization of such densities is complicated due to a limited control of the profile shape. During high-β phases of these discharges MHD-like events were observed, which may indicate a stability limit. In addition, linear GENE simulations suggest that the density and temperature gradients in that phases were large enough to destabilise kinetic ballooning modes (KBMs). Although these plasmas are stable to ideal-MHD instabilities, including ballooning modes, gyrokinetic effects on the latter render them unstable. |
Monday, October 17, 2022 9:42AM - 9:54AM |
BO03.00002: A reduced-turbulence regime in the Large Helical Device upon injection of low-Z materials powders Federico Nespoli, Kenji Tanaka, Suguru Masuzaki, Naoko Ashikawa, Mamoru Shoji, Erik P Gilson, Robert A Lunsford, Tetsutaro Oishi, Katsumi Ida, Mikiro Yoshinuma, Yuki Takemura, Toshiki Kinoshita, Gen Motojima, Masaki Osakabe, Naoki Kenmochi, Gakushi Kawamura, Chihiro Suzuki, Alexander Nagy, Alessandro Bortolon, Novimir A Pablant, Albert V Mollen, Naoki Tamura, David A Gates, Tomohiro Morisaki Recently an improved confinement regime with reduced turbulent fluctuations was observed in the Large Helical Device upon injection of boron powder into the plasma [F. Nespoli et al., Nature Physics 2022]. Dynamic transport analysis and modeling of neoclassical fluxes suggest the confinement increase is due to reduced turbulent transport. |
Monday, October 17, 2022 9:54AM - 10:06AM |
BO03.00003: Studying the neutral gas flow in detached W7-X discharges using EMC3-EIRENE Dieter Boeyaert, Heinke G Frerichs, Oliver Schmitz, Felix Reimold, Victoria Winters, Yuhe Feng, Dirk Naujoks, Thierry Kremeyer During operation of W7-X a key point is to pump the neutrals out of the experiment. Therefore, maximizing the neutral pressure in the subdivertor is important. The present neutral particles are generated due to plasma-wall interaction at the divertor targets. In ref. [1] it is shown for attached conditions that only a small part of the neutral particle flux is going through the pumping gap. Most particles recycle back into the plasma. On top, a part of the particle flux going through the pumping gap leaks back into the divertor chamber decreasing the subdivertor neutral pressure. While increasing the plasma density and going in that way from attachment to detachment, experiments during the campaign of 2018 of ref. [2] show that the subdivertor pressure builds up, stagnates, and drops again. The compression ratio decreases during this transition. A dedicated study of the physics behind detachment in W7-X was performed in ref. [3]. Starting from these findings, this contribution focuses on examining the dynamics of the pressure evolution to understand what it depends on and how it scales. In a first step, the neutral fluxes at the pumping gap are studied. In all simulations, special attention is put on ensuring a small numerical error on the simulated neutral quantities. |
Monday, October 17, 2022 10:06AM - 10:18AM |
BO03.00004: Optimizing stellarators for turbulent impurity and energy transport using linear gyrokinetic simulations Stefan Buller, Matt Landreman, Rahul Gaur In neoclassically optimized stellarators, turbulence may be needed to avoid impurity accumulation. Comparisons between standand and turbulence reduced scenarios in Wendelstein 7-X indeed show order of magnitude higher impurity concentrations in the latter scenarios (A. Langenberg et al 2021 Nucl. Fusion 61 116018). As lower levels of turbulences generally correlate with increased energy confinement, this poses a potentially contradictory set of requirements on the optimization of stellarator geometries. It remains to be seen if it is possible to design stellarator magnetic fields that simultanously achieve high enough outward impurity transport and low enough energy transport. |
Monday, October 17, 2022 10:18AM - 10:30AM |
BO03.00005: Design of an arrangement of cubic magnets for a quasi-axisymmetric stellarator experiment Kenneth C Hammond, Caoxiang Zhu, Keith Corrigan, David A Gates, Robert Lown, Robert Mercurio, Tony Qian, Michael Zarnstorff The usage of permanent magnets to shape the confining magnetic field of a stellarator has the potential to reduce or eliminate the need for non-planar coils. As a proof-of-concept for this idea, we have developed a procedure for designing an array of cubic permanent magnets that works in tandem with a set of toroidal-field coils to confine a stellarator plasma. All of the magnets in the design are constrained to have identical geometry and one of three polarization types in order to simplify fabrication while still producing sufficient field accuracy. We present some of the key steps leading to the design, including the geometric arrangement of the magnets around the device, the procedure for optimizing the polarizations according to the three allowable magnet types, and the choice of magnet types to be used. We apply these methods to design an array of rare-Earth permanent magnets that can be paired with a set of planar toroidal-field coils to confine a quasi-axisymmetric plasma with a toroidal magnetic field strength of about 0.5 T on axis. |
Monday, October 17, 2022 10:30AM - 10:42AM |
BO03.00006: Stellarator profile predictions using Trinity3D and GX Tony Qian, Braden Buck, Rahul Gaur, Noah R Mandell, Patrick S Kim, William D Dorland We present stellarator transport calculations using a first-principles gyrokinetic approach, based on a new multi-scale transport framework composed of the Trinity3D transport solver and the GX gyrokinetic code. GX computes the full nonlinear turbulent fluxes from the gyrokinetic equations, using a pseudo-spectral formulation. Trinity3D leverages an implicit time step to model transport scale evolution from micro turbulent scale fluxes. It recomputes the equilibria by using self-consistent profiles in VMEC. Our model includes alpha heating, collisional equilibration between species, and bremsstrahlung radiation. Since Trinity solves the time dependent power balance equation, it is also possible to introduce time dependent sources, for example experimental auxiliary heating. We will present our attempt to explain the ion-clamping phenomena observed experimentally in W7X, and how it is alleviated by peaking the density profile. We will also present simulations of a hypothetical stellarator reactor heated to ignition. |
Monday, October 17, 2022 10:42AM - 10:54AM |
BO03.00007: Optimizing stellarators to the infinite-n ideal ballooning mode with an adjoint method Rahul Gaur, Stefan Buller, Matt Landreman, William D Dorland We have developed an infinite-n, ideal-ballooning solver for general 3D MHD equilibria. This solver is based on the numerical scheme developed by Sanchez et al for the ballooning code COBRAVMEC. Unlike COBRAVMEC, our code also scans each surface for multiple values of the ballooning parameter to find the maximum growth rate. We present tests for various 2D and 3D equilibria. |
Monday, October 17, 2022 10:54AM - 11:06AM |
BO03.00008: Contributions of fluctuation-driven Poynting flux to the energy transport in a self-organized reversed-field pinch plasma Derek J Thuecks, Karsten J McCollam Fluctuation measurements reveal the outward electromagnetic energy flux needed to drive the dynamo EMF supporting magnetic self-organization in a reversed-field pinch plasma. The radial Poynting flux due to tearing mode fluctuations is measured with an insertable probe during magnetic relaxation. This flux corresponds to transient power levels much larger than the input power. A simple Poynting's theorem model for an incompressible, resistive MHD plasma with resistive boundary is developed, predicting that the fluctuation-induced Poynting flux out of the plasma corresponds approximately to the power lost from the equilibrium magnetic field due to the dynamo EMF. In RFP experiments on the MST device, probe measurements of this flux are roughly as predicted by the model upon substitution of time-resolved equilibrium measurement data. Nonlinear MHD simulations using the NIMROD code are planned to examine this transport process. |
Monday, October 17, 2022 11:06AM - 11:30AM |
BO03.00009: Analysis and modeling of densification during startup of PFRC-2 Eugene S Evans, Charles P Swanson, Sangeeta P Vinoth, Eric Palmerduca, Gabriel A Gonzalez, Samuel A Cohen Understanding the startup behavior of the PFRC-2 by odd-parity rotating magnetic fields is critical to producing discharges at high magnetic fields with high density ($n_{e}>10^{13}$/cm$^{3}$), temperature ($T_{e},T_{i}>100$~eV), absorbed power fraction ($>0.5$), and reproducibility. Experiments on the PFRC-2 have shown the existence of two distinct phases of density rise: a slow rise to $1-3\times 10^{11}$/cm$^{3}$ ($\tau\sim 150$~$\mu$s) followed by an abrupt transition to a rapid brief density rise to $5\times 10^{12}$/cm$^{3}$ characterized by $\tau\sim 5-7$~$\mu$s. At lower field, higher pressure, and higher power conditions, rapid densification phase may occur in less than 50~$\mu$s; the opposite conditions result in a longer delay, exceeding 3~ms for certain gases. The variation in delay before the onset of rapid densification increases with the delay time. A zero-dimensional global balance model which captures relevant atomic, molecular, and plasma processes has been developed to compare against the experimental data, and guide investigations into which processes drive plasma behavior before and during rapid densification. Empirical scaling laws and implications for high-power PFRC-2 operation and future PFRC-based reactor design will be discussed. |
Monday, October 17, 2022 11:30AM - 11:42AM |
BO03.00010: Fundamental Scaling of Adiabatic Compression of Field Reversed Configuration Thermonuclear Fusion Plasmas David Kirtley Helion’s Trenta prototype compressed Field Reversed Configuration (FRC) plasmas to thermonuclear fusion conditions, reaching 9 keV plasma temperatures [1]. FRC plasmas are fundamentally high-beta and if heated through pulsed, adiabatic compression, they operate in a unique collisionality regime that supports both thermonuclear fusion conditions as well as maintains an ion to electron temperature ratio. |
Monday, October 17, 2022 11:42AM - 11:54AM |
BO03.00011: TriForce algorithms for enabling long-time-scale FRC plasma simulations Ayden J Kish, Michael J. Lavell, Robert Masti, Andrew T Sexton, John G Shaw, Adam B Sefkow TriForce is a computational environment using a hybrid fluid-kinetic model to execute higher-fidelity simulations in shorter time frames. At its core are a particle-in-cell model and a meshless hydrodynamic model that can be coupled to perform modeling across multiple spatiotemporal scales and approximation regimes. The key to performing these calculations over large numbers of time steps is the management of accumulating numerical error. Presented here are details regarding the implementation and verification of multiple new packages in the TriForce Library for Integrated Numerical Kinetics, including those for active particle statistics management, charge conservation, and 2nd-order particle-force interpolation. Additionally, an update is given on fully-integrated electro-magnetic PIC simulations of PFRC, a field reversed configuration that demonstrates plasma heating by odd-parity rotating magnetic fields. |
Monday, October 17, 2022 11:54AM - 12:06PM |
BO03.00012: Implementation and Validation of Collisions in a New Particle-in-Cell Code Michael J. Lavell, Ayden Kish, Andrew T Sexton, Robert Masti, John G Shaw, Adam B Sefkow Elastic and inelastic collisions in a new particle-in-cell (PIC) code are computed with the binary Monte Carlo scattering method and used in plasma simulations of the Princeton Field Reversed Configuration (PFRC). We discuss the implementation of Coulomb scattering, neutral scattering, electron impact ionization, and fusion with shared memory and Graphical Processing Unit acceleration. We measure the simulated electrical conductivity in solid density copper, stopping of relativistic electrons in solid aluminum targets, plasma densification due to the breakdown of helium by an applied electric field, and neutron spectrums generated by thermonuclear and beam-target fusion numerical tests. An update is given on fully-integrated electromagnetic PIC simulations of PFRC, a field reversed configuration that demonstrates plasma heating by odd-parity rotating magnetic fields.
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Monday, October 17, 2022 12:06PM - 12:18PM |
BO03.00013: Pressure driven dynamics and global energy transport in finite beta RFP computations Urvashi Gupta, Carl R Sovinec Finite-beta visco-resistive non-linear MHD computations are applied to self-consistently model RFP relaxation and energy transport. Linear resistive MHD analysis of the dominant fluctuations from the quasi-steady conditions shows that these RFP equilibria are tearing unstable. RFPs have bad curvature implying that pressure has a role to play in tearing dynamics. To assess the pressure drive, linear eigenfunctions are used to compare stabilizing and destabilizing contributions to the perturbed kinetic energies. While most of the stabilizing contribution from the field-line bending force is annihilated by resistive diffusion, the parallel current term alone does not drive tearing. Pressure drive is always comparable and provides a significant destabilizing contribution for tearing modes. Fluctuation induced outward energy transport from the core of the RFP comes largely from parallel thermal conduction. Second order correlations of magnetic fluctuations and parallel heat flux however, do not result in net outward energy transport at all radii. Taking into account higher order correlations shows that third order correlations are larger and cancel the inward transport from quadratic correlations near the reversal surface. |
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