Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session VI2: MHD of Magnetic Confinement Systems |
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Chair: Richard Fitzpatrick, University of Texas Room: Bissonet |
Thursday, October 30, 2014 3:00PM - 3:30PM |
VI2.00001: High Performance Field Reversed Configurations Invited Speaker: Michl Binderbauer The field-reversed configuration (FRC) is a prolate compact toroid with poloidal magnetic fields [1]. FRCs could lead to economic fusion reactors with high power density, simple geometry, natural divertor, ease of translation, and possibly capable of burning aneutronic fuels. However, as in other high-beta plasmas, there are stability and confinement concerns. These concerns can be addressed by introducing and maintaining a significant fast ion population in the system. This is the approach adopted by TAE and implemented for the first time in the C-2 device. Studying the physics of FRCs driven by Neutral Beam (NB) injection, significant improvements were made in confinement and stability. Early C-2 discharges [2] had relatively good confinement, but global power losses exceeded the available NB input power. The addition of axially streaming plasma guns, magnetic end plugs as well as advanced surface conditioning leads to dramatic reductions in turbulence driven losses and greatly improved stability [3]. As a result, fast ion confinement significantly improved and allowed for build-up of a dominant fast particle population. Under such appropriate conditions we achieved highly reproducible, long-lived, macroscopically stable FRCs with record lifetimes [4]. This demonstrated many beneficial effects of large orbit particles and their performance impact on FRCs Together these achievements point to the prospect of beam-driven FRCs as a path toward fusion reactors. This presentation will review and expand on key results and present context for their interpretation. \\[4pt] [1] L.C. Steinhauer, Phys. Plasmas \textbf{18}, 070501 (2011).\\[0pt] [2] M.W. Binderbauer, \textit{et al.}, Phys. Rev. Lett. \textbf{105}, 045003 (2010).\\[0pt] [3] M. Tuszewski \textit{et al}., Phys. Rev. Lett.\textbf{ 108}, 255008 (2012).\\[0pt] [4] H.Y. Guo, \textit{et al.}, submitted to Nature Communications (2014). [Preview Abstract] |
Thursday, October 30, 2014 3:30PM - 4:00PM |
VI2.00002: Unification of Kinetic Resistive Wall Mode Stabilization Physics in Tokamaks Invited Speaker: S.A. Sabbagh Joint experiments and analysis on the DIII-D tokamak and the NSTX spherical torus have led to a unification of understanding of resistive wall mode (RWM) stability physics between the devices. Unstable and/or marginally stable modes have been found at significant levels of plasma rotation in both devices, and share common dynamics observed during mode growth and rotation. Large collapses of plasma stored energy (up to 60 percent) limit performance in DIII-D high normalized beta plasmas at high minimum safety factor, q$_{\mathrm{min}}$, at plasma normalized beta, $\beta _{\mathrm{N}}$, near 3.5 and lead to full disruptions in NSTX. Kinetic RWM stabilization theory [1] implemented in the MISK code can quantitatively explain the observed destabilization, with the results having important complementarity between devices. DIII-D experimental high $\beta _{\mathrm{N}}$ plasmas are less stable at high q$_{\mathrm{min}}$ than at lower q$_{\mathrm{min}}$, which is reproduced by MISK, including the marginal stability point in terms of plasma rotation profile, $\beta _{\mathrm{N}}$, and q$_{\mathrm{min}}$. Trapped ion bounce resonance stabilization is dominant compared to ion precession drift resonance stabilization at q$_{\mathrm{min}} = $ 1.2. As q$_{\mathrm{min}}$ is increased to 1.6 and 2.8 in the experiment, ion bounce resonance stabilization decreases significantly, while the ideal MHD instability drive increases, leading to the marginal stability condition. In NSTX, variations of plasma rotation profile and collisionality alter RWM stability, and reproduce marginal stability conditions, but with the ion precession drift stabilization dominant to the bounce resonance stabilization. This understanding is critical for future design and operation of magnetic fusion devices, including ITER high normalized beta steady-state scenarios, to minimize minor and major plasma disruptions.\\[4pt] [1] B. Hu and R. Betti, Phys. Rev. Lett. \textbf{93}, 1050002 (2004). [Preview Abstract] |
Thursday, October 30, 2014 4:00PM - 4:30PM |
VI2.00003: Coupled neoclassical-magnetohydrodynamic simulations of axisymmetric plasmas Invited Speaker: Brendan C. Lyons Neoclassical effects (e.g., the bootstrap current and neoclassical toroidal viscosity [NTV]) have a profound impact on many magnetohydrodynamic (MHD) instabilities, including tearing modes, edge-localized modes (ELMs), and resistive wall modes. High-fidelity simulations of such phenomena require a multiphysics code that self-consistently couples the kinetic and fluid models. We present the first results of the DK4D code, a dynamic drift-kinetic equation (DKE) solver being developed for this application. In this study, DK4D solves a set of time-dependent, axisymmetric DKEs for the non-Maxwellian part of the electron and ion distribution functions ($f_{NM}$) with linearized Fokker-Planck-Landau collision operators. The plasma is formally assumed to be in the low- to finite-collisionality regimes. The form of the DKEs used were derived in a Chapman-Enskog-like fashion, ensuring that $f_{NM}$ carries no density, momentum, or temperature. Rather, these quantities are contained within the background Maxwellian and are evolved by an appropriate set of extended MHD equations. We will discuss computational methods used and benchmarks to other neoclassical models and codes. Furthermore, DK4D has been coupled to a reduced, transport-timescale MHD code, allowing for self-consistent simulations of the dynamic formation of the ohmic and bootstrap currents. Several applications of this hybrid code will be presented, including an ELM-like pressure collapse. We will also discuss plans for coupling to the spatially three-dimensional, extended MHD code M3D-$C^1$ and generalizing to nonaxisymmetric geometries, with the goal of performing self-consistent hybrid simulations of tokamak instabilities and calculations of NTV torque. [Preview Abstract] |
Thursday, October 30, 2014 4:30PM - 5:00PM |
VI2.00004: Numerical Studies and Metric Development for Validation of MHD Models on the HIT-SI Experiment Invited Speaker: Chris Hansen Biorthogonal Decomposition (BD) decomposes large data sets, as produced by distributed diagnostic arrays, into principal mode structures without assumptions on spatial or temporal structure. We present an application of the BD technique to define a few scalar metrics that capture the level of agreement between macroscopic dynamics in different data sets. These metrics have been applied to validation of the Hall-MHD model using experimental data from the Helicity Injected Torus with Steady Inductive helicity injection (HIT-SI) experiment. Each metric provides a measure of correlation between mode shapes extracted from experimental data and simulations for an array of 192 surface mounted magnetic probes. In collaboration with the Plasma Science and Innovation (PSI) Center, extensive simulations have been performed and compared to experimental data using BD and other metrics to determine validity of the Hall-MHD model in the parameter regime of HIT-SI operation (T $\sim 10$'s eV, n $\sim 10^{19}$ m$^{-3}$). Numerical validation studies have been performed using NIMROD [1], which models the injectors as boundary conditions on the flux conserver, and PSI-TET [2], which models the entire plasma volume. Results from these studies will be presented, illustrating application of the BD method. A simplified (constant, uniform density and temperature) Hall-MHD model has accurately modeled the current amplification achieved when the injectors are driven with a frequency of 14.5 kHz. However at higher frequencies (30 kHz $<$ f$_{inj}$ $<$ 70 kHz) this simplified model does not reproduce the experimental current amplification. In addition, simulations have yet to accurately reproduce the internal q profile, an important factor in equilibrium stability, indicating additional physics may be required to achieve full agreement.\\[4pt] [1] C. Akcay, Physics of Plasmas (2013).\\[0pt] [2] C. Hansen, PhD Dissertation: University of Washington (2014). [Preview Abstract] |
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