Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Plasma Physics
Volume 64, Number 11
Monday–Friday, October 21–25, 2019; Fort Lauderdale, Florida
Session JO6: MF: MHD and Disruption Physics |
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Chair: Darren Craig, Wheaton College Room: Grand D |
Tuesday, October 22, 2019 2:00PM - 2:12PM |
JO6.00001: Discovery of Discrete Alfven-Sound Eigenmodes in Tokamaks C. Z. Cheng, Gerrit Kramer, Mario Podesta, Raffi Nazikian Theoretical and experimental studies have confirmed that the discrete Alfven eigenmodes such as TAEs can be destabilized by fast particles and cause significant fast particle loss in the tokamaks. Most Alfven eigenmodes have frequencies above the lowest TAE continuum. Experiments have also found BAEs with frequencies below the TAE continuum gap. Here we present the discovery of new Alfven-Sound Eigenmodes (ASE) with frequencies below the TAE continuum gap. In finite pressure plasmas slow modes interact with Alfven waves and breaks up the continuous spectrum below the TAE gap to form Alfven-Sound (AS) frequency gaps. Analytical theory of AS gaps will be presented. Using the full MHD NOVA code several new ASEs are discovered to exist with frequencies in the AS gaps without suffering continuum damping. The existence of the ASEs is robust for normal and reversed q-profiles, broad range of plasma beta values and plasma shaping. The stability of ASEs due to fast ions will be presented. [Preview Abstract] |
Tuesday, October 22, 2019 2:12PM - 2:24PM |
JO6.00002: Study of transition between even and odd toroidal Alfv\'en eigenmodes on EAST using NIMROD Yawei Hou, Charlson Kim, Ping Zhu, Zhihui Zou, Youjun Hu, Xingting Yan Linear simulations of toriodal Alfv\'en eigenmodes (TAEs) driven by energetic particles (EPs) on EAST are performed using the hybrid-kinetic MHD model implemented in the NIMROD code. The "slowing down" distribution is adopted for modeling the equilibrium distribution of the energetic ions from the deuterium neutral beam injection on EAST. The frequency, the dominant poloidal mode number, the radial location, and the detailed 2D mode structure of the TAE/RSAE/EPM modes are consistent between the eigenvalue analysis and the NIMROD calculation. As the EP $\beta$ fraction increases, a transition between even and odd TAEs occurs, along with that between the ballooning and anti-ballooning mode structures. When the EP $\beta$ fraction is close to the transition threshold, both types of TAEs coexist. [Preview Abstract] |
Tuesday, October 22, 2019 2:24PM - 2:36PM |
JO6.00003: Investigations of the visco-resistive kink(m=1,n=1) mode using the two fluid CUTIE code Jervis Mendonca, Debasis Chandra, Abhijit Sen, Anantanarayanan Thyagaraja We have made numerical simulations of the visco resistive kink (m=1,n=1) mode using the CUTIE code, thereby extending our earlier visco-resistive MHD results into the two fluid regime. We report that two fluid effects are extremely influential in the dynamics of the mode. We began our investigations with linear studies on the effects of flow on the the mode. We noticed a change in the two fluid regime which was seen earlier in the case of (2,1) tearing modes by us. Particularly, thesymmetry in the normalised growth rate and frequency of the (1,1) mode, as a function of the flow Alfven Mach numbers, is broken when we include a finite density gradient. We also have observed that by imposing equilibrium poloidal flows, we increase the growth rate of the mode, thereby destabilising, possibly due to the canceling of the intrinsic poloidal flow.The cases with an imposed helical and axial flow also differ from the corresponding single fluid results. These investigations were extended to the nonlinear two fluid regime. We add that imposed poloidal flow can even increase the (1,1) mode's nonlinear saturated energy levels. In conclusion, we remark that these results in the two fluid model complement our results from the single fluid model, and are of great importance for tokamaks. [Preview Abstract] |
Tuesday, October 22, 2019 2:36PM - 2:48PM |
JO6.00004: 3D MHD spectroscopy towards real-time detection of multi-modes' stabilities Tong Liu, Zhirui Wang, M.D. Boyer, B.-H Park, Stefano Munaretto, Nikolas Logan, Jong-Kyu Park, Zheng-Xiong Wang A 3D MHD spectroscopy method, utilizing the internal 3D coils to scan the frequency and poloidal spectrum of the applied 3D fields, is developed to perform active detection of multiple eigenmodes' stability in DIII-D and K-STAR experiments. Efficient wave packet coil waveforms have been developed to achieve fast updating of the plasma stability every 100ms. A new time dependent model is developed to extract multi-modes stability from the plasma response measured by multiple 3D sensors in the experiments. The time domain method and frequency domain method using the multi-pole transfer function are both applied in the data analysis to better understand their accuracy, robustness and efficiency while extracting mode stability. This work verifies the feasibility of real-time detection of plasma stability, which is essential to the disruption avoidance in the advanced tokamak operation. *Work supported under DE-FC02-04ER54698 and DE-AC02-09CH11466 [Preview Abstract] |
Tuesday, October 22, 2019 2:48PM - 3:00PM |
JO6.00005: The physics of locked tearing mode stabilization by rotating 3D fields in the presence of static error fields S. Inoue, M. Okabayashi, N.C. Logan, N.Z. Taylor, E.J. Strait, J. Shiraishi, M. Takechi, G. Matsunaga, A. Isayama, S. Ide Effective screening-out of static error fields (EFs) using rotating resonant magnetic perturbation (RMP), i.e., “self-stabilized” regimes, are found by nonlinear resistive MHD simulation [Inoue$+$, NF16{\&}PPCF17] and validated using DIII-D experimental observations of tearing mode (TM) locking to uncorrected EFs. Simulations with a single rational surface have predicted the dominant single TM behaviors measured in DIII-D when applying rotating external fields. These behaviors include a standing wave type response when the internal TM is locked to the intrinsic EF and a propagating wave response when it is locked to the external rotating fields. Despite the qualitative consistency between AEOLUS-IT simulation and DIII-D experiment, details of stabilized regime measurements, where double and triple helicity responses due to toroidicity and poloidal shaping were also discovered [Okabayashi$+$, IAEA18], do not agree. Recent multi-helicity simulations suggest a self-stabilizing mechanism via anti-phasing between double-helicity structures. These improve consistency with observations and improve detailed understanding of locked TM control. This detailed cross-validation suggests that the physical process of the TM/RMP interaction proceeds quasi-/non-linearly and with multiple helicity structures, which goes beyond the linear stability model prediction. *Supported by the US DOE under DE-FC02-04ER54698 {\&} DE-AC02-09CH11466. [Preview Abstract] |
Tuesday, October 22, 2019 3:00PM - 3:12PM |
JO6.00006: Multi-parallel-component fluid theory for magnetically confined plasmas Linjin Zheng Braginskii’s two fluid theory has been widely used in this field both for theoretical analyses and numerical simulations. It extends the ideal MHD to count for the different responses from the ion and electron species. Note that Braginskii’s two fluid theory relies on the high collisionality assumption, while in reality the magnetically confined plasmas in thermonuclear fusion turn to be collisionless. One cannot simply introduce the parallel fluid velocities, since some charged particles are trapped by the equilibrium field or perturbed waves and some others are circulating. To solve this difficulty the multi-parallel-component fluid theory is developed, which modifies the Braginskii two fluid theory by relaxing the collisional dominance assumption. In the perpendicular direction the particle spatial localization is solo resulted from the strong magnetic field, while the finite Larmor radius (FLR) effects are taken into account in the next order. In the parallel direction the particle mobility feature is fully retained by introducing the multi-parallel-component fuild model. Comparision with Braginskii’s two fluid theory will be detailed and the application of new theory will be discussed. [1] Linjin Zheng, IOP Concise Physics: Magnetically Confined Fusion Plasma Physics. [Preview Abstract] |
Tuesday, October 22, 2019 3:12PM - 3:24PM |
JO6.00007: Scalable implicit, adaptive MFEM-based solvers for reduced resistive MHD Qi Tang, Luis Chacon, Tzanio Kolev, John Shadid, Xian-Zhu Tang The extended magnetohydrodynamics (XMHD) equations are advanced continuum models to understand the evolution of complex plasma dynamics in tokamak disruptions. However, solving XMHD numerically is challenging due to its disparity in time and length scales, its strongly hyperbolic nature and its nonlinearity. Therefore, scalable, adaptive implicit algorithms based on efficient preconditioning strategies are necessary for XMHD. In this work, we design and develop several finite-element schemes for a simple model, the reduced resistive MHD equations. Both explicit and implicit schemes are developed using the scalable C++ framework MFEM (mfem.org). The implicit scheme is based on the JFNK method with a physics-based preconditioner as proposed in [Chacon et al. JCP 2002]. The preconditioner is generalized for the finite element discretization, and algebraic multigrid methods are used to invert certain operators to achieve scalability. Our preconditioner effectively treats stiff hyperbolic components in the system. Both explicit and implicit solvers are implemented in parallel with adaptive mesh refinement and dynamic load-balancing. Benchmark results will be presented to demonstrate the accuracy and scalability of the implicit scheme in the presence of strong scale disparity. [Preview Abstract] |
Tuesday, October 22, 2019 3:24PM - 3:36PM |
JO6.00008: DIII-D disruption prediction using deep convolutional neural networks on raw imaging data R.M. Churchill Predictions for oncoming disruptions are made using a novel deep learning algorithm applied directly to the raw sensor output of the Electron Cyclotron Emission imaging (ECEi) diagnostic on DIII-D. The algorithm, deep convolutional neural networks with dilated convolutions, allows learning directly on high-dimensional, multi-scale data, such as ECEi. Using this deep learning algorithm, we obtain promising results of a 91\% F1 score for predicting 300ms before the disruption, using only data from the ECEi diagnostic. Predictions at each time slice use long sequences of prior ECEi data, allowing identification of pre-disruption markers in time for disruption avoidance. The fine ECEi time resolution and long time coverage allow the algorithm to learn pre-disruption markers simultaneously from turbulence, MHD, and transport timescales. This work also opens the possibility of incorporating the raw data from multiple diagnostics to enhance disruption prediction, in addition to the global physics parameters often used in machine learning disruption prediction. Future directions on how to transfer these machine learning models to future devices such as ITER will be discussed. [Preview Abstract] |
Tuesday, October 22, 2019 3:36PM - 3:48PM |
JO6.00009: Self-consistent modelling of electron runaway during tokamak disruptions Tunde Fulop, Linnea Hesslow, Ola Embréus, Mathias Hoppe, Oskar Vallhagen, Lucas Unnerfelt Recent progress in modelling the dynamics of runaway electrons (REs) during disruptions mitigated by massive material injection indicate a substantial increase in the avalanche multiplication gain during an ITER current quench compared to previous estimates [Hesslow et al, Nucl. Fusion 59, 084004 (2019)]. This is due to the increased number of target electrons available for the avalanche process in weakly ionized plasmas, which is only partially compensated by the increased friction force on REs. This results in more stringent requirements either on the maximum runaway seed population that can be allowed to survive the thermal quench, or on the density profile of the injected material needed to achieve successful mitigation. We present ongoing efforts to model runaway beam formation and evolution during the thermal quench and the development of a reduced kinetic model for RE dynamics in the presence of material injection. [Preview Abstract] |
Tuesday, October 22, 2019 3:48PM - 4:00PM |
JO6.00010: Numerical study of the non-linear interaction of runaway electrons and MHD during cold VDEs and in stochastic fields V. Bandaru, M. Hoelzl, F.J. Artola, G. Papp, G.T.A. Huijsmans The runaway electron (RE) fluid model in the MHD code JOREK is applied to study the nonlinear interaction of REs and MHD activity during ITER cold VDEs and during the formation of stochastic fields. The fluid model treats REs as a seperate species, with its density evolved taking into account the Dreicer, hot-tail via initial distribution and avalanche generation as well as RE transport. ITER cold VDE simulations with REs show a significant slowing down of the vertical motion due to the formation of REs and the possibility of internal kink modes being destabilized due to $q$ falling below unity in the core of the plasma. The presence of REs also leads to a significantly different dynamics of the 3D mode structure during the VDE. To study RE-MHD interaction in a stochastic field, we use a diffusion model to mimic the fast radial loss of REs, thereby avoiding the numerical limitations in advecting REs with speed of light. The implications and efficacy of this approach will be discussed. [Preview Abstract] |
Tuesday, October 22, 2019 4:00PM - 4:12PM |
JO6.00011: Compressional Alfven Eigenmodes Driven by Runaway Electrons in a Tokamak Chang Liu, Andrey Lvovskiy, Carlos Paz-Soldan, Eric Fredrickson, Dylan Brennan, Amitava Bhattacharjee This work provides the first study of resonant interactions between runaway electrons (REs) and compressional Alfven eigenmodes (CAEs) in a tokamak. Kinetic instabilities driven by MeV REs during tokamak disruptions have been recently observed on DIII-D [A. Lvovskiy et al., Plasma Phys. Control. Fusion 60, 124003 (2018)]. These instabilities correlate with intermittent RE loss from the plasma and they are hypothesized to be responsible for a non-sustained post-disruption RE current. In the present work, CAEs driven by REs are proposed as a possible candidate for the instability. Their mode structure is modeled using the modified code modelling excitation of CAEs by fast ions [E. D. Fredrikson et al., Phys. Plasmas 20, 042112 (2013)]. The growth rate is calculated from a simulation of runaway electron distribution function based on bounce-averaging, which includes the enhanced RE pitch-angle scattering due to ion partial screening. Radial diffusion of REs to the edge is explained via interactions with CAEs. The results match the experiment qualitatively, and provide a way to predict the dynamics of REs and study means of their control for disruption mitigation in ITER. [Preview Abstract] |
Tuesday, October 22, 2019 4:12PM - 4:24PM |
JO6.00012: Experimental observations of 3D neon transport following shattered pellet injection in Super H-modes R. Sweeney, R. Raman, N. Eidietis, R. Granetz, J. Herfindal, E. Hollmann, M. Lehnen, R. Moyer, D. Shiraki Stable DIII-D Super H-modes with 1.8~MJ thermal energies are terminated by a shattered neon pellet to study radiation asymmetries during mitigated disruptions. Asymmetric neon distributions cause radiation peaking that might melt ITER components [Lehnen NF \textbf{55} (2015) 123027]. Ne-I images of the injection show field-aligned and cross-field structures, and penetration to the $q=2$ surface before the thermal quench (TQ). Near the injection in the co-rotation direction, an Absolute eXtreme UltraViolet fan array (AXUV-1) also exhibits signs of cross-field transport. Approximately twice the distance in the counter-rotation direction, AXUV-2 and interferometry measurements corroborate a 0.5-1~ms delay in the arrival of neon ions relative to AXUV-1, indicating a peaked Ne distribution during this time. When the Ne reaches AXUV-2, the Ne distribution is helical, and this helical structure is evident throughout the TQ. Interferometry further supports this helical structure and reveals strong inboard-to-outboard density asymmetries. Parallel diffusion does not appear suitable to explain these observations, so parallel convection models are under investigation, and implications for radiation peaking in ITER will be discussed. [Preview Abstract] |
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