Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session UO5: MHD and Transport |
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Chair: Nathaniel Ferraro, General Atomics Room: 200 |
Thursday, November 19, 2015 2:00PM - 2:12PM |
UO5.00001: The MHD spectral web: Connecting all instabilities of stationary plasmas Hans Goedbloed Quite a lot is known about the spectra of MHD instabilities in plasmas with background flow, in particular through numerical studies. They exhibit bewildering distributions of the complex eigenvalues, with isolated global modes as well as local modes clustering towards complicated continuous spectra. This calls for the development of a general theory generating physically meaningful structures in the complex $\omega$-plane connecting the eigenvalues. Whereas the simplicity of the energy principle of static equilibria no longer applies, proper consideration of the {\em two} quadratic forms of the potential energy and the averaged Doppler--Coriolis shift leads a new approach to the analysis of stationary plasmas, called the {\em spectral web}, that provides the desired structures. Thus, for the first time, the full complex spectrum of stationary plasmas is obtained together with a connecting structure. This permits to consider the enormous diversity of MHD instabilities of laboratory and astrophysical plasmas with arbitrary flow and rotation profiles from a single unifying view point. I will illustrate that with results obtained on these instabilities with the new spectral code ROC. [Preview Abstract] |
(Author Not Attending)
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UO5.00002: Electromagnetically Sustained Liquid Metal Flow for Feedback Stabilization Studies Seyyed Mohammad Mirhoseini, Francesco Volpe Liquid metal walls in fusion reactors, whether nearly static or rapidly flowing, will be subject to instabilities that will make them locally bulge, thus entering in contact with the plasma, or deplete, hence exposing the underlying solid substrate. To prevent this, research has begun at Columbia University to create liquid metal flows and demonstrate their stabilization by electromagnetic forces, adjusted in feedback with thickness measurements. Here we present initial results regarding the sustainment of a flow of Galinstan (a gallium, indium, tin alloy) by a special pump consisting of a ferromagnetic rotor, with permanent magnets mounted on it. The magnetic field is partly ``frozen'' in the liquid metal surrounding the rotor. Therefore, as the field rotates, the liquid metal rotates as well, although with a slip factor. This solution was preferred to conventional pumps, which would enter in electrical contact with the metal flow. The pump, 3D-printed at Columbia, allows to adjust the flow-velocity from few mm/s to several cm/s. [Preview Abstract] |
Thursday, November 19, 2015 2:24PM - 2:36PM |
UO5.00003: Stability of a two-volume MRxMHD model in slab geometry Li Huey Tuen, Matthew Hole, Robert Dewar, Graham Dennis Ideal MHD models are inadequate to describe various physical attributes of toroidal plasmas with no continuous symmetry, such as magnetic islands and stochastic regions. A new MHD formulation, MRxMHD [1], uses only a finite number of ideal-MHD flux surface interfaces, with relaxed plasma regions in between, thus allowing a stepped-pressure profile, magnetic islands, and stochastic regions. In toroidally asymmetric plasma, the existence of interfaces in MRxMHD is contingent on the field pitch, or rotational transform, of flux surfaces being irrational; a KAM [2] argument shows that some good equilibrium flux surfaces continue to exist for small perturbations to an integrable system (foliated by flux surfaces), provided that the rotational transforms on both sides of each interface are sufficiently irrational. Building upon the MRxMHD stability model by Hole [3], we study the effects of irrationality of the rotational transform at interfaces in MRxMHD on plasma stability. Investigating the plasma stability of a two-volume MRxMHD periodic-slab, we find that the 2D system stability conditions are dependent on the interface and resonance layer magnetic field pitch at minimised energy states. While tearing instabilities exist at low order rational resonances, investigating instability of high-order rationals requires study of pressure-driven instabilities. \\[4pt] [1] Hudson, S. R., et al. (2012), \textit{Physics of Plasmas }\textbf{19}, 112502\\[0pt] [2] McGann, M. (2013), PhD thesis\\[0pt] [3] Hole, M.J. \textit{et al, }(2007), \textit{Nucl. Fusion }\textbf{47, }746. [Preview Abstract] |
Thursday, November 19, 2015 2:36PM - 2:48PM |
UO5.00004: Quiescent H-Mode 3D MHD Free-Boundary Equilibrium W. Anthony Cooper, Jonathan P. Graves, Basil P. Duval, Laurie Porte, Olivier Sauter, Trach-Minh Tran, Daniele Brunetti, David Pfefferle, Madhusudan Raghunathan, Jonathan M. Faustin, Hamish Patten, Andreas Kleiner, Holger Reimerdes Free boundary magnetohydrodynamic equilibrium states with spontaneous three dimensional deformations of the plasma-vacuum interface are computed with the 3D VMEC solver [Hirshman {\em et al.}, Comput.~Phys.~Commun.~\textbf{43} (1986) 143]. The structures we have obtained have the appearance of saturated ideal external kink/peeling modes. Large edge pressure gradients yield toroidal mode number $n=1$ corrugations when the edge bootstrap current is large and $n=4$ distortions when this current is small. The deformations of the plasma boundary region induces a nonaxisymmetric Pfirsch-Schl\"uter current that drives a field-aligned current ribbon which is consistent with experimental observations reported. We claim that the equilibrium states we compute model the Edge Harmonic Oscillation [K.H.~Burrell {\em et al.}, Phys.~Plasmas {\bf 22} (2005) 021805. W.M.~Solomon {\em et al.}, Phys.~Rev.~Lett.~{\bf 113} (2014) 135001] observed on DIII-D and the Outer Mode [E.R.~Solano {\em et al.}, Phys.~Rev.~Lett.~{\bf 104} (2014) 135001] found in JET during Quiescent H-mode operation. [Preview Abstract] |
Thursday, November 19, 2015 2:48PM - 3:00PM |
UO5.00005: Vlasov tokamak equilibria with shearad toroidal flow and anisotropic pressure George Throumoulopoulos, Apostolos Kuiroukidis, Henri Tasso By choosing appropriate deformed Maxwellian ion and electron distribution functions depending on the two particle constants of motion, i.e. the energy and toroidal angular momentum, we reduce the Vlasov axisymmetric equilibrium problem for quasineutral plasmas to a transcendental Grad-Shafranov-like equation. This equation is then solved numerically under the Dirichlet boundary condition for an analytically prescribed boundary possessing a lower X-point to construct tokamak equilibria with toroidal sheared ion flow and anisotropic pressure. Depending on the deformation of the distribution functions these steady states can have toroidal current densities either peaked on the magnetic axis or hollow. These two kinds of equilibria may be regarded as a bifurcation in connection with symmetry properties of the distribution functions on the magnetic axis. [Preview Abstract] |
Thursday, November 19, 2015 3:00PM - 3:12PM |
UO5.00006: Can the Non-linear Ballooning Model describe ELMs? S.A. Henneberg, S.C. Cowley, H.R. Wilson The explosive, filamentary plasma eruptions described by the non-linear ideal MHD ballooning model is tested quantitatively against experimental observations of ELMs in MAST. The equations describing this model were derived by Wilson and Cowley [1] for tokamak-like geometry which includes two differential equations: the linear ballooning equation which describes the spatial distribution along the field lines and the non-linear ballooning mode envelope equation, which is a two-dimensional, non-linear differential equation which can involve fractional temporal-derivatives, but is often second-order in time and space. To employ the second differential equation for a specific geometry one has to evaluate the coefficients of the equation which is non-trivial as it involves field line averaging of slowly converging functions. We have solved this system for MAST, superimposing the solutions of both differential equations and mapping them onto a MAST plasma. Comparisons with the evolution of ELM filaments in MAST will be reported in order to test the model.\\[4pt] [1] H.R. Wilson and S.C. Cowley, Phys. Rev. Lett., 92, 175006 (2004) [Preview Abstract] |
Thursday, November 19, 2015 3:12PM - 3:24PM |
UO5.00007: Turbulence studies using a Doppler Backscattering (DBS) system during ELM mitigation and suppression on EAST Chu Zhou, Adi Liu, Mingyuan Wang, Jianqiang Hu, Jin Zhang, Hong Li, Xiaohui Zhang, Tao Lan, Jinlin Xie, Wandong Liu, Changxuan Yu, Edward Doyle ELM mitigation and suppression using Resonant Magnetic Perturbations (RMP), Supersonic Molecular Beam Injection (SMBI), and lithium pellet injection has been demonstrated on EAST. A new eight-channel DBS system has been installed for turbulence measurements in such plasmas. The frequency range is 55 to 75 GHz, covering the entire H-mode pedestal, with a turbulence wavenumber range of 4-12 /cm. The turbulence evolution has been measured during ELM mitigation and suppression by the different ELM control methods (RMP, SMBI, lithium pellet, etc.), so as to study the relationship between the pedestal turbulence and the ELM mitigation and suppression, and to determine whether a single trend of pedestal turbulence exists with ELM control. [Preview Abstract] |
Thursday, November 19, 2015 3:24PM - 3:36PM |
UO5.00008: Mind the Gap: Exploring the Physics of Null Points Using Unconventional Coordinate Systems Brendan Shanahan, Ben Dudson, Fabio Avino, Jarrod Leddy, Peter Hill, Ivo Furno Simulations of instabilities and turbulence in X-point configurations are challenging due to the limitations of field-aligned coordinate systems: X-point dynamics are often interpolated from flux surfaces, which could exclude relevant physics. Here we explore the physics of null regions in multiple geometries using unconventional coordinate systems in BOUT++. Specifically, we have investigated the physics of blob propagation and compared with experimental measurements within the TORPEX device, indicating an increase in inertially limited filament propagation in the null region caused by longer connection lengths. The null point dynamics of filaments crossing the separatrix in tokamak geometry will also be presented using a novel field-aligned coordinate system, with applications to ELM and blob theory. Finally, recent work on implementation and applicability of the Flux Coordinate Independent (FCI) approach to complex magnetic geometry modelling will be discussed. [Preview Abstract] |
Thursday, November 19, 2015 3:36PM - 3:48PM |
UO5.00009: Characteristics of magnetic and electrostatic turbulence in the edge plasma of HL-2A tokamak Min Xu, Lin Nie, Rui Ke, Yi Yu, Tao Lan, Wulu Zhong, Xiaolan Zou, Dong Guo, Boda Yuan, Zhanhui Wang, Yifan Wu, Xuru Duan Simultaneous measurement of magnetic and electrostatic turbulence in the HL-2A tokamak edge plasma has been carried out by using a multifunctional reciprocating probe. This probe is able to simultaneously measure the 3-dimensional magnetic fluctuation, toroidal rotation velocity, as well as plasma density, potential, and electron temperature. The measured magnetic turbulence is broad band, and peaks in the frequency ranges 20-50kHz and 250-350kHz. It is generally associated with 0.2-0.3 Gauss fluctuation. The coherence between floating potential and magnetic fluctuation is low in these two frequency ranges and is relatively high in 100-250kHz. The particle flux induced by magnetic turbulence is relatively small as compared to that induced by electrostatic turbulence. One thing worth noting is that the electrostatic turbulence with high frequencies (100-500kHz) is equally important as turbulence with lower frequencies (peaks around 30-40kHz) in particle, momentum, and electron heat transport. Other statistics such as measured Reynolds stress and Maxwell stress will also be presented. [Preview Abstract] |
Thursday, November 19, 2015 3:48PM - 4:00PM |
UO5.00010: Integrated multi-scale simulations of drift-wave turbulence: coupling of two kinetic codes XGC1 and XGCa Salomon Janhunen, Robert Hager, Seung-Hoe Ku, Choong-Seock Chang, Jan Hesthaven, Jong Choi, Fan Zhang, Manish Parashar A novel technique for acceleration of gyrokinetic total $f$ particle simulations in diverted geometry has been developed, based on the XGC1 code and its axisymmetric version XGCa. Both XGC1 and XGCa calculate particle motion in a 5-dimensional (5D) phase space, but while XGC1 is equipped with a full turbulence solver, XGCa has an axisymmetric Poisson solver and is generally used for the simulation of neoclassical transport. Here, acceleration in transport calculations is achieved through relaxed constraints on numerical requirements in XGCa, such as mesh resolution and total number of markers. Coupled simulations have been performed for ITG turbulence, where long-term evolution is obtained by periodically calling XGC1 to obtain turbulence-driven transport while evolving the neoclassical equilibrium with XGCa. We present results from simulations with long-term evolution of the microscopic plasma state while using this technique in the presence of sources and sinks. We also introduce in-memory techniques used in the coupling between the fine-scale and coarse models, applicable for massively parallel simulations of long term evolution of kinetic plasma equilibria in the presence of turbulent and neoclassical transport processes. [Preview Abstract] |
Thursday, November 19, 2015 4:00PM - 4:12PM |
UO5.00011: ETG-dominated transport regimes in near-edge DIII-D L-mode plasmas: Validation of multiscale gyrokinetic simulations Tom Neiser, Frank Jenko, Lothar Schmitz, Daniel Told, Alejandro Banon Navarro, Troy Carter, Zheng Yan, George McKee A prerequisite for the development of a self-consistent theoretical description of the L-H transition is the ability to quantitatively characterize near-edge L-mode plasmas. It is shown here for the first time that regimes exist in the L-mode near-edge that appear to be dominated by sub-ion-scale turbulence driven by electron temperature gradient (ETG) modes. These are results of gyrokinetic simulations of a DIII-D L-mode discharge in the near edge region (r/a $=$ 0.8) with the GENE code (www.genecode.org). Instructed by a linear analysis, we performed nonlinear simulations of ITG and ETG turbulence, pointing to a dominance of ETG turbulence regarding the anomalous radial heat flux. Direct comparison with experimental data is encouraging. Respective multi-scale simulations, covering both ion and electron scales are underway and will be presented. Implications for L-H transition modeling will also be discussed. [Preview Abstract] |
Thursday, November 19, 2015 4:12PM - 4:24PM |
UO5.00012: Gyrokinetic electromagnetic isotope effect in ITER-hybrid plasmas and validation Tobias Goerler, Jeronimo Garcia, Frank Jenko A number of high-realism simulations with the gyrokinetic turbulence code GENE have been performed recently for comparison with experimental measurements in, e.g., ASDEX Upgrade and DIII-D. Some of these successful validation studies will be reviewed briefly as basis for subsequent predictive simulations for a particular ITER hybrid scenario [K.Besseghir et al., PPCF 55, 125012 (2013)]. Here, comprehensive local GENE simulations have been employed considering the multi-component character of such plasmas including impurities, fuel ions, helium ash, up to two fast ion species as well as electromagnetic fluctuations, inter- and intra-species collisions, and external shear effects. The fluxes are in general in good agreement with those in the above ITER study performed with the CRONOS code suite. A particular subject of interest is the turbulent transport comparison between deuterium-tritium (DT) plasmas and pure deuterium (DD) fuel as mostly used in present-day experiments. Here, a strong heat flux drop from DD to DT plasmas can be observed which is in line with experimental evidence found at TFTR and JET. This contribution may hence help to gain a better understanding of this so-called isotope effect and improve projections for future ITER DD- and DT-plasma studies. [Preview Abstract] |
Thursday, November 19, 2015 4:24PM - 4:36PM |
UO5.00013: Verification and Validation of Gyrokinetic Particle Simulation of Fast Electron Driven Beta-induced Alfv\'{e}n Eigenmodes on HL-2A Tokamak Junyi Cheng, Yang Chen, Wenlu Zhang, Zhihong Lin, Wei Chen, Limin Yu, Xuantong Ding A verification and validation study is carried out for a sequence of fast electron driven beta-induced Alfven eigenmodes in HL-2A tokamak plasma. The fast electron driven beta induced Alfv\'{e}n eigenmode (e-BAE) in toroidal plasmas is investigated for the first time using global gyrokinetic particle simulations, where the fast electrons are described by the drift kinetic model. The phase space structure shows that only the processional resonance is responsible for the e-BAE excitations while fast-ion driven BAE can be excited through all the channels such as transit, drift-bounce, and processional resonance. For weakly nonlinear driven case, frequency is observed to be in phase with the particle energy flux, and mode structure is almost the same as linear stage . While in the strongly driven nonlinear case, BAAE is excited after the BAE mode saturated. The simulation of e-BAE with the HL-2A tokamak parameters is taken. Three modes n/m$=$ 1/2 , 2/5 ,1/3 in HL-2A can be excited in our simulations. [Preview Abstract] |
Thursday, November 19, 2015 4:36PM - 4:48PM |
UO5.00014: Electromagnetic effects on the energy flows saturating microturbulence Garth Whelan, Moritz Pueschel, Paul Terry In kinetic plasma turbulence mode coupling in perpendicular wavenumber excites large-scale stable modes, allowing both the perpendicular cascade and stable-mode damping to saturate the instability. Using GENE, we evaluate the dominant triad energy transfer function via zonal flows, distinguishing between energy transfer to stable modes and transfer to higher wavenumber. We find that in cyclone base case ITG turbulence, the zonal flows are excited primarily by modes with poloidal wavenumber equal to or below the wavenumber responsible for the peak in transport, while modes with larger poloidal wavenumber produce a smaller nonlinear energy transfer out of zonal flows. We investigate the dissipation that balances the net excitation by varying collisionality and the rate of geodesic acoustic mode damping. Increasing the temperature gradient sharpens the nonlinear zonal flow drive peak around the peak in transport. As plasma beta is increased, proportionally more energy is transferred to stable modes within the wavenumber region of instability, providing an effect responsible for the increased nonlinear stabilization of ITG turbulence with plasma beta. We also investigate Kelvin-Helmholtz like saturation mechanisms of ETG turbulence. [Preview Abstract] |
Thursday, November 19, 2015 4:48PM - 5:00PM |
UO5.00015: Subdominant Eigenmode Excitation in Stellarator Turbulence M.J. Pueschel, B.J. Faber, C.C. Hegna, P.W. Terry, D.R. Hatch Owing to their complex geometry, stellarators are known to give rise to a large number of unstable eigenmodes for any single flux tube. As has recently been demonstrated for HSX cases [B.J. Faber et al., Phys. Plasmas 22, 072305 (2015)], these eigenmodes have very different properties, may come in pairs, and can easily switch from subdominant to dominant upon small adjustments in geometry or input parameters. In addition, the question of stable eigenmodes has so far not been addressed in stellarators, which may be excited nonlinearly and affect the turbulent dynamics. In tokamaks, the subdominant microtearing mode tends to be responsible for a majority of the magnetic transport, whereas its role in stellarators is yet to be determined. Here, gyrokinetic GENE simulations in a geometry similar to Wendelstein 7-X are performed, solving for the full linear eigenvalue spectrum. In the unstable range, eigenmode structures are compared, and the limitations of iterative solvers are discussed. Additional focus lies on turbulent excitation: nonlinear simulations and mode structures are projected onto the linear eigenmodes, clarifying the role of subdominantly unstable as well as stable linear eigenmodes in the quasi-saturated state, with possible consequences for quasilinear modeling. [Preview Abstract] |
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