Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session TO5: MHD, Stability and Auxiliary Heating |
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Chair: Roger Raman, University of Washington Room: Governor's Square 10 |
Thursday, November 14, 2013 9:30AM - 9:42AM |
TO5.00001: Gyrokinetic framework for Neoclassical Tearing Modes Natalia Tronko, Howard Wilson, Alain Brizard We are developing a new theoretical framework based on the Hamiltonian Gyrokinetics [1] for description of the Neoclassical Tearing Modes dynamics. The main advantage of this approach is the possibility of systematically including effects of magnetic geometry as well as magnetic field fluctuations due to the presence of a magnetic island. Moreover it makes it possible to systematically compute the expression for the polarization current as well as including FLR effects. This work represents an extension of previous drift-kinetic models for NTMs [2].\\[4pt] [1] A.J. Brizard and T.S. Hahm, Rev.of Mod.Physics 79, 2007.\\[0pt] [2] H.Wilson, J. W. Connor et al., Phys. Plasmas 3, 248 (1996). [Preview Abstract] |
Thursday, November 14, 2013 9:42AM - 9:54AM |
TO5.00002: Investigation of the stochastic model for sawteeth Marie-Christine Firpo, Wahb Ettoumi, Ricardo Farengo, Hugo Ferrari, Pablo Luis Garcia-Martinez, Agustin Lifschitz Tokamak sawteeth have often been considered as a manifestation of magnetic reconnection in a laboratory plasma. However, measurements have repeatedly shown that the very fast crash phase may be associated with little reconnection, as the central $q$-profile remains below one and almost unchanged before and after the sawtooth collapse. One is thus left with the need to search for an explanation of the fastness of the sawtooth crash outside of the pure frame of magnetic reconnection. To account for incomplete reconnection, Lichtenberg argued in a seminal paper that the fast disruptive relaxation could be caused by the intrinsic large-scale stochasticity caused by overlapping magnetic islands. Nevertheless, the well known nickel trace experiments in JET [Wesson et al. PRL 1997] appeared to contradict the simple notion of stochasticity and thermal redistribution. Using a full orbit following code for the nickel ions, we demonstrate that the profile flattening of nickel ions during the sawtooth crash phase may be well reproduced using a stochastic model for the magnetic field and the electric field deduced from an ideal MHD hypothesis, but not in the case of integrable magnetic field lines. A chaotic indicator for the nickel motion quantifies the discrepancy between the two scenarios. [Preview Abstract] |
Thursday, November 14, 2013 9:54AM - 10:06AM |
TO5.00003: Magnetohydrodynamically generated toroidal and poloidal velocities in confined plasma Jorge A. Morales, Wouter J.T. Bos, Kai Schneider, David C. Montgomery The spatio-temporal self-organization of visco-resistive magnetohydrodynamics in a toroidal geometry is studied in the presence of curl free imposed toroidal magnetic and current density fields, with uniform transport coefficients. The computation involves no microscopic instabilities or kinetic theory and is purely magnetohydrodynamic (MHD) in nature. The computation makes use of the recently developed ``penalty method'' to enforce visco-resistive boundary conditions. It is observed that a flow is generated and that an up-down asymmetry of the geometry causes the generation of a nonzero toroidal angular momentum. An analysis of the dynamics is presented and the physical origin of the toroidal velocity is illustrated. It is shown that the safety factor has an influence on the sign of the toroidal velocity and on its profile. The result of the inversion of the imposed toroidal magnetic field is also discussed. [Preview Abstract] |
Thursday, November 14, 2013 10:06AM - 10:18AM |
TO5.00004: Quasi-Periodic Transport Events during the Transition from Low to High Confinement Modes in the KSTAR Tokamak and the Large Helical Device G.S. Yun, J.H. Lee, J. Leem, W. Lee, H.K. Park, T. Akiyama, N.C. Luhmann, Jr., J.G. Bak Quasi-periodic bursts of RF radiations in a broad frequency range ($\sim$ 100~MHz -- 1~GHz) with a typical period of $\sim$ 1~ms are routinely observed during the early transition phase from low (L) to high (H) confinement modes in the KSTAR tokamak plasmas. No noticeable change in the $D_{\alpha}$ emission was observed at the time of RF bursts, suggesting that the RF bursts are not related to the so-called ``dithering'' behaviour in the L-H transition. Interestingly, each RF burst corresponds to a tiny transport event localized in the edge region according to 2D images of the plasma edge measured by electron cyclotron emission imaging (ECEI) diagnostic, which is also supported by the con-current burst of ion saturation currents measured at the divertor. These observations may indicate the existence of radially-localized non-ambipolar transport events during the L-H transition. Similar quasi-periodic RF bursts have been observed in the Large Helical Device (LHD), suggesting the existence of a common mechanism for L-H transition in tokamak and stellarator devices. [Preview Abstract] |
Thursday, November 14, 2013 10:18AM - 10:30AM |
TO5.00005: Voltage scale for electro-thermal runaway Y.Y. Lau, D. Chernin, Peng Zhang, R.M. Gilgenbach, Adam Steiner Contact problems account for 40{\%} of all electrical/electronic failures [1]. Current crowding leads to intense local heating in both bulk [2] and thin film contacts [3], and is a concern to high power microwave sources, pulsed power systems, field emitters, thin film devices, and interconnects, etc. We investigate electro-thermal instability (ET) due to the increase in electrical conductivity as temperature increases, which may lead to thermal runaway at fixed voltage. We deduce a voltage scale for ET onset [4], $V_{s} =\sqrt {\kappa /{\sigma }'_{0} } $ [in volts], where $\kappa $ is the thermal conductivity [in W/(m-K)] and ${\sigma }'_{0} $ is the rate of change of the electrical conductivity with respect to temperature [in 1/(ohm-m-K)]. $V_{s} $ depends only on material properties and is independent of geometry or the operating voltage. It measures the intrinsic tolerance of the material to ET. The calculated $V_{s} $ are consistent with the well-known properties of several common materials, such as Si, Ge, C (graphite), and SiC [4].\\[4pt] [1] \textit{Review of Federal Programs for Wire System Safety}, NSTC Final Report, 2000. [2] Zhang, PhD dissertation, UM, Ann Arbor (2012). [3] Zhang et al., IEEE TED 59, 1936 (2012); J. Phys. D: Appl. Phys. 46, 065502 (2013); ibid 46, 209501 (2013); IEEE JEDS (in the press, 2013). [4] Lau, et. al., PPPS Proc 2013. [Preview Abstract] |
Thursday, November 14, 2013 10:30AM - 10:42AM |
TO5.00006: Particle simulation of radio frequency waves in fusion plasmas Animesh Kuley, Bao Jian, Zhihong Lin Radio frequency (RF) waves can provide heating, current and flow drive, as well as instability control, and others nonlinear phenomenon for steady state operations of fusion experiments. A RF particle simulation has been developed in this work to provide a first-principles tool for studying the RF nonlinear interactions with plasmas. In this model, ions are considered as fully kinetic particles using the Vlasov equation and electrons are treated as guiding centers using the drift kinetic equation. This model has been implemented in a global gyrokinetic toroidal code GTC with realistic toroidal geometry using real electron-to-ion mass ratio. Linear simulations of ion plasma oscillation, ion Bernstein wave, and lower hybrid wave have been verified. Also we verified the propagation of the lower hybrid wave in cylindrical and toroidal geometry and its Landau damping by electron. Our goal is to develop a nonlinear toroidal particle code to study the radio frequency wave heating and current drive in fusion plasmas. [Preview Abstract] |
Thursday, November 14, 2013 10:42AM - 10:54AM |
TO5.00007: Plasma Heating and Current Drive by Stochastic Acceleration of Relativistic Electrons at the WEGA Stellarator Heinrich Laqua, Enrico Chlechowitz, Vladimir Fuchs, Matthias Otte, Torsten Stange Relativistic electrons with parallel energies of up to 2 MeV have been continuously (10 s) generated by a stochastic interaction with the rf-field (6-26kW) of a 2.45 GHz open waveguide antenna without any loop voltage. These ``run-away'' electrons have been detected by their synchrotron, x- and $\gamma $-ray emission and have also generated a toroidal plasma current in the kA range. They are perfectly confined in the stellarator magnetic field of 0.5 T. The particle trajectories form their own nested drift surfaces which are shrunken inward and shifted outward with respect to the magnetic flux surfaces. This geometrical effect connects the antenna region, where the electrons are accelerated, with the plasma core, where a low temperature (20eV, 0.2-5 10$^{\mathrm{18~}}$m$^{\mathrm{3}})$ bulk plasma is generated. The acceleration process was modelled by a random walk diffusion model and a Fermi Ulan map Monte-Carlo simulation. Both calculations show similar results for the heating and current drive efficiencies. They also reproduce the temporal behaviour of the plasma current and the synchrotron radiation, when the RF-power is modulated and show the need for a random phase interaction between the relativistic electrons and the antenna field. [Preview Abstract] |
Thursday, November 14, 2013 10:54AM - 11:06AM |
TO5.00008: 3-D microwave simulation in fusion plasmas Thomas Williams, Roddy Vann, Martin O'Brien, Alf Koehn The propagation of EM radiation past wavelength-sized 3D inhomogeneities is not well understood, yet is of importance for both microwave heating and diagnostic applications in tokamaks. To improve this understanding, a new cold-plasma code has been written to extend full-wave simulations of propagation and mode conversion in magnetized plasmas to 3D. Studies of propagation past density filaments (``blobs'') are presented and compared with 2D simulations. This work supports MAST experiments using the SAMI diagnostic to image microwave emission from the plasma edge due to mode conversion from electron Bernstein waves. Significant fluctuations in the SAMI data mean that detailed modelling is required to improve its interpretation, since analytic and experimental work suggests that electron density fluctuations and magnetic shear can affect the mode conversion efficiency. [Preview Abstract] |
Thursday, November 14, 2013 11:06AM - 11:18AM |
TO5.00009: MHD spectroscopy of moving plasmas Hans Goedbloed MHD spectroscopy of the waves and instabilities of moving laboratory and astrophysical plasmas is a valuable diagnostic for the determination of the magnetic geometry of these plasmas. Whereas presently available high-resolution spectral codes are routinely exploited to compute the occurring swarms of isolated complex eigenvalues, so far, no structures connecting them were known to exist. Such structures have recently been obtained by means of the construction of the {\em spectral web,} consisting of a dual set of curves in the complex $\omega$-plane with the eigenvalues on the intersections. For static plasmas, one set of the curves degenerates into the real and imaginary axes to which the eigenvalues are restricted to lie while monotonically connected through Sturm--Liouville properties of the real eigenfunctions. For rotating plasmas, the spectral web has a surprisingly involved geometry, but it does provide insight into the connection between local and global instabilities related to a novel kind of monotonicity properties of the complex eigenfunctions. All this will be illustrated by the spectral webs for force-free magnetic fields, internal kink modes of tokamaks, magneto-rotational instabilities of accretion disks, and jets ejected from compact objects. [Preview Abstract] |
Thursday, November 14, 2013 11:18AM - 11:30AM |
TO5.00010: Theoretical Origins of Nonresonant m/n=1/1 Modes in Weakly Reversed Shear D.P. Brennan, J.M. Finn, M.R. Halfmoon, C.C. Kim, A.D. Turnbull It is important for us to understand the rich physics of the stability of weakly reversed shear configurations in toroidal confinement experiments. At infinite aspect ratio, a nonresonant $m/n=1/1$ eigenfunction, delimited within the minimum in $q >\sim 1$, is a stable solution to the MHD stability equation. This non-resonant $1/1$ mode has a top-hat structure, and its analysis is mathematically related to that of the usual resonant $1/1$ mode with $q<1$. At finite aspect ratio, this mode is manifested as a continuous coupled component of the stable and unstable modes, becoming the dominant $m=1$ component as $q_{min}$ approaches unity from above. The coupling to $m>1$ components varies with aspect ratio. A stable discrete mode exists, surrounded in frequency by a continuum of stable modes. The frequency of the mode approaches marginality with increasing $\beta$. The continuum modes are discretized by the discrete computational treatment, but remain identifiable. Energetic particles can resonate with and drive the stable discrete mode unstable. The interaction of the particles with the modes changes the eigenfunction and frequency of the mode, depending on the physics of the particle interaction, giving results in good agreement with previous simulations and experiments. [Preview Abstract] |
Thursday, November 14, 2013 11:30AM - 11:42AM |
TO5.00011: Meshless Method for Solving Fixed Boundary Problem of Axisymmetric Plasma Equilibrium Ryota Imazawa, Yasunori Kawano, Kiyoshi Itami This study is to solve Grad-Shafranov (GS) equation with the fixed plasma boundary by utilizing the meshless method for the first time. The previous studies have utilized the finite element method (FEM) to solve the equilibrium inside the fixed separatrix. In order to avoid the difficulty of FEM (e.g. mesh problem, difficulty of coding, expensive calculation cost, etc.), this study proposes the new method to apply the meshless methods, especially RBF-MFS and Kansa's method to inhomogeneous and nonlinear partial differential equations (PDE). Although the RBF-MFS and Kansa's method are applicable to the inhomogeneous PDE, the application of these methods to the GS equation is not straight-forward. Since the current profile is usually parameterized by the normalized poloidal flux, the inhomogeneous term of the GS equation contains the normalized poloidal flux, not just a poloidal flux. This is the difficulty for solving the GS equation. Accuracy and calculation time of the meshless method and FEM are compared in the condition of the same total number of nodes. The results show that the error of magnetic field obtained by the meshless methods is one hundredth of that by FEM and that the calculation time of the meshless method is one tenth of that of FEM. Moreover, this study shows that the meshless methods can be easily accelerated by parallel computing. [Preview Abstract] |
Thursday, November 14, 2013 11:42AM - 11:54AM |
TO5.00012: Active feedback stabilization of flute instability in a mirror trap Ilan Be'ery, Omri Seemann, Amnon Fruchtman, Amnon Fisher, Amiram Ron The flute instability in a table-top mirror machine has been stabilized by a feedback system consisting of optical probes, digital signal processor, and needle electrodes. The total response time of the system is 5$\mu $s, which is considerably faster than the typical flute growth time. Simulation and a dynamic model of the plasma's response to the needle actuators were tested against cyclic bias experiments. The plasma density is increased by the stabilization by a factor of two and is limited by other decay processes. [Preview Abstract] |
Thursday, November 14, 2013 11:54AM - 12:06PM |
TO5.00013: Radial Localization of Toroidal Alfven Eigenmode in Tokamak Plasmas Zhixuan Wang, Zhihong Lin, William Heidbrink, Benjamin Tobias, Michael Van Zeeland Toroidal Alfven eigenmode (TAE) with radially extended structures can be driven unstable by pressure gradients of energetic particles (EP). These unstable Alfveneigenmodes (AE) have been routinely observed in fusion experiments to induce a large EP transport, whichcould degrade overall plasma confinement and damagefusion devices.In the well-accepted paradigm,the growth rate of the AEs can be calculated from a perturbative EP contribution to a fixedmode structure and real frequency given by magnetohydrodynamic (MHD) properties of thermal plasmas. However, linear and nonlinear kinetic effects of both EP and thermal plasmasare important and should be treated on the same footing. The gyrokinetic simulation has thus emerged as anecessary and powerful tool for studying the linear andnonlinear dynamics of AEs. In the current work, the gyrokinetic toroidal code(GTC) linear simulation of the tokamakexperiment finds a radial localization of the TAE dueto the non-perturbative EP contribution. The EP-drivenTAE has a radial mode width much smaller than thatpredicted by the MHD theory. The TAE radial positionpeaks at and moves with the location of the strongest EPpressure gradients. Experimental data confirms that the eigenfunction drifts quicklyoutward radially. The non-perturbativeEP contribution also breaks the radial symmetry of the mode structure and induces a TAE frequency dependence on the toroidal mode number, in excellent agreement with the experimental measurements. [Preview Abstract] |
Thursday, November 14, 2013 12:06PM - 12:18PM |
TO5.00014: Rapid Frequency Chirps of TAE mode due to Finite Orbit Energetic Particles Herb Berk, Ge Wang The tip model for the TAE mode in the large aspect ratio limit, conceived by Rosenbluth et. al. [1] in the frequency domain, together with an interaction term in the frequency domain based on a map model [2], has been extended into the time domain. We present the formal basis for the model, starting with the Lagrangian for the particle wave interaction. We shall discuss the formal nonlinear time domain problem and the procedure that needs to obtain solutions in the adiabatic limit.\\[4pt] [1] M.N. Rosenbluth, et. al.~Phys. Fluids B, \textbf{4}, 2189, (1992)\\[0pt] [2] Berk, H.L., Breizman, B.N. and Ye, H.~Phys. Fluids B,~\textbf{5},1506, 1993 [Preview Abstract] |
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