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 GO7: Waves, Turbulence, Transport and Theory |
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Chair: Ami Dubois, University of Wisconsin-Madison Room: Galerie 6 |
Tuesday, October 28, 2014 9:30AM - 9:42AM |
GO7.00001: Ion Heating Due to Low-Frequency Wave Propagation in Partially Ionized Plasmas With a Strong Density Gradient in the Hot Helicon Experiment (HELIX) Stephanie Sears, Robert VanDervort, Greg Lusk, Mark Soderholm, John McKee, Earl Scime Ion heating is observed in the solar corona but is still poorly understood. Alfv\'{e}n wave damping is one of the most plausible mechanisms proposed to explain coronal ion heating. With time-resolved Laser Induced Fluorescence (LIF), we have measured the increase in ion temperature in HELIX in the presence of low-frequency waves generated by a small antenna near the high-density core. We have taken LIF measurements across the plasma column to characterize how the local density and density gradient affect ion heating. Wavelet analysis of localized, small-scale (smaller than an ion gyroradius) magnetic probe measurements suggest that plasma turbulence also plays a role in ion heating. [Preview Abstract] |
Tuesday, October 28, 2014 9:42AM - 9:54AM |
GO7.00002: The stability of freely-propagating ion acoustic waves in 2D systems Thomas Chapman, Richard Berger, Jeffrey Banks, Stephan Brunner The stability of a freely-propagating ion acoustic wave (IAW) is a basic science problem that is made difficult by the need to resolve electron kinetic effects over a timescale that greatly exceeds the IAW period during numerical simulation. Recent results examining IAW stability using a 1D+1V Vlasov-Poisson solver indicate that instability is a fundamental property of IAWs that occurs over most if not all of the parameter space of relevance to ICF experiments [1]. We present here new results addressing the fundamental question of IAW stability across a broad range of plasma conditions in a 2D+2V system using LOKI [2,3], ranging from a regime of relatively weak to a regime of relatively strong ion kinetic effects. \\[4pt] [1] T. Chapman, S. Brunner, J. W. Banks, R. L. Berger, B. I. Cohen, and E. A. Williams, Phys. Plasmas 21, 042107 (2014).\\[0pt] [2] J. W. Banks and J. A. F. Hittinger, IEEE Trans. Plasma Sci. 38, 2198 (2010).\\[0pt] [3] J. W. Banks, R. L. Berger, S. Brunner, B. I. Cohen, and J. A. F. Hittinger, Phys. Plasmas 18, 052102 (2011). [Preview Abstract] |
Tuesday, October 28, 2014 9:54AM - 10:06AM |
GO7.00003: Intermittent Profile Collapse in a Basic Heat Transport Experiment B. Van Compernolle, G.J. Morales, J.E. Maggs, R. Sydora Results of a basic heat transport experiment involving an off-axis heat source are presented. Experiments are performed in the Large Plasma Device (LAPD) at UCLA. A ring-shaped electron beam source injects low energy electrons ($<$ 20 eV) along the magnetic field. The injected electrons are thermalized within a short distance and provide an off-axis heat source that results in a long, hollow, cylindrical filament of elevated electron temperature embedded in a colder plasma. The electron heat transport is studied as a function of heating power. At low heating power classical heat transport is observed. At high heating powers drift-wave fluctuations dominate the transport. At intermediate heating powers a regime has been found in which intermittent collapses of the temperature profile occur. The heating causes the radial temperature profiles to steepen until a threshold is reached at which time drift waves grow and cause a rapid collapse. After the profile collapses the drift wave activity disappears. On a longer time-scale the profile slowly recovers and steepens again and the process repeats. The repetition frequency of the collapses is a sensitive function of heating power, with only a few collapses at low heating powers, and many in rapid succession at higher heating powers. [Preview Abstract] |
Tuesday, October 28, 2014 10:06AM - 10:18AM |
GO7.00004: 3D Gyrokinetic Simulation of Heat Transport in a Magnetized Ring-Shaped Temperature Filament R. Sydora, B. Van Compernolle, G.J. Morales, J.E. Maggs 3D electromagnetic gyrokinetic particle simulations of drift-Alfven fluctuations and thermal transport have been carried out using plasma conditions similar to those in the recent off-axis heat source experiment performed in the Large Plasma Device (LAPD) at UCLA. The novel heat source resulted in a long, hollow cylindrical temperature filament of elevated electron temperature(Te) embedded in a colder plasma. The drift wave fluctuations and thermal transport and profiles have been characterized experimentally. The gyrokinetic simulations (cylindrical geometry) are initialized using experimental parameters including the radial and axial extent of the hollow cylindrical Te-filament. The inner ring diameter is taken to be approximately four times its width. The Te gradient is mainly determined by the ratio of the peak temperature in the ring to the background temperature and above a certain gradient threshold, drift-Alfven fluctuations were excited and induced a rapid thermal collapse on time scales consistent with experiment. The spatio-temporal pattern of the electrostatic potential, density, and magnetic fluctuations have been analyzed in the linear and nonlinear, saturated state of evolution. Detailed comparisons with experiment are presented. [Preview Abstract] |
Tuesday, October 28, 2014 10:18AM - 10:30AM |
GO7.00005: Profile evolving tokamak plasma edge turbulence simulations Bo Li, D.R. Ernst, C. Sun, A. Zhou, H. Zhang We have developed a new flux-driven 3D two-fluid code for the simulation of resistive ballooning modes in the tokamak edge and SOL. The plasma pressure and potential profiles are evolved self-consistently using the drift-reduced Braginskii equations. The simulation domain is radially global with both closed and open field line regions. We find that the plasma potential structures play an important role in the edge turbulence and transport. The simulations show that the nonlinear evolution of the curvature-driven mode produces the large-scale convective cells. These radially broad eddies result in the flattened pressure profiles in the turbulent, steady state. [Preview Abstract] |
Tuesday, October 28, 2014 10:30AM - 10:42AM |
GO7.00006: Non-diffusive suprathermal ion transport associated with blobs in TORPEX plasmas Alexandre Bovet, Fabio Avino, Ambrogio Fasoli, Ivo Furno, Kyle Gustafson, Paolo Ricci We present unprecedented space and time-resolved measurements of the transport of a suprathermal ion beam injected in the toroidal device TORPEX. Experiments are performed in turbulence dominated by an ideal-interchange mode using a Li6+ ion source and a set of energy analyzers. Depending on the suprathermal ion energy, the transport exhibits subdiffusive or superdiffusive behaviors. The fast ion current fluctuations are quasi-Gaussian in the former regime and strongly intermittent in the latter. In the superdiffusive case, using conditional sampling, we show that the transport is associated with intermittent field-aligned blobs. In the subdiffusive case, supra-thermal ions average the turbulent structures during their gyro-motion and their vertical drift. These results complement our investigations of the supra-thermal ion transport using 3D time-averaged measurements, which are in agreement with numerical simulations. Numerical modeling is performed by computing the trajectories of tracers in a turbulent electrostatic field generated by a 2D global fluid model. Gyro- and drift-averaging reduce the transport. The evolution of the radial distribution of fast ions can be modeled by a fractional diffusion equation describing fractional L\'{e}vy motion. [Preview Abstract] |
Tuesday, October 28, 2014 10:42AM - 10:54AM |
GO7.00007: Coherent and Turbulent Fluctuation Dynamics in a Linear Magnetized Plasma with Biasing Tiiffany Desjardins, Mark Gilmore, Dustin Fisher, Jose-Miguel Reynolds-Barredo The Helicon-Cathode (HelCat) Device at the University of New Mexico is a linear plasma device that exhibits a wide range of plasma dynamics. HelCat has intrinsic fluctuations that vary from coherent to fully turbulent, depending on variables such as magnetic field strength, source power, and neutral background fill. In addition, biased grid and ring electrodes are found to strongly affect the fluctuation dynamics. A detailed study of the transition from a coherent state to a fully turbulent states with the variation of operating parameters and electrode bias is underway. It is seen that with increased magnetic field, fluctuation mode and character changes, and the plasma may become chaotic, before becoming turbulent. With biasing, it is possible to fully suppress instabilities and in some cases excite new ones. In addition to experimental measurements, a linear eigenmode solver is used to accurately identify the instabilities resent. A basic overview of results and analysis will be presented. [Preview Abstract] |
Tuesday, October 28, 2014 10:54AM - 11:06AM |
GO7.00008: On the validity of drift-reduced plasma fluid models Jarrod Leddy, Ben Dudson, Michele Romanelli Fluid approximations are often used in the simulation of plasmas to reduce the dimensionality of the system from the full kinetic description. A further simplification can be made to reduce the full velocity vector field in the Braginskii equations to parallel velocity and parallel vorticity by taking the curl of the velocity equation. This so-called drift-reduced simplification assumes small drift velocities perpendicular to the magnetic field lines. The Hazeltine 4-field model makes use of this reduction and assumes an isothermal plasma minimising the full fluid equations to solve for pressure, poloidal flux, parallel velocity, and parallel vorticity. This model has been implemented using the BOUT$++$ framework and benchmarked against the full Braginskii code CENTORI. It was predicted that as gradients and drives were increased, the resulting increase in drift velocities would render the model inaccurate; however, initial results indicate that linear growth rates continue to match analytical values. Further study will reveal how the nonlinear behaviour is affected in this regime. [Preview Abstract] |
Tuesday, October 28, 2014 11:06AM - 11:18AM |
GO7.00009: Can time-spectral methods improve turbulence modelling? Jan Scheffel In computational fusion physics, the widely separated time and space scales often demand extremely long computer simulations and vast memory resources, using finite time steps. Gyrokinetic turbulence modelling at high Reynolds or Lundquist numbers may be allocated millions of CPU hours for parallel processing on supercomputers. It is thus worthwhile to explore new avenues that may alleviate requirements on computer power. Indeed, time-stepping may be completely avoided for initial-value problems. In the recently developed Generalized Weighted Residual Method GWRM [1], temporal, spatial and parameter domains are all handled using a solution ansatz in the form of a sum of Chebyshev polynomials. The coefficients of the ansatz are determined using a weighted residual method for which a new efficient equation solver has been developed [2]. In addition, the temporal and spatial computational region has been successfully treated using subdomain methods in a number of test problems, more efficiently than relevant finite difference methods. The GWRM, however, relies on solution of linear systems of equations in each subdomain, and memory requirement is an issue. In this presentation we will discuss recent subdomain approaches for efficient and convergent modelling of drift-wave turbulence. \\[4pt] [1] Scheffel J, Partial Differential Equations: Theory, Analysis and Applications (Nova Science Publishers) p 1-49, 2011. \\[0pt] [2] Scheffel J and H{\aa}kansson C, Appl. Numer. Math. 59(2009)2430. [Preview Abstract] |
Tuesday, October 28, 2014 11:18AM - 11:30AM |
GO7.00010: Chaotic motion of charged particles in toroidal magnetic configurations Xavier Leoncini, Benjamin Cambon, Michel Vittot, Remi Dumont, Xavier Garbet We study the motion of a charged particle in a toroidal magnetic field and discuss its chaotic nature. First considering an idealized magnetic configuration, we add a non generic perturbation corresponding to a magnetic ripple, breaking one of the invariant of the motion. Chaotic motion is then observed and opens questions about the link between chaos of magnetic field lines and chaos of particle trajectories. Second, we return to an axi-symmetric configuration and tune the safety factor (magnetic configuration) in order to recover chaotic motion. In this last setting with two constants of the motion, the presence of chaos implies that no third global constant exists. We are facing a mixed phase space with both regular and chaotic regions and point out the difficulties in performing a global reduction such as gyrokinetics. [Preview Abstract] |
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