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 GM9: Mini-Conference: Plasma Energization - Interactions Between Fluid and Kinetic Scales II |
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Chair: Hui Li, Los Alamos National Laboratory Room: 100/101 |
Tuesday, November 17, 2015 9:30AM - 10:00AM |
GM9.00001: Gyrokinetic Particle Simulation of Kinetic MHD Zhihong Lin, Ge Dong, Peng Jiang The excitation and evolution of macroscopic magnetohydrodynamic (MHD) instabilities that limit the performance of fusion plasmas often depend on kinetic effects at microscopic scales as well as the nonlinear coupling of multiple physical processes. As the first step toward integrated simulation coupling multiple physical processes, effects of magnetic islands on neoclassical transport and microturbulence has been studied in GTC simulations. Simulations find that different toroidal modes are linearly coupled together and that toroidal spectra become broader when the island width increases. The real frequencies and growth rates of different toroidal modes approach each other with the averaged value independent of the island width. The linear mode structures are enhanced at the island separatrices and weakened at the island centers, consistent with the flattening of the pressure profile inside the islands. Furthermore, GTC simulations of neoclassical transport find that the balance between the perpendicular and parallel transport sets the density gradient inside the magnetic island. As a result, the radial distribution of the bootstrap current change significantly. The bootstrap current decreases dramatically inside the island. On the other hand, the steepening of the density gradient outside the islands leads to a larger bootstrap current in the vicinity of the separatrix. The volume averaged values of the bootstrap current does not change. [Preview Abstract] |
Tuesday, November 17, 2015 10:00AM - 10:30AM |
GM9.00002: The Conversion of Large-Scale Turbulent Energy to Plasma Heat In Astrophysical Plasmas Gregory Howes Turbulence in space and astrophysical plasmas plays a key role in the conversion of the energy of violent events and instabilities at large scales into plasma heat. The turbulent cascade transfers this energy from the large scales at which the motions are driven down to small scales, and this essentially fluid process can be understood in terms of nonlinear wave-wave interactions. At sufficiently small scales, for which the dynamics is often weakly collisional, collisionless mechanisms damp the turbulent electromagnetic fluctuations, and this essentially kinetic process can be understood in terms of linear wave-particle interactions. In this talk, I will summarize the possible channels of the turbulent dissipation in a weakly collisional plasma, and present recent results from kinetic numerical simulations of plasma turbulence. Finally, I will discuss strategies for the definitive identification of the dominant dissipation channels using spacecraft measurements of turbulence in the solar wind. [Preview Abstract] |
Tuesday, November 17, 2015 10:30AM - 10:50AM |
GM9.00003: Kinetic Versus Fluid Effects in Turbulence: A Study of Distribution Function Dynamics Jason TenBarge, James Juno Upcoming and proposed spacecraft missions will supply unprecedented particle distribution function data sets; however, our current predictions of the shape of the distribution function in a turbulent bath are mostly informed by simplistic models using quasi-linear theory or simplistic numerical simulations that do not employ a full, strong turbulence cascade. The few existing numerical simulations of turbulence that have focused on distribution functions dynamics have ascribed all observed features to kinetic effects, ignoring the possibility that linear waves alter the distribution and the adiabatic changes that could also occur. Inspired by the Turbulence Dissipation Challenge, we use the novel Gkeyll framework, which contains both a mutli-fluid model and the full Vlasov-Maxwell system, to compare the results of a fluid to a fully kinetic approach to studying simulations of a spectrum of kinetic Alfv\'{e}n waves and a 2.5D Orszag-Tang vortex. We focus on the particle distribution function dynamics to identify both similarities and differences in the fluid and full kinetic simulations in an attempt to identify what are truly kinetic effects versus potentially adiabatic changes. [Preview Abstract] |
Tuesday, November 17, 2015 10:50AM - 11:20AM |
GM9.00004: Energy Transfer and Saturation in Kinetic Turbulence P.W. Terry, M.J. Pueschel, G.G. Whelan, Z. Williams Kinetic turbulence in magnetic confinement fusion devices saturates by transfer of energy both to large-scale damped modes and to small-scale perpendicular wavenumbers through interaction with large-scale zonal flows. The energy branching ratio between the perpendicular wavenumber cascade and large scale-stable modes is not well known for different values of collisionality, driving gradients, plasma beta, and other parameters, nor is there an understanding of saturation and its scalings that accounts for the key players in this physics. We describe efforts to understand the energetics of saturation across regimes that range from fluid to kinetic using a combination of analytic theory and gyrokinetic simulation with advanced diagnostics that include mode decomposition of spectral energy transfer and the scale-dependent ratio of spectral energy transfer to dissipation. From simulation, we analyze the damping of large scale zonal flows, the effect of stable modes in collisionless regimes, and the role of magnetic fluctuations at finite beta. From analytic theory we study scalings of turbulence and zonal flow saturation levels with collisionality and gradient drive. [Preview Abstract] |
Tuesday, November 17, 2015 11:20AM - 11:40AM |
GM9.00005: Integrated accretion disk angular momentum removal and astrophysical jet acceleration mechanism Paul Bellan A model has been developed for how accretion disks discard angular momentum while powering astrophysical jets. The model depends on the extremely weak ionization of disks. This causes disk ions to be collisionally locked to adjacent disk neutrals so a clump of disk ions and neutrals has an effective cyclotron frequency $\alpha\omega_{ci}$ where $\alpha$ is the fractional ionization. When $\alpha\omega_{ci}$ is approximately twice the Kepler orbital frequency, conservation of canonical momentum shows that the clump spirals radially inwards producing a radially inward disk electric current as electrons cannot move radially in the disk. Upon reaching the jet radius, this current then flows axially away from the disk plane along the jet, producing a toroidal magnetic field that drives the jet. Electrons remain frozen to poloidal flux surfaces everywhere and electron motion on flux surfaces in the ideal MHD region outside the disk completes the current path. Angular momentum absorbed from accreting material in the disk by magnetic counter-torque $-J_{r}B_{z}$ is transported by the electric circuit and ejected at near infinite radius in the disk plane. This is like an electric generator absorbing angular momentum and wired to a distant electric motor that emits angular momentum. [Preview Abstract] |
Tuesday, November 17, 2015 11:40AM - 12:10PM |
GM9.00006: Studies of the linear and nonlinear properties of Alfv\'{e}n waves in LAPD Troy Carter, Seth Dorfman, Walter Gekelman, Shreekrishna Tripathi, Bart Van Compernolle, Steve Vincena, Giovanni Rossi, Frank Jenko An overview will be given of recent experimental research into linear and nonlinear properties of Alfv\'{e}n waves in the Large Plasma Device (LAPD). The nonlinear three-wave interaction process at the heart of the parametric decay instability is studied by launching counter-propagating Alfv{\'e}n waves from antennas placed at either end of LAPD, producing a damped ion acoustic mode.\footnote{S. Dorfman and T.A. Carter, Phys. Rev. Lett. 110, 195001 (2013)} The decay of a lone, large amplitude Alfv\'{e}n wave has been observed, producing co-propagating daughter waves with characteristics consistent with kinetic Alfv\'{e}n waves. The process has an amplitude threshold and the frequency of the daughter modes varies with the amplitude of the pump. A new plasma source based on LaB$_6$ cathode has been added to LAPD, enabling much higher density (x50), electron temperature (x2) and ion temperature (x6). This provides the opportunity to study the physics of waves and instabilities with space and astrophysically relevant $\beta$. Topics under investigation include the physics of Alfv\'{e}n waves in increased $\beta$ plasmas, electromagnetic effects in drift-Alfv\'en wave turbulence and the excitation of ion-temperature-anisotropy driven modes such as the mirror and firehose. [Preview Abstract] |
Tuesday, November 17, 2015 12:10PM - 12:30PM |
GM9.00007: Canonical field theory Setthivoine You A new canonical field theory has been developed to help interpret the interaction between plasma flows and magnetic fields. The theory augments the Lagrangian of general dynamical systems to rigourously demonstrate that canonical helicity transport is valid across single particle, kinetic and fluid regimes, on scales ranging from classical to general relativistic. The Lagrangian is augmented with two extra terms that represent the interaction between the motion of matter and electromagnetic fields. The dynamical equations can then be re-formulated as a canonical form of Maxwell's equations or a canonical form of Ohm's law valid across all non-quantum regimes. The field theory rigourously shows that helicity can be preserved in kinetic regimes and not only fluid regimes, that helicity transfer between species governs the formation of flows or magnetic fields, and that helicity changes little compared to total energy only if density gradients are shallow. The theory suggests a possible interpretation of particle energization partitioning during magnetic reconnection as canonical wave interactions. [Preview Abstract] |
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