Ryusuke Numata

AstroGK: Astrophysical gyrokinetics code

The gyrokinetic simulation code AstroGK is developed to study fundamental aspects of kinetic plasmas and for applications mainly to astrophysical problems. AstroGK is an Eulerian slab code that solves the electromagnetic gyrokinetic-Maxwell equations in five-dimensional phase space, and is derived from the existing gyrokinetics code GS2 by removing magnetic geometry effects. Algorithms used in the code are described. The code is benchmarked using linear and nonlinear problems. Serial and parallel performance scalings are also presented.

Gyrokinetic simulations of the tearing instability

Linear gyrokinetic simulations covering the collisional -- collisionless transitional regime of the tearing instability are performed. It is shown that the growth rate scaling with collisionality agrees well with that predicted by a two-fluid theory for a low plasma beta case in which ion kinetic dynamics are negligible. Electron wave-particle interactions (Landau damping), finite Larmor radius, and other kinetic effects invalidate the fluid theory in the collisionless regime, in which a general non-polytropic equation of state for pressure (temperature) perturbations should be considered. We also vary the ratio of the background ion to electron temperatures, and show that the scalings expected from existing calculations can be recovered, but only in the limit of very low beta.

Ion and electron heating during magnetic reconnection in weakly collisional plasmas

Magnetic reconnection and associated heating of ions and electrons in strongly magnetised, weakly collisional plasmas are studied by means of gyrokinetic simulations. It is shown that an appreciable amount of the released magnetic energy is dissipated to yield (irreversible) electron and ion heating via phase mixing. Electron heating is mostly localized to the magnetic island, not the current sheet, and occurs after the dynamical reconnection stage. Ion heating is comparable to electron heating only in high-beta plasmas, and results from both parallel phase mixing and perpendicular phase mixing due to finite Larmor radius effects; in space, ion heating is localized to the interior of a secondary island (plasmoid) that arises from the instability of the current sheet.

Numerical modeling of Large Plasma Device Alfvén wave experiments using AstroGK

Collisions between counterpropagating Alfven waves represent the fundamental building block of plasma turbulence, a phenomenon of great importance to a wide variety of fields, from space physics and astrophysics to controlled magnetic fusion. Proposed experiments to study Alfven wave collisions on the Large Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angeles, will benefit significantly from numerical modeling capable of reproducing not only the linear dispersive effects of kinetic and inertial Alfven waves, but also the nonlinear evolution of the Alfvenic turbulence. This paper presents a comparison of linear simulation results using the astrophysical gyrokinetics code, AstroGK, to the measured linear properties of kinetic and inertial Alfven waves in the LAPD plasma. Results demonstrate that: (1) finite frequency effects due to the ion cyclotron resonance do not prevent satisfactory ...

Gyrokinetic simulations of collisionless reconnection in turbulent non-uniform plasmas

We present nonlinear gyrokinetic simulations of collisionless magnetic reconnection with non-uniformities in the plasma density, the electron temperature, and the ion temperature. The density gradient can stabilize reconnection due to diamagnetic effects but destabilize driftwave modes that produce turbulence. The electron temperature gradient triggers microtearing modes that drive rapid small-scale reconnection and strong electron heat transport. The ion temperature gradient destabilizes ion temperature gradient modes that, like the driftwaves, may enhance reconnection in some cases.

Nonlinear three-dimensional simulation for self-organization and flow generation in two-fluid plasmas

A three-dimensional Hall-Magnetohydrodynamics (Hall-MHD) simulation code has been developed to study the self-organization process in two-fluid plasmas. An appreciable amount of flow is created in the direction perpendicular to the magnetic field, which is in a sharp contrast with the relaxed states in single-fluid MHD.

AC loss of HTSC bulks for magnetic levitation

Abstract The new simulation method was developed based on the T -method to simulate the AC loss of high temperature superconducting magnetic flywheel when the magnetic rotor rotates with the inhomogeneous component of the magnetic field. The dependencies of AC loss upon some conditions such as the wave number, the amplitude of inhomogeneous component and so on were analyzed.

Electron and Ion Heating during Magnetic Reconnection in Weakly Collisional Plasmas

Gyrokinetic simulations of magnetic reconnection are presented to investigate plasma heating for strongly magnetized, weakly collisional plasmas. For a low plasma beta case, parallel and perpendicular phase mixing strongly enhance energy dissipation yielding electron heating. Heating occurs for a long time period after a dynamical process of magnetic reconnection ended. For a higher beta case, the ratio of ion to electron dissipation rate increases, suggesting that ion heating (via phase-mixing) may become an important dissipation channel in high beta plasmas.

Numerical analysis on the contribution of the singular perturbation by the Hall term to the spectrum of MHD turbulence using a shell model

We have developed a new shell model for the Hall magnetohydrodynamic (MHD) system to investigate the spectral properties of the plasma turbulence. Through the numerical simulation of the shell model, in the Hall MHD case. we find that the energy spectral index of the flow field indicates -5/3 in the whole inertial range, while the energy spectral index of the magnetic field indicates -5 3 (-7/3) in the large- (small-)scale region of the inertial range. The simulation of the conventional MHD case was also carried out. and we find that the energy spectral index of both tlie flow and magnetic fields indicate -5/3.