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.Comment: 9 pages, 3 figures, accepted for publication in JPSJ Suppl. [Proceedings of the 12th Asia Pacific Physics Conference

Bifurcation in electrostatic resistive drift wave turbulence

The Hasegawa-Wakatani equations, coupling plasma density and electrostatic potential through an approximation to the physics of parallel electron motions, are a simple model that describes resistive drift wave turbulence. We present numerical analyses of bifurcation phenomena in the model that provide new insights into the interactions between turbulence and zonal flows in the tokamak plasma edge region. The simulation results show a regime where, after an initial transient, drift wave turbulence is suppressed through zonal flow generation. As a parameter controlling the strength of the turbulence is tuned, this zonal flow dominated state is rapidly destroyed and a turbulence-dominated state re-emerges. The transition is explained in terms of the Kelvin-Helmholtz stability of zonal flows. This is the first observation of an upshift of turbulence onset in the resistive drift wave system, which is analogous to the well-known Dimits shift in turbulence driven by ion temperature gradients.Comment: 21 pages, 11 figure

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.Comment: 7 pages, 10 figures, submitted to Po

Stability Analysis of Time Stepping for Prolonged Plasma Fluid Simulations

The Hasegawa-Wakatani system of equations may be used to predict and study the behavior of plasma flow. Stability analysis of the flow requires results over prolonged time series, which places a great strain on computational resources. Results can only be achieved for a wide choice of parameters by using numerical methods that allow long time steps and do not pollute the results with numerical instabilities. The report presents an analysis of several linear multistep methods and concludes that much of the understanding of the stability of linear systems also applies to the study of nonlinear problems such as the Hasegawa-Wakatani system of equations. In particular, methods such as the backward differentiation formulas should be used with the stiff systems generated by the discrete formulation of the Hasegawa-Wakatani system of equations

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 spectra

Marginally Stable Current Sheets in Collisionless Magnetic Reconnection

Non-collisional current sheets that form during the nonlinear development of magnetic reconnection are characterized by a small thickness, of the order of the electron skin depth. They can become unstable to the formation of plasmoids, which allows the magnetic reconnection process to reach high reconnection rates. However, no work has so far investigated the marginal stability conditions for the development of plasmoids when the forming current sheet is purely collisionless. We analyze the geometry that characterizes the reconnecting current sheet, and what promotes its elongation. Once the reconnecting current sheet is formed, we identify the regimes for which it is plasmoid unstable. Our study shows that plasmoids can be obtained, in this context, from current sheets with an aspect ratio much smaller than in the collisional regime, and that the plasma flow channel of the marginally stable current layers maintains an inverse aspect ratio of $0.1$