Thermal Magnetic Fluctuations and Anomalous Electron Diffusion

Abstract

We investigate electron transport for a thermal plasma using a magnetostatic plasma model. Three cases are investigated; (1) fixed ions, only magnetic fluctuations; (2) fixed ions, magnetic and electrostatic fluctuations; (3) mobile ions, magnetic, and electrostatic fluctuations. For (1), magnetic islands as well as many unclosed field lines occurred; for (2) and (3) electrostatic convective cells occurred along with magnetic islands giving a much more turbulent situation. Generally the transport due to electrostatic convective cells dominated.Anomalous plasma diffusion due to thermally excited convective cells has been studied in detail. 1 Even for thermal equilibrium, plasma diffusion across a strong magnetic field can be dominated by zero-frequency fluctuations (convective cells). It has recently been pointed out 2 that in addition to the electrostatic convective cells, zero-frequency magnetic fluctuations exist in a two-dimensional situation which produce random magnetic islands as well as many open field lines for a shearless zero-order magnetic field. Since the particles can follow the magnetic field they can diffuse across the system; it is important to compare this diffusion with collisional diffusion and convective cell diffusion. Furthermore, since the motion of charged particles due to the convective cells can destroy the current filaments responsible for the magnetic fluctuations, a strong coupling between the electrostatic and magnetostatic fluctuations is expected to occur. This coupling should determine the correlation time of the thermal magnetic fluctuations. It is also possible for the fluctuating magnetic fields to give rise to shorting of the charges associated with the convective cells and affect their lifetime. This effect is only important at high ft In order to study the above processes in detail, a set of two-and-one-half-dimensional simulations have been carried out using a magnetostatic particle code with a uniform external magnetic field in the z direction. 3 The simulation parameters were the following: a 64x64 grid, 4096 ions and electrons, Sl e /(jo Pe = 1 (electron gyrofrequency/plasma frequency), m i /m e =3Q, T e /T { = 2, X De /6=l (electron Debye length/grid spacing), V Te /c-j$ (electron thermal speed/speed 753 of light) and, 0=0.04 (plasma pressure/magnetic pressure); note that fi^m e /m i here. To study the particle diffusion and the coupling between the electrostatic and magnetostatic fluctuations in detail, simulations are carried out in three steps. First, all the electrostatic fluctuations are suppressed in the code and the ions are treated as a stationary background (case 1). This will correspond to the case considered in Ref. 2. In Fig. I, test-particle diffusion ((Ax) 2 ) with time is shown. We observe that ((Ax) 2 ) increasx250 < I 0 (b) 1 1 Cose 2 / y Case 3 ^J ~~i i 1 FIG. 1. Test-particle diffusion ((Ax) 2 ) vs time for (a) fixed ions, only magnetic fluctuations (case 1); (b) fixed ions, magnetic and electrostatic fluctuations (case 2); mobile ions, magnetic and e...

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