Theory and numerical simulation on plasma diffusion across a magnetic field

The diffusion of two and two and a half-dimensional plasmas across magnetic fields have been studied theoretically and by numerical simulation. Only diffusion at thermal equilibrium is studied. It is found that there are three regions: for sufficiently weak magnetic fields the diffusion coefficient is the classical one with D⊥ going like B−2; for moderate magnetic fields (ωce≈ωpe) the diffusion rate is enhanced and B−1 is almost independent of B; finally, for very large fields (ωciωpi) the diffusion coefficient goes like B−1. The enhanced diffusion at moderate and high magnetic fields is dominated by collective modes; i.e., by thermally excited convective modes. Theory and simulation are in good agreement. It is also shown that the diffusion coefficient behaves essentially the same way for a three-dimensional plasma when the magnetic field lines are closed.