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 $D_{\perp }$ going like $B^{\text{−2}}$ ; for moderate magnetic fields ${\mathrm{(\omega }}_{\mathrm{ce}}{\mathrm{\approx \omega }}_{\mathrm{pe}})$ the diffusion rate is enhanced and $B^{\text{−1}}$ is almost independent of $B$ ; finally, for very large fields ${\mathrm{(\omega }}_{\mathrm{ci}}{\mathrm{>\omega }}_{\mathrm{pi}})$ the diffusion coefficient goes like $B^{\text{−1}}$ . 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.