Field Diffusion and Plasma Diffusion

Abstract

The diffusion time is the time during which the magnetic field penetrates into a stationary conductor. If the plasma is confined by the pressure of the magnetic field, diffusion of the magnetic field into the plasma violates the pressure balance and the plasma begins to move. To be more specific we should not speak of magnetic field diffusion into the plasma, but rather of plasma diffusion into the magnetic field. If the ratio of the kinetic pressure to the magnetic pressure β = p/p M is small, then the difference in the magnetic pressure inside and outside the plasma is small. The time needed for the field to diffuse into the plasma is equal to the skin or diffusion time given above and is determined by this small difference. However, total equilibrium requires a much longer time, since the plasma must be uniformly distributed over the entire region which it originally did not occupy. This plasma diffusion time is not determined by small pressure differences, but by the total magnetic pressure; it is 1/β times larger than the skin time. Correspondingly we can assume that the plasma diffusion coefficient is many times smaller than the magnetic field diffusion coefficient.