Collisional diffusion of a plasma in multipolar and picket fence devices

A collisional diffusion model of a low‐β plasma in a two‐dimensional magnetic field is worked out under the assumption of joint diffusion of electrons and ions in the plane of the magnetic field. This model is applied to multidipole and picket fence discharges; density and potential profile are computed. The main features observed experimentally are in qualitative agreement with this computation: a leak width proportional to the hybrid gyroradius and the existence of potential hills between the cusps and poor plasma confinement.