Theory of the plasma–sheath transition in an oblique magnetic field

Bohm's criterion for the formation of a thin (λd → 0) sheath requires that the ions are pre‐accelerated in a quasineutral “presheath”. Usually the presheath mechanism is based on geometric ion current concentration, on ion friction (collisions), or on ionization. The present paper considers a presheath which depends essentially on magnetic deflection of the ion orbits. To this end a hydrodynamic model of the presheath accounting for an oblique magnetic field as well as for collisions and ionization is investigated. It is shown that the magnetic field alone is not sufficient to provide a presheath mechanism. The “magnetic presheath” is intrinsically controlled by elementary processes, and “compressed” by a strong magnetic field into a thin layer with a characteristic extension of the ion gyroradius ρi.