NON-STEADY-STATE BOUNDARY-CONDITIONS FOR A SHEATH IN A TILTED MAGNETIC-FIELD

The paper provides an analysis of the boundary conditions for plasma i n contact with a wall, for the case where the magnetic field intersect s the walls at an angle alpha much less than 1. This case is of intere st for diverters of fusion devices, some plasma processing devices, an d spacecraft in the geomagnetic field. We specifically address the iss ue of the boundary conditions for a plasma whose parameters are varyin g in time. We note that, in case alpha much less than 1, the time with in which the ion traverses the sheath is approximately equal to 2 pi/a lpha omega(Ci), where omega(Ci) is the ion gyrofrequency. Therefore, e ven for macroscopic plasma motion, with frequency omega below the ion gyrofrequency, the non-steady effects in the sheath can become importa nt for omega > alpha omega(Ci). We derive boundary conditions for this case and find that main effects of non-stationarity come from variati ons of the electron temperature.