MAGNETIC AND COLLISIONAL EFFECTS ON PRESHEATHS

The effects of both magnetic field and collisions on presheath propert ies are experimentally investigated in plasmas with electron temperatu res much greater than ion temperatures. Measurements of plasma potenti al in collisionless plasmas show presheath thicknesses at boundaries o blique to magnetic field to be approximately (C-s/omega(ci))sin psi wh ere C-s is the ion sound speed, omega(ci) is the ion gyrofrequency, an d Psi is the angle between the magnetic field and the normal to the wa ll boundary. Measurements of plasma potential in collisional plasmas f ind presheaths consisting of two distinctive regions. With ion-neutral collision mean-free path lambda(n)<(C-s/omega(ci))sin psi the preshea th region next to the sheath has collisional characteristics and a thi ckness of approximately (0.5-0.6)lambda(n). The corresponding presheat h region adjacent to the bulk plasma has magnetic characteristics and a thickness of approximately (0.5-0.9)(C-s/omega(ci)sin psi. Equipoten tial contours in the collisional region of this presheath are found to be parallel to the boundary, while those in the magnetic region are n ot. (C) 1995 American Institute of Physics.