ANALYSIS OF THE TOKAMAK SHEATH REGION WITH IN-SHEATH IONIZATION AND TRANSPORT OF SURFACE EMITTED PARTICLES

A self-consistent numerical analysis has been performed of the oblique incidence magnetic field tokamak sheath, with in-sheath ionization an d transport of surface desorbed and reflected hydrogen molecules and a toms, and sputtered and evaporated surface atoms. The analysis uses th e newly developed BPHI Monte Carlo/cloud-in-cell kinetic code together with particle-surface interaction models. For typical plasma boundary conditions (T(e) = 100 eV, n(e) = 1 x 10(20) m-3), nearly all desorbe d hydrogen molecules are ionized in the sheath; this lowers sheath pot ential by up to 30% and increases heat transmission by up to 150%. Bac kscattered hydrogen, and elastic collisions between primary ions and h ydrogen molecules, in contrast, have a negligible effect on sheath par ameters. The magnetic sheath region has a significant effect on sputte red tungsten atoms, resulting in high local redeposition, but is not i tself changed by the sputtering. Ionization of thermally evaporated su rface atoms (from overheating) can increase sheath heat transmission s ignificantly, a process which may contribute to surface hot-spot forma tion.