Analysis of the tokamak sheath region with in-sheath ionization and transport of surface emitted particles

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Abstract
A self‐consistent numerical analysis has been performed of the oblique incidence magnetic field tokamaksheath, with in‐sheath ionization and transport of surface desorbed and reflected hydrogen molecules and atoms, and sputtered and evaporated surface atoms. The analysis uses the 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 =1×1020 m−3), nearly all desorbed hydrogen molecules are ionized in the sheath; this lowers sheath potential by up to 30% and increases heat transmission by up to 150%. Backscattered hydrogen, and elastic collisions between primary ions and hydrogen molecules, in contrast, have a negligible effect on sheath parameters. The magnetic sheath region has a significant effect on sputtered tungsten atoms, resulting in high local redeposition, but is not itself changed by the sputtering.Ionization of thermally evaporated surface atoms (from overheating) can increase sheath heat transmission significantly, a process which may contribute to surface hot‐spot formation.