Sheath superheat transmission due to redeposition of thermally emitted material

A self-consistent three-dimensional numerical analysis using the newly developed BPHI-3D code was performed for the Debye/magnetic tokamak-type oblique incidence magnetic field sheath, with near-surface ionization and transport of thermally emitted surface material. The analysis uses Monte Carlo, kinetic treatment for deuterium-tritium and impurity ions/neutrals, Boltzmann/guiding-center electrons, and particle-in-cell time-independent Poisson solver. For typical predicted fusion edge plasma conditions for a liquid lithium divertor most evaporated lithium atoms-from a similar to 1 cm(2) overheated spot-are ionized in the similar to 1 mm-wide magnetic sheath. These ions are strongly redeposited due to the sheath electric field. While this redeposition minimizes core plasma contamination, it increases the peak heat flux to the surface. A runaway situation is then possible due to superheating/evaporation positive feedback. Carbon may behave likewise as seen in code results obtained for a TORE SUPRA [Aymar , Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1989)] carbon limiter. A semianalytic formula for sheath parameters as a function of emitted surface material flux is developed and verified with the code. (C) 2000 American Institute of Physics. [S1070-664X(00)05206-X].