Combined sheath and thermal analysis of overheated surfaces in fusion devices

The phenomenon of runaway surfaces overheating in fusion experiments due to plasma-sheath/surface-emission coupling has been analyzed. A semi-analytic model has been used as well as the BPHI-3D kinetic sheath code (J.N. Brooks, D, Naujoks, Phys. Plasmas 7 (2000) 2565) coupled with the THERM code which solves the non-stationary heat conduction equation also in 3D geometry. Runaway heating due to initial overheating and subsequent sheath breakdown and superheat has been analyzed for two materials - lithium and carbon. For typical liquid lithium divertor conditions the critical exposure time for thermal runaway is of order 10 ms - generally greater than plasma transient periods (e.g., ELMS) or flowing liquid exposure times. Critical exposure times for carbon are much longer (similar to1-2 s), as expected due to thermal property differences, and this may explain various 'hot spot' formations in carbon systems. It is also shown that, especially for carbon materials, effects such as flake formation and deterioration of heat conductivity can play a crucial role. (C) 2001 Elsevier Science B.V. All rights reserved.