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. Brook
s, D, Naujoks, Phys. Plasmas 7 (2000) 2565) coupled with the THERM code whi
ch 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 t
imes for carbon are much longer (similar to1-2 s), as expected due to therm
al 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.