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 (∼1–2 s), as expected due to thermal property differences, and this may explain various `hot spotʹ 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.