EVALUATION OF TUNGSTEN AS A PLASMA-FACING MATERIAL FOR STEADY-STATE MAGNETIC FUSION DEVICES

Tungsten in the form of bulk-material, and relatively thick (1 mm) chemically deposited and plasma-sprayed coatings on molybdenum, has been evaluated as a plasma-facing material for near future steady state magnetic fusion devices, focusing on issues related to the divertor plate design. These issues are: (1) thermal outgassing; (2) plasma erosion; (3) deuterium retention; (4) disruption erosion; and (5) surface modifications. Total outgassing quantities from bulk tungsten and chemically deposited coatings are substantially smaller than those from graphites. Effects of redeposition and impurities on the erosion behavior due to deuterium plasma bombardment have been analyzed. Trace amounts of oxygen-containing impurities in the plasma can reduce the threshold energy for physical sputtering, affecting the overall erosion behavior of tungsten at energies below 500 eV. However, it has been found that at electron temperatures around 5 eV or lower, fragmentation of these impurities followed by positive ionization is significantly reduced, whereby plasma erosion data basically agree with sputtering theories and ion beam data. Thermal desorption measurements after plasma bombardment have indicated that the deuterium retention quantity in tungsten materials is of the order of 10(14-15) D atoms/cm2. At simulated disruption with an energy deposition of 6 MJ/m2, the effective heat deposition is found to be reduced to about 1%, due to a combined effect of molten layer protection and vapor shielding. Steady state plasma bombardment removes surface impurities and smooths the surface topography along with surface erosion whereas disruption causes microscopic cracking and surface melting.