Thermal shock resistance of a tungsten-diamond composite under extreme heat loads

In nowadays magnetic confinement fusion devices, plasma facing components are armoured with refractory high Z metals like tungsten (W). Despite W high melting point, moderate neutron activation, low tritium retention and erosion, its thermomechanical properties make it prone to recrystallization, cracking and/or melting under thermal shocks.Carbon was discarded as plasma-facing material in fusion for its high level of erosion and tritium retention. However, carbon as Chemical Vapor Deposition (CVD) diamond has low retention and outstanding performances under thermal shocks because of its high thermal conductivity. Unfortunately, it is comparable to graphite for erosion. For this reason, tungsten-diamond (W-diamond) test samples made of a ~mm thick CVD diamond substrate for a fast distribution of heat with a ~10 µm W surface coating for protection against erosion were manufactured. According to modelling, W-diamond is expected to have a thermal behaviour similar to pure CVD diamond.The objective of this paper is to explore the domain of viability of the metal-diamond interface under thermal shocks of a new armor material not as limiting as bulk W for present day machines with low irradiation levels like WEST, W7X and JT60-SA. Various configurations were considered: types and thickness of diamond substrates, pure W coatings and a molybdenum interlayer. Different levels of thermal shocks were also performed with a laser on these samples:● up to 104 cycles of 1 ms at 1-3 GW.m-2 (~32-95 MW.m-2.s0.5) to simulate fast transients like edge-localized modes or disruptions,● up to 20 cycles of 1 s at 100 MW.m-2 to simulate divertor reattachment.Post-mortem analysis showed pristine W coatings and metal-diamond interfaces for all samples after slow transients at 100 MW.m-2 and after fast transients up to 1.7-2 GW.m-2. Bulk ITER-grade W samples tested in the same conditions showed cracked and melted surfaces at the lowest thermal loads.