The investigation of structure, chemical composition, hydrogen isotope trapping and release processes in deposition layers on surfaces exposed to DIII-D divertor plasma

Abstract

An ATJ graphite sample, arranged to receive parallel heat flux on a small region of the surface, was exposed to DIII-D divertor plasma using the DiMES (Divertor Material Evaluation System) mechanism. The sample was exposed to 600 ms of outer strike point plasma in a single discharge. Actual divertor tiles are subjected to such high heat flux due to misalignments, subjecting leading edges to the parallel flow of plasma, or during disruptions. The sample was constructed to collect the eroded material directed downward into a trapping zone onto a Si disk collector. The surface heat flux onto the divertor floor at the location of the sample during the exposure was 200 W/cm 2. Taking the local field pitch into account, the parallel heat flux was about 5 kW/cm 2. After exposure, the graphite sample and Si collector disk were analyzed using SEM, NRA, RBS, Auger spectroscopy, IR and Raman spectroscopy. Thermal desorption was studied also. The deposited coating on the graphite sample is an amorphous carbon layer. Just upstream of the high heat flux zone, the redeposition layer has a globular structure. The deposition layer on the Si disk is composed also from carbon but has a diamond-like structure. The areal density of C and D in the deposited layer on Si disk varied in poloidal and toroidal directions. The maximum D/C areal density ratio is about 0.23, maximum carbon density is about 3.8 × 10 18 cm −2, maximum D areal density is about 3 × 10 17 cm −2. The thermal desorption spectrum had a peak at 1250 K.