Deuterium pumping and erosion behavior of selected graphite materials under high flux plasma bombardment in PISCES-A — effects of surface pores and machined grooves

Abstract

The deuterium plasma recycling and chemical erosion behavior of selected graphite materials have been investigated using the PISCES-A facility. These materials include: Pyro graphite; 2D-graphite weave; 4D-graphite weave; and POCO graphite. Deuterium plasma bombardment conditions are: fluxes around 7 × 10 17 ions s −1 cm −2; exposure time in the range from 10 to 100 s; bombarding energy of 300 eV; and graphite temperatures between 20 and 120 °C. To reduce deuterium plasma recycling, several approaches have been investigated. Erosion due to high-fluence helium plasma conditioning significantly increases the surface porosity of POCO graphite and 4D-graphite weave whereas little changes for 2D-graphite weave and Pyro graphite. The increased pore openings and refreshed in-pore surface sites are found to reduce the deuterium plasma recycling and chemical erosion rates at transient stages. It is also found that deuterium recycling even at the steady state tends to be suppressed as the surface porosity increases by materials selection. Surface topographical modification by machined-grooves noticeably reduces the steady state deuterium recycling rate and the impurity emission from the surface. These surface topography effects are attributed to continuous co-deposition of reemitted deuterium, chemically sputtered hydrocarbon and physically sputtered carbon under deuterium plasma bombardment. The co-deposited groove surface exhibits a characteristic morphology with dens-packed dendritic microstructures.