Fuel removal from tile gaps with oxygen discharges: reactivity of neutrals

Carbon removal in the so-called remote areas, where no energetic species can reach the surface, is investigated with low-temperature glow discharges in pure oxygen. Plasma-deposited amorphous hydrogenated carbon thin films are used as a model system for redeposited films. Erosion measurements are performed on flat substrates as well as on two different three-dimensional (3D) test structures. One design consists of 19mm deep gaps with widths ranging from 0.5 to 4mm to simulate ITER tile gaps. A second design consists of a flat box-like structure where particles can enter only through a narrow slit. Measurements are performed for substrate temperatures ranging from 290 to 580 K. Erosion rates follow an Arrhenius-type dependence on substrate temperature with an effective activation energy of 0.25 eV. While at room temperature the surface loss probability of the dominant eroding species is 50% it becomes smaller at elevated temperatures. At elevated temperatures, the films are not only removed faster but simultaneously erosion penetrates deeper into the gaps.