Chemical erosion yields and photon efficiency measurements in the JET gas box divertor

Hydrocarbon injection into the JET gas box divertor in ohmic and L-mode discharges has been used to investigate the intrinsic hydrocarbon erosion yields and to determine the variation of the photon efficiency with the edge ion flux. Constant methane erosion yields were measured (5% for CD4, 3% for CH4) with no sign of any flux dependence. The erosion yield of ethane/ethene was lower but increased with plasma density (increasing edge ion flux, decreasing edge electron temperature) such that it dominated the molecular carbon source at high density. The measured photon efficiencies (the number of molecular dissociations per emitted photon) were not constant, but were found to increase with increasing density (increasing edge ion flux, decreasing edge electron temperature).