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 dis charges has been used to investigate the intrinsic hydrocarbon erosion yiel ds and to determine the variation of the photon efficiency with the edge io n flux. Constant methane erosion yields were measured (5% for CD4, 3% for C H4) with no sign of any flux dependence. The erosion yield of ethane/ethene was lower but increased with plasma density (increasing edge ion flux, dec reasing edge electron temperature) such that it dominated the molecular car bon source at high density. The measured photon efficiencies (the number of molecular dissociations per emitted photon) were not constant, but were fo und to increase with increasing density (increasing edge ion flux, decreasi ng edge electron temperature). (C) 2001 JET Joint Undertaking. Published by Elsevier Science B,V. All rights reserved.