REAL-TIME, INSITU MEASUREMENTS OF SECONDARY-ELECTRON EMISSION IN DITE

Using a new application of the retarding field analyser (RFA), it is possible to measure, in-situ, the coefficients of secondary electron release from chosen, conducting target substrates. Since the measurements are made in the tokamak, the yields are due to primary particles characterized by velocity distributions which are close to those experienced by solid surfaces intercepting the edge plasma. Operating the RFA in the standard mode also permits measurement, under identical discharge conditions, of T(i), T(e), n(e) and V(sheath) at the probe. Measurements of the coefficients of ion and electron secondary electron emission (delta-i, delta-e) are presented for both graphite and molybdenum target substrates. For electron energies in the range 0 to approximately 60 eV, values of delta-e near to unity have been measured for Mo and C with the graphite yields being slightly lower than those for molybdenum. delta-i is found to be 0.3-0.5 for molybdenum, in good agreement with ion beam measurements at low energy (approximately 300-500 eV).