PARTICLE-INDUCED DESORPTION OF HYDROGEN FROM A-C-H AT LOW IMPACT ENERGIES

The release of hydrogen from the carbonized walls of a tokamak by plasma bombardment is an important process intervening in hydrogen recycling. The desorption of hydrogen from amorphous, hydrogen containing carbon films (a-C:H) by low energy hydrogen (deuterium) and noble gas ions has been studied in an UHV test device coated with a-C:H films. The dynamics and quantities of the released fluxes as well as the implanted fluxes by exposure to low energetic (200-350 eV) hydrogen and noble gas ions have been measured. Evidence is presented for the preferential incorporation of low energy hydrogenic species in a growing a-C:H layer from CH4/H2 mixtures. In subsequent RG discharges in pure hydrogen the desorption of hydrogen from the film leads to a change of the recycling coefficient R from initial values R > 1 to stationary values R almost-equal-to 1. The more effective removal of hydrogen by glow discharges in noble gases yields finally a strong receptivity for hydrogen of the a-C:H layer. This behaviour can be observed while operating a glow discharge in pure hydrogen following such a conditioning procedure. Starting reimplantation of hydrogen from values R < 1 the a-C:H coating approaches a stationary state in which the hydrogen concentration is almost constant and the recycling coefficient reaches again values of about R = 1. Results from TEXTOR show the relevance of the experiments for tokamak operation.