Damage to N-NBI systems due to positive ion back-streaming

Sputtering yields of molybdenum (Mo) are computed for deuterium, oxygen and carbon as impurity ions from the residual gas of the vacuum system, as well as Mo and Cs using Yamamura formula. The incident ion energy ranges from 10 keV to 1 MeV, the back-streaming ion beam energy from the extraction voltage up to ITER full acceleration potential. The sputtering yield of Cs against Mo reached ten atoms per ion and was the largest among the investigated ion species. The effect due to retention of deuterons in Mo was studied for the case of grazing angle incidence of D+ at D- ion extraction energy to confirm the small effect onto the sputtering yield; the result obtained for 10 keV D+ for 10% retention of D atoms in Mo increased the sputtering yield only by 2%. The back-streaming D+ ions at 1 MeV energy should make a layer of high concentration deuterium atoms at 6 µm depth from the Mo surface. Possible failure mode associated with the particle implantation is discussed.