H-isotope retention and thermal/ion-induced release in boronized films for the Tokamak Fusion Test Reactor

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Abstract
Over the past decade, it has been clearly demonstrated that the composition of the very near surface (∼100 nm) of plasma‐interactive components plays a determinant role in most processes which occur in the plasma edge of tokamaks. Two very successful techniques to effect control of the plasma‐wall interaction are (1) i n‐s i t udeposition of amorphous carbon or boron‐carbon films and (2) the use of He/C conditioning discharges or He glow discharge cleaning to modify the near surface of bulk graphite components. We have deposited boronized layers onto Si using plasma‐assisted chemical vapor deposition(CVD) and sputter deposition. The plasma‐enhanced chemical vapor deposition (PCVD) deposition conditions were as close as possible to those used in the TokamakFusion Test Reactor (TFTR), and some filmsdeposited in TFTR have also been studied. Using these two deposition techniques, B x CH y films have been produced with x varying from 1/2–4, and y from ∼1 (sputtered) to ∼3 (PCVD). Most films also contained significant amounts of O. Thermal and ion‐induced release of H isotopes from BC films is qualitatively similar to that measured for graphite. Implanted H saturates in these films at an H/host atom ratio of 0.7 which is considerably higher than that of graphite (∼0.4). As‐deposited PCVD films are already saturated with H, while sputteredfilms are not. Sputtered BC films therefore possess an inherent H‐pumping capability which could prove to be extremely beneficial to TFTR.

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