Microscopic damage of metals exposed to the helium discharges in TRIAM-1M tokamak and its impact on hydrogen recycling process

High-energy charge-exchange particles bombarding the plasma-facing wall may cause not only surface sputtering but also damage the materials inside due to their rather high energy. In the case of burning plasma, we should take into account the effects of helium because it is well known that helium atoms have much stronger effects on material damage than hydrogen atoms. In this paper, therefore, microscopic damage of metals exposed to long pulse discharges of helium plasma in TRIAM-1M was studied and the impact on the hydrogen recycling process was discussed by comparing it with helium ion irradiation experiments. Considerably larger amounts of dislocation loops and very dense fine bubbles were formed by the irradiation of rather low-energy charge-exchange neutrals of helium. According to the irradiation experiments with low-energy helium ions, formation of dense helium bubbles drastically enhances hydrogen trapping and makes the desorption difficult. These results indicate that the hydrogen recycling phenomenon during the burning plasma discharge must be quite different from that of the hydrogen plasma discharge experiments.