Bench test of lithium first wall in HT-7 Tokomak

The movable lithium limiter was proposed for the HT-7 superconducting Tokomak for the purpose of suppression of plasma impurities, reduction of particle recycling and performance improvement of the plasma. The temperature dependence of its physical properties, such as melting, volatilization, and deposition as well as their interdependence was bench tested to optimize the operating conditions. The results show that the temperature strongly affects the melting, volatilization, and deposition of lithium, and that lithium films have strong gettering capability. For instance, at a temperature below 300° C, volatilization of Li is negligible, however, as the temperature rises up from 300° C to 360° C, volatilization rate rapidly increases, peaking at 370° C. The 300 nm thick Li film, deposited by evaporation or by plasma discharge, considerably getters the vacuum system because of its strong surface absorption. We suggest that Ar protection and low temperature be necessary when lithium is used as the moveable limiter in HT-7 Tokomak.