The control of the impurity level in magnetically confined plasmas is a critical issue for future fusion devices. All the graphite tiles have been replaced by molybdenum tiles as limiter materials in the 2011 spring campaign in order to further reduce the recycling and hydrogen content of the plasma. A lithium coating technique has been applied as an important wall conditioning method to the HT-7 tokamak. The effective ion charge Zeff and impurity behavior with full metallic first walls of high-Z materials and lower hydrogen recycling have been investigated in a series of lithium coating experiments in this paper. Plasma performance and impurity behavior without wall coatings are studied in the early stage of the campaign. Comparison of Zeff with different plasma-facing components has been made. A typical lithium coating experiment has been analyzed in order to understand the effect of lithium coating. The evolution of main impurity line radiation, Zeff and the H/(H + D) ratio is analyzed in detail as lithium coating is repeated, indicating that lithium coating is a very effective tool to control impurity level and reduce hydrogen recycling. Furthermore, a boronization is conducted at the end of this campaign in order to make comparison with lithium coating. Experimental results show that lithium coating has much more advantages in edge recycling control, though it does not reduce impurity level as effectively as boronization.
Fusion applications for lithium: wall conditioning in magnetic confinement devices