Influence of Li and B coatings of metal walls on deuterium retention and plasma confinement in HT-7

In winter 2010, the carbon plasma-facing components (PFCs) of HT-7 were changed for molybdenum PFCs. The surface area of the wall that was covered with Mo was about 1.28m ². In the following experimental campaign, the plasma was first operated with Mo PFCs; then 28 successive lithium coatings were carried out (for a total of 350g Li); finally, 3g of carborane were filmed to the PFCs. In each phase, the characteristics of hydrogenic species retention were investigated as well as the energy confinement and recycling properties, offering a good opportunity for comparing similar discharges. With Mo PFCs, significant wall outgassing is generally observed and, in all cases, less than 10% of the puffed fuel is trapped, corresponding to a retention rate of around a few Pals ^-1 (10 ¹⁸atoms ^-1); energy confinement is poor, mainly due to huge central radiation. When the PFCs are covered by lithium, about 40-80% of the puffed gas is trapped and the retention rate is 10 ²-10 ³Pals ^-1 (10 ²⁰-10 ²¹atoms ^-1); good confinement properties are recorded, and the radiation remains located at the edge. After boronization, about 70-80% of the puffed deuterium is trapped and the retention rate is a few 10 ²Pals ^-1 (10 ²⁰atoms ^-1); the energy confinement is not as good as with Li coating, and significant radiation is measured in the plasma core.