Leak tightness tests of mechanical seal joints for HL-2M tokamak under thermal cycle loads

The mechanical seal joints used in HL-2M tokamak mainly include vacuum coupling radius seals (VCRs) and conflat flanges (CF), which are usually employed as a part of the cooling flow channels of the advanced divertor. Some of the joints which are located in the vacuum vessel would be used together with the cooling pipes (CPs) as the vacuum boundary structure. Their sealing performance must be extremely reliable, otherwise leak may occur, thus affecting the safe operation of the tokamak machine. One of the worst conditions is that the joints are baked from room temperature (RT) to 350 degrees C, which is kept for a long time, then cooled to RT again. In addition, they will go through many cycles of baking during the lifetime of the tokamak. Therefore, it is essential to carry out the baking tests of the mechanical seal joints for the leak tightness performance under thermal cycle loads. The experimental results show that the sealing performance of VCRs is still good after 28 thermal cycles, while for CF25, its airtightness will gradually fail after more than 20 thermal cycles, so it should be used within limited times during its lifetime.