Fuel Retention in LHD

A global particle balance study has been investigated in the Large Helical Device (LHD) in which the first wall and the divertor tiles are made of stainless steel (SUS-316L) and carbon, respectively. The carbon area is less than 10% of the stainless steel area. The analyzed discharges have been conducted under an intrinsic helical divertor (HD) or a local island divertor (LID). The HD is an open divertor at this stage, and the LID is a closed divertor equipped with a baffle structure and a pump system. In the HD configuration, fuel retention up to 75% of injected hydrogen was observed, and the retained hydrogen affected the plasma density control. On the other hand, almost all fueled hydrogen was evacuated by the pumps in the LID configuration. After each experimental campaign, detailed analyses of the in-vessel material probes (SUS-316L stainless steel) and a divertor tile exposed to various plasma discharges during each experimental campaign were conducted. The areal density of the retained hydrogen both in the material probes and the divertor tile was in the range 10²¹ to 10²² H/m² , and it corresponded to the averaged areal density that was observed after an experimental day with high-density discharges.