Mechanical Design and Fabrication of the Lower Divertor for DIII-D

The lower divertor of the DIII-D tokamak is being modified to provide improved density control of the tokamak plasma during operation in a high triangularity double null configuration. This divertor replaces the low triangularity advanced divertor installed in 1990. The design and analysis of the lower divertor is complete and hardware is being fabricated. Installation of the new divertor is scheduled to be completed by the end of 2005. The primary component of the lower divertor is a toroidally continuous flat plate. The plate is water cooled for heat removal. Three rows of graphite tiles are mechanically attached to the plate to shield it from plasma impingement. Owing to a concern over excessive erosion caused by plasma impingement, the through tile-face bolt holes have been eliminated from graphite in areas of high heat flux. The plate is water cooled for heat removal between shots and heated to 350degC with hot air and inductive current during vessel baking. The divertor plate is supported 100 mm from the vacuum vessel floor by two rows of 24 supports that must react the vertical loads due to halo currents. These supports are radially flexible to allow for differential radial thermal expansion between the divertor ring and the floor. The space below the plate forms a pumping plenum connecting the floor strike point to the lower cryopump. Upgraded floor tiles inboard of the plate will be installed to improve the target for the plasma strike point for outer leg pumping. The divertor plate is to be fabricated in four 90 deg sectors from type 316 stainless steel. Each sector consists of two plate halves with three machined coolant channels and is joined together by spot welds and perimeter seam TIG welds. The vacuum tight 90 deg plate sectors are welded together inside the vessel to form a toroidally continuous ring. The water cooling/air bake-out lines connecting the 4 sectors into two 180 deg cooling circuits will be welded in situ. Several plasma diagnostics wil- l require some modification or relocation for integration into the divertor system