Design, fabrication and installation of the lower divertor for DIII-D

The geometry of the DIII-D tokamak lower divertor was recently modified to improve tokamak plasma density control during operation in a high triangularity double-null configuration. The primary component of the lower divertor is a toroidally continuous flat cooling plate that was fabricated by the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). Three rows of graphite tiles are mechanically attached to the plate to shield it from plasma impingement. The plate is water-cooled for heat removal between shots and is heated to 350 °C with hot air and inductive current during vessel baking. The divertor plate is supported 100 mm from the vacuum vessel floor to allow for cryo-pumping. The vacuum tight 90° plate sectors were positioned and welded together inside the vessel forming a toroidally continuous ring. Plasma facing tile designs have evolved from previous installations. To limit erosion caused by plasma impingement on sharp edges, the through tile-face bolt holes were eliminated from graphite in areas of high heat flux. Upgraded floor tiles were installed to improve the target for the plasma strike point for outer leg pumping. Thermal analysis was done for the Union Carbide ATJ grade graphite divertor shelf and vessel floor tiles and results are presented.