Graphite Tile Thermal Performance on the New DIII-D Lower Divertor

The lower divertor of the DIII-D tokamak has been modified to provide improved density control of the tokamak plasma during operation in a high triangularity double-null configuration. Union Carbide ATJ grade graphite tiles covering the new lower divertor and vessel floor were designed to have better tile-to-tile alignment and to withstand higher heat flux than existing tiles.

Gaps between tiles were successfully reduced from 2.5 to 0.4 mm and tile top surface alignment was greatly improved from 1.0 to 0.1 mm. Small tile gaps along with good vertical edge alignment greatly reduce the number and size of thin edges visible to the plasma, thus minimizing possible carbon introduction into the plasma. Close tile-to-tile alignment was the result of the very flat divertor plate surface, carefully controlled tile positioning, well-machined graphite tiles, and hand filing.

Tiles were specified to survive 27 MJ of energy deposited per toroidal row of tiles during a 10 s shot period. When this energy is applied over the narrow triangular heat flux profiles originally specified, modeling shows that the tiles exceed maximum allowable tensile stress. Modeling does show that the tiles are able to absorb the 27 MJ per row without exceeding stress limits in cases where the heat flux profile is less focused than the original design specification.

This paper will compare tile design analysis with operational experience obtained during the first 12-week operations campaign with the new divertor.