Description
Dedicated experiments on the DIII-D tokamak demonstrate detached divertor operation in the new compact Shape and Volume Rise (SVR) divertor using both increased deuterium fueling (density-driven detachment) and extrinsic nitrogen seeding across a broad range of operational scenarios with concurrent modeling. Parameters explored include toroidal field ($B_T$) direction, fueling location (main chamber vs. private flux region), and heating strategy (stationary vs. ramped power), with auxiliary heating up to ~14 MW.
In the density-driven detachment pathway, the density threshold for detachment is lower in the favorable $B_T$ direction (ion drifts into the divertor) than in the unfavorable direction, opposite to observations in the extreme-closure, unpumped DIII-D SAS divertor. In both $B_T$ directions, the transition to detachment is smooth, in contrast to the sharp, cliff-like transition previously observed in the favorable $B_T$ direction in SAS. This $B_T$ dependence is interpreted to arise from the radial E x B force, which pushes plasma away from the pump in the favorable $B_T$ direction (sustaining higher divertor density) and toward the pump in the unfavorable direction (sustaining lower divertor density). As detachment is approached, pedestal pressure decreases gradually, followed by a rapid drop once the target electron temperature falls below ~8 eV. This behavior is independent of $B_T$ direction and detachment pathway (density- or nitrogen-driven). Initial SOLPS-ITER simulations constrained by Thomson scattering reproduce pedestal profiles but fail to capture divertor target conditions when particle drifts are neglected, highlighting the critical role of drift physics in accurately modeling divertor exhaust.
The compact SVR divertor geometry is designed to support high-performance pedestal operation by enabling larger core plasma volumes and higher plasma currents, leading to divertor densities and pressures approaching those expected in fusion pilot plants. However, the reduced divertor volume places the X-point closer to the targets, increasing the risk of core performance degradation during detachment. These results provide early insight into integrating detached divertor operation with high-performance pedestals in shallow divertor geometries and inform improvements to the predictive capability of edge modeling tools for next-generation divertor design.
Work supported by US DOE under DE-FC02-04ER54698, DE-AC52-07NA27344, DE-AC05-00OR22725, DE-NA0003525, DE-SC0019003, DE-SC0014264.