Limiting factors for achieving peeling-limited pedestals in present devices

An optimized pedestal regime called the Super-H Mode is leveraged to access peeling limited pedestals and study limitations on the integrated tokamak exhaust and performance gap. Analysis of DIII-D experiments demonstrates separatrix and mid-pedestal collisionality as key parameters in determining the state of both the pedestal and the divertor regions. Experiments were performed on DIII-D to determine the conditions where peeling and ballooning modes become strongly coupled. Primary actuators for these experiments included D<span style="font-size:10.8333px">2 puffing to match pedestal density in semi-open and closed divertors as well as varied strike point location to change the pumping efficiency in the divertor. Pedestal structure is modified by fueling and pumping such that the closed divertor requires higher gas puffing for a matched pedestal density due to the higher pumping efficiency. Due to ~2x increased pumping efficiency in the closed divertor configuration, approximately 5x higher fueling rates were required to achieve similar pedestal conditions as the open divertor. The difference in pumping efficiency and fueling for the same pedestal density indicates a change in pedestal particle transport, which leads to ~5x lower collisionalities at the separatrix and ~10x lower at mid-pedestal, allowing access to a peeling-limited pedestal.

Work supported by the U.S. Department of Energy under DE-FC02-04ER54698, DE-SC0014264, DE-AC02-09CH11466, and DE-AC05-00OR22725.