Comparison of wall/divertor deuterium retention and plasma fueling requirements on the DIII-D, TdeV and ASDEX Upgrade tokamaks

We present a comparison of the wall deuterium retention and plasma fueling requirements of three diverted tokamaks, DIII-D, TdeV and ASDEX Upgrade, with different fractions of graphite coverage of stainless steel or Inconel outer walls and different heating modes. Data from particle balance experiments on each tokamak demonstrate well-defined differences in wall retention of deuterium gas, even though all three tokamaks have complete graphite coverage of divertor components and all three are routinely boronized. This paper compares the evolution of the change in wall loading and net fueling efficiency for gas during dedicated experiments without helium glow discharge cleaning on the DIII-D and TdeV tokamaks. On the DIII-D tokamak, it was demonstrated that the wall loading could be increased by >1250 Torr l (equivalent to 150 x plasma particle content) plasma inventories resulting in an increase in fueling efficiency from 0.08 to 0.25, whereas the wall loading on the TdeV tokamak could only be increased by <35 Torr l (equivalent to 50 x plasma particle content) plasma inventories at a maximum fueling efficiency ∝1. Data from the ASDEX Upgrade tokamak suggests qualitative behavior of wall retention and fueling efficiency similar to DIII-D. (orig.)