Tolerable ELMs at high density in DIII-D

The energy released at each edge-localized mode (ELM) is found to decrease in relation to the pedestal pressure, by more than a factor of five, as the line-averaged density in DIII-D H-mode is raised from about half the Greenwald density limit to near the Greenwald limit. The pedestal pressure remains nearly constant over this range demonstrating an attractive regime for future larger tokamaks. The reduction in ELM energy, in both low and high triangularity configurations, is seen to scale more with the pedestal electron temperature than the pedestal density. At low density both the electron density and temperature inside the separatrix drop due to the ELM instability; however at high density the density perturbation remains similar while the temperature profile is unaffected. ELMs at high density are also characterized by smaller magnetic fluctuations consistent with a higher toroidal mode number ELM instability.