Limits to the H-mode pedestal pressure gradient in DIII-D

The spatial and temporal evolution of the total pedestal pressure profile has been measured during the pedestal evolution between successive edge localized modes (ELMs) of type-I ELMing H-mode discharges in DIII-D. Measurements are used to test a model that predicts that kinetic ballooning modes (KBMs) provide a strong constraint on the pedestal pressure gradient obtained during an inter-ELM cycle and cause the pedestal width to scale as the square root of the pedestal poloidal beta. Discharges in two different parameter regimes are examined for evidence that the evolution of the pressure gradient reaches a limit prior to the onset of an ELM. Both discharges show evidence of rapid evolution of the pressure profile very early in the recovery phase from an ELM. In one discharge, the pressure gradient reached approximate steady state within ∼3 ms after the ELM event. In the other discharge, the pressure gradient just inboard of the last closed flux surface reached steady state early in the ELM recovery phase even as the pedestal expanded into the core and the maximum pressure gradient continued to rise during the remainder of the ELM cycle. Simple quantitative theoretical metrics show that pressure gradients in both discharges reached levels that were large enough to excite KBMs. In addition, the peeling–ballooning theory for the onset of type-I ELMs and the EPED1 model for pedestal height and width make predictions consistent with the data of both discharges.