Pedestal formation via different trajectories in the stability space in response to the timing scan of neutral beam heating in DIII-D

The frequency of type-I ELMs decreases as the initiation of the neutral beam injection (NBI) heating is delayed with respect to the time when plasma current (I_p) reaches flat-top in the ITER Baseline Scenario discharges in DIII-D. Henceforth, the time gap between the NBI initiation and I_p flat-top will be referred to as “heating delay.” As the heating delay is modified, pedestal formation follows different trajectories in the edge current density-pedestal pressure gradient (j_edge−∇p_e^ped) space from the L-H transition toward the first ELM event. During the stationary phase after the first ELM, the ELM frequency (f_ELM) decreases by a factor of ∼2 as the heating delay is increased. A longer pedestal recovery time in the inter-ELM period is observed for the low f_ELM discharges as compared to the high f_ELM discharges. Both low and high f_ELM discharges show nearly identical profiles of electron density and temperature and have a similar MHD stability just before an ELM crash. However, a marked difference is observed in the magnetic spectrogram of the high and low f_ELM discharges in response to the variation in the heating delay. The main difference is in the 200-400 kHz range of the magnetic spectra. A quasi-coherent mode (QCM) at 220 kHz and weaker broadband fluctuations are observed in the high f_ELM discharges, while only strong broadband fluctuations are prevalent in the low f_ELM discharges. ELM-synchronized analysis shows that the time evolution of these modes is different for the high and low f_ELM discharges. The localization of both these modes is confirmed at the maximum gradient region of the pedestal. We hypothesize that these modes cause important pedestal transport and that the difference in the pedestal recovery of the high and low f_ELM discharges is a result of the difference in transport driven by these modes, as they change with changes in the heating delay. It is demonstrated experimentally for the first time that discharges with similar pedestal parameters can carry the history of the heating delay into the stationary phase and that changes in turbulent-driven transport are a likely cause of changes in f_ELM observed with variations of heating delay.