Profile gradient evolution during the transition from Edge Harmonic Oscillations to Limit Cycle Oscillations in wide-pedestal QH-mode

In recent DIII-D wide-pedestal QH-mode plasmas, a long-lived and predator-prey type Limit Cycle Oscillations (LCO) regime is observed to follow the disappearance of the Edge Harmonic Oscillations (EHO)¹. During the fast transition (~3-4 ms) from the EHO termination to LCO, both electron density and temperature gradients significantly increase near the top of pedestal, i.e. 0.8< rho <0.9, while the gradients inside pedestal change slightly. At almost same time, the intermediate-k (k = 4-6 cm^-1) density fluctuation magnitude increases at the top of pedestal (rho ~0.85). The possibility that the pressure gradient near the top of pedestal leads to the transition from EHO to LCO regime is investigated. Stability analysis and the evolution of T_i, E_r, and ExB rotation shear rate during the transition are also examined. The investigation will help to understand the transition and the mechanism to the LCO dominated regime. ¹K. Barada, et al., Phys. Rev. Lettrs. 135002 (2018)