Observation and Characterization of EHO-like modes during the Inter-ELM periods of type-I ELMing DIII-D H-mode Discharges

Edge harmonic oscillations (EHOs) are normally observed in quiescent H-mode (QH-mode) plasmas. Reported here for the first time are the observations of EHO-like modes during the inter-ELM period of regular type-I ELMing H-mode discharges of the DIII-D tokamak. The observed modes, with a character similar to that of EHOs, are being investigated for the role they may play in particle transport during the inter-ELM period. ELITE calculations show that the MHD stability is close to the peeling boundary in the inter-ELM period, similar to QH-mode pedestal. The EHO-like modes (~10 kHz and its harmonics) are observed during each inter-ELM period in the measurement of density fluctuation (ñ) by Doppler backscattering (DBS) and beam emission spectroscopy, with similar modes observed by magnetic probes and in the ion saturation current measurements by Langmuir probes. The DBS measurements confirm that the EHO-like modes are pedestal localized, i.e., they weaken towards the separatrix and the pedestal top. Analysis of several discharges indicate a critical electron pedestal pressure gradient following an ELM crash above which the EHO-like modes are excited.