Characterization of the pedestal MHD in a negative triangularity plasma on DIII-D with Electron Cyclotron Emission Imaging

The Electron Cyclotron Emission Imaging (ECEI) system is used to characterize the edge MHD temperature fluctuations in a Negative Triangularity (NT, δ=-0.3) lower single null plasma discharge on DIII-D. The discharge exhibits small ELM-like crashes at ~ 400 Hz, a clear density pedestal with n_e,ped ~ 3x10¹⁹ /m³ and a weak temperature pedestal with  T_e,ped ~250 - 300 eV. ECEI observed clear MHD turbulence fluctuations ( n=6 -17 at ρ~0.96) during the inter-ELMs and ELM crashes. The fluctuation amplitude is observed to increase by a factor of 2 from the inter-ELMs phase to the ELM-crash phase. The dominant mode number decreases from n~17 (25 kHz)  to n~6 (9kHz)   when the pedestal transitioned from the inter-ELM phase to the ELM-crash phase. The phase velocity of the mode is ~ 0.5 v_{ion,diamagnetic} /2+V_ExB at ρ~0.95-0.98, which is consistent with peeling-ballooning modes. Preliminary agreement of mode number and radial location is found between the linear BOUT++ simulation and the ECEI observation using the interELM profile and equilibrium.