Pedestal confinement degradation in DIII-D ELMy H-mode plasmas with density ramp

In a DIII-D ELMy H-mode experiment with closed (small-angle-slot) divertor, as line-integrated electron density is increased by gas injection, electron pressure at the pedestal top and its gradient in the pedestal decrease, accompanied with increase of collision frequency υ_e. Experimental pedestal pressure p_exp agrees with EPED prediction p_eped at low υ_e but is below p_eped as υ_e increases. The ratio of p_exp and p_eped correlates with υ_e as p_exp/p_eped∝υ_e^-0.38. Two branches of low-k (k_θρ_s~0.1) magnetic and density fluctuations are detected by Faraday-effect polarimeter and Beam-Emission-Spectroscopy (BES) in the pedestal, respectively, one at low frequency (3-7 kHz) propagating in ion diamagnetic direction while the other at high frequency (200-500 kHz) propagating in electron diamagnetic direction. Fluctuation amplitudes for both branches increase as υ_e increases and pedestal degrades. The high-frequency branch has been identified as micro-tearing-modes [1] while the low-frequency branch is found consistent with kinetic-ballooning-modes [2], both of which can be destabilized by collisions. These observations indicate collision-destabilized turbulence may be critical to explain the pedestal degradation.

[1] Phys. Plasmas 28, 022506, 2021

[2] APS DPP 2020, TP15.015