Magnetic fluctuation dominated electron heat transport in DIII-D ELMy H-mode pedestal

Experimental evidence from DIII-D shows that micro-tearing modes (MTMs) can dominate electron heat transport in ELMy H-mode pedestal. Pedestal electron temperature gradient is observed to decrease as pedestal-top electron collisionality υ_e^* is increased from ~0.4 (low) to ~1 (high), and is clamped during the entire inter-ELM period at high collisionality. Power balance analysis shows pedestal electron heat diffusivity increases 76% from 0.41 (low υ_e^*) to 0.72 m²/s (high υ_e^*). Experimentally-measured, internal, line-averaged magnetic fluctuations originating from MTMs increase from 3.1±0.4 to 4.1±0.5 Gauss. Estimated stochastic electron heat diffusivity using the measured magnetic fluctuation amplitudes and stochastic-field theory increases 43%, comparable to the increase of experimental electron heat diffusivity. Measured pedestal ion heat diffusivity and electron density profile evolution show little change, consistent with transport predicted by theory of micro-tearing modes. Local, nonlinear gyrokinetic simulations find MTMs are dominant in the pedestal steep gradient region at both low and high collisionalities. Simulated MTM magnetic fluctuation amplitudes and electron heat flux increase at high collisionality, qualitatively agreeing with experimental observations. This work supports gyrokinetic simulations that claim MTMs can generate significant electron heat transport in the pedestal. Furthermore, this work provides new insight into long standing issues at high collisionality, such as, H-mode pedestal degradation with high gas puffing and high density.



*Work supported by US DOE under DE-SC0019004 and DE-FC02-04ER54698.