Ruifeng Xie, UT-Austin

"Experimental Observation and Study of Core Transport Barrier in DIII-D Discharges with Dominant Electron Cyclotron Heating"

Abstract: This talk details the experiment investigating hollow electron temperature (T_e) profiles observed in DIII-D H-mode discharges driven solely by off-axis electron cyclotron heating (ECH).

These off-axis T_e peaks appear just inside the ECH deposition location after the L-H transition and grow to be approximately 30% higher than T_e(0). Electron heat transport properties near these off-axis temperature peaks have been studied using perturbative experiments.

Modeling with a linear transport analysis code, which solves the electron thermal diffusion equation, finds that the hollow experimental profile can be reproduced with a local electron diffusivity being an order of magnitude lower than the average value across the plasma, suggesting the presence of an electron internal transport barrier (ITB) inside the ECH heating location. Using equilibrium reconstructions created with the EFIT code using kinetic pressure and current constraints from TRANSP modeling, the formation of these ITBs appears to be correlated with off-axis values of the safety factor q being near a low-order rational value.