Investigation of Core Transport Barriers in DIII-D discharges with off-axis Te Profile peaks

Hollow electron temperature (T_e) profiles of sustained duration have been 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 modulated ECH to infer electron diffusive and convective transport coefficients. Modeling with a transport analysis code GEvol, which solves the electron thermal diffusion equation, finds that the hollow experimental profile can be reproduced with a local electron diffusivity, χ_e 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 1.