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

DIII-D discharges that transition to H-mode solely with off-axis electron cyclotron heating (ECH) often exhibit strong off-axis peaking of electron temperature profiles at the heating location. Electron heat transport properties near these off-axis temperature peaks have been studied using modulated ECH deposited at ρ ≈ 0.3. The Fourier analyzed electron temperature data has been used to infer electron diffusive and convective transport coefficients. Initial comparisons with the quasi-linear transport model TGLF find that the data is consistent with a narrow region with an electron diffusivity an order of magnitude lower than the average χ_e across the plasma, suggesting an electron internal transport barrier (ITB) just inside the heating location. Using precise equilibrium reconstructions created with the EFIT code with constraints from TRANSP modeling, the formation of these ITBs appear to be correlated with off-axis values of the minimum safety factor  being near 1. The H-mode appears to be key for these unusual Te profiles, with the change in the current profile after the L-H transition leading to the q-profile-related barriers.