Investigation of Core Transport Barriers in DIII-D Discharges with off-axis Te 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. The Fourier analyzed electron temperature data have been used to infer electron thermal diffusive and convective transport coefficients. Comparisons with a linear transport code GEvol find that the data are consistent with a narrow region with electron diffusivity $\chi_e$ an order of magnitude lower than the average value across the plasma, suggesting an electron internal transport barrier (ITB) near the ECH heating location. Using 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 safety factor $q$ being near 1.