Investigation of Core Transport Changes in DIII-D H-mode Plasmas with off-axis T_e Profile peaks

DIII-D discharges that transition to H-mode solely with off-axis electron cyclotron heating (ECH) exhibit strong off-axis peaking of the electron temperature (T_e) profiles. These off-axis T_e peaks appear inside the ECH deposition location, grow to be approximately 30% higher than T_e at the magnetic axis after the L-H transition, and persist for multiple electron energy confinement times τ_e. Perturbative transport studies near the off-axis T_e peaks indicate a region with 90% reduction in electron heat diffusivity and the presence of electron internal transport barriers (ITBs). Detailed profile analysis and modeling results suggests that the formation of ITBs is related to changes in the q profile near low-order rational values. Furthermore, the ECH driven H-mode discharges demonstrate more rapid electron heating rate near the ECH deposition location than the L-mode discharges despite having lower auxiliary ECH heating power. Additional modeling attributes this difference to the modification of electron heat transport in the core at the L-H transition, which also sustains the off-axis T_e peaks. Understanding this transport behavior in ECH-only H-mode plasmas has implications for future reactor relevant machines, like ITER, that aim to establish the H-mode state with dominant electron heating.