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 diffusivity. Comparisons with numerical solutions of the
time-dependent electron thermal equation find that the data are consistent with
a narrow region with electron diffusivity χe​ an order of magnitude lower
than the average value across the plasma, suggesting an electron internal
transport barrier (ITB) near the ECH heating location. Detailed profile
analysis and equilibrium reconstructions suggest that the formation of these
ITBs are correlated with off-axis values of the safety factor q being near 1.
Furthermore, the ECH driven H-mode discharges demonstrate more rapid electron
heating rate near the ECH deposition location than L-mode discharges with
higher 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 electron temperature peaks