Investigation of Core Transport Barriers in DIII-D Discharges with off-axis Te Profile Peaks

TL;DR

This study investigates off-axis electron temperature peaks and associated transport barriers in DIII-D tokamak discharges, revealing localized low diffusivity regions linked to safety factor values near 1 and differences in heating rates between H-mode and L-mode.

Contribution

It provides new evidence of electron internal transport barriers forming near off-axis heating locations, correlated with safety factor values and transition dynamics.

Findings

Identification of narrow regions with significantly reduced electron diffusivity

Correlation of ITB formation with safety factor q near 1

Faster electron heating rates in H-mode discharges compared to L-mode

Abstract

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 $\chi_e$ an order of magnitude lower than the average value across the plasma, suggesting an electron internal transport barrier (ITB) near the ECH heating location. 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.