Isotope dependence of the radial electric field in the DIII-D tokamak*

DIII-D experiments in hydrogen and deuterium plasmas have identified differences in the L-mode radial electric field profiles between isotopes just before the L-H transition, which are observed by four independent radial electric field diagnostics: impurity charge exchange spectroscopy, Doppler BackScattering, Beam Emission Spectroscopy, and Langmuir probes. The electric field differences between isotope plasmas are investigated from the perspective of both the open and closed field line regions. In the open field line region, a hotter outer strike point is found to generate a more positive radial electric field at the separatrix in hydrogen plasmas compared to deuterium. In the closed field line region, the higher radial electric field is attributed to changes in poloidal rotation and turbulent Reynolds stress between isotopes, with Reynolds stress profiles consistent with predictions from E×B shear eddy tilting. The measured differences in L-mode Er and E×B shear are modeled to quantitatively assess their impact on the observed L-H transition power threshold.