Investigations into low to high confinement mode transition isotope effect using the DIII-D tokamak

Kyle Callahan
Tuesday, February 20, 2024
11:00 am

Abstract: Recent experiments at the DIII-D tokamak have identified systematic isotopic differences in the L-mode heat transport and radial electric fields between hydrogen and deuterium plasmas. The heat transport isotope effects observed are uncovered using the nonlinear gyrokinetic code CGYRO, and found to be from a combination of physical effects which include: main ion dilution, electron non-adiabaticity, and mass dependent E×B shear stabilization. The flux matched gyrokinetic simulations, when compared to experimental turbulence measurements from the Beam Emission Spectroscopy and Correlation Electron Cyclotron systems using synthetic diagnostics, illustrate reasonable agreement between theory and experiment. The observed radial electric field isotopic differences are seen by all four edge radial electric field profile diagnostics at DIII-D, which include: impurity charge exchange spectroscopy, Doppler BackScattering, Beam Emission Spectroscopy, and Langmuir probes. The origin of the electric field differences between hydrogen and deuterium plasmas is investigated from the perspective of both the open and closed field line regions. Investigations into the open field line region, with the aid of both Langmuir probe measurements and UEDGE modeling, indicate that a hotter outer strike point resulting from a shifted density profile inside the separatrix correlates with a higher (more positive) radial electric field in hydrogen plasmas compared to deuterium. In the closed field line region, the higher radial electric field is attributed to changes in poloidal rotation between isotopes. These rotation differences are due to changes in the time-independent turbulent Reynolds stress between hydrogen and deuterium, which are consistent with Reynolds stress induced from E×B shear eddy tilting. It is hypothesized that the measured differences in L-mode Er and E×B shear may contribute to the observed L-H transition isotope effect.

Host: Laszlo Bardoczi

FRH 4135