UCLA

Over the course of this PhD thesis, the dominant physics underlying the isotope dependence of the low- to high confinement (L to H-mode) transition in the DIII-D tokamak has been studied in detail. Historically, the pronounced isotope dependence of the L-H power threshold has been attributed to differences in thermal transport and in the radial electricfield at the plasma edge. In this thesis, extensive gyrofluid and local gyrokinetic modeling via the TGLF and CGYRO codes attribute the observed increased thermal transport in hydrogen to three important effects: 1) a reduced critical gradient for on Temperature Gradient (ITG) modes, caused by reduced carbon sputtering and impurity dilution in hydrogen plasmas; (2) electron non-adiabaticity (leading to increased transport fluxes in hydrogen), and (3) the main ion mass dependence of E×B shear stabilization (leading to reduced edge turbulence suppression in hydrogen). Turbulence predictions from CGYRO gyrokinetic simulations are compared to experimental measurements including electron temperature, density and E×B velocity fluctuations, and are found to be in good agreement with available experimental turbulence data. In addition to validating edge transport predictions, this thesis has focused on the isotopic dependence of the edge radial electric field. A two times larger edge radial electric field in hydrogen plasmas, compared to deuterium, is believed to play a fundamental role in setting the requisite conditions which trigger the L-H transition. Two effects are found to contribute to these isotopic differences in Er: A larger radial gradient of the turbulent Reynolds stress in hydrogen, and an increased outer strike point electron temperature and space potential on open field lines. Dedicated experiments were also performed to actively reduce the L-H power threshold in hydrogen plasmas (a topic of great relevance for the initial non-nuclear ITER experiments). The goal of this work was to artificially increase Zeff via carbon seeding to match the deuterium experiments, and possibly reduce the L-H power threshold based on gyrokinetic predictions of reduced ITG-driven thermal transport. While ITG turbulence was indeed observed to be reduced as expected with carbon seeding, no discernible change in L-H power threshold was found. A detailed investigation determined that the Reynolds stress and edge radial electric field were nearly unchanged in the presence of carbon seeding and ITG stabilization. Hence, taken together these experiments point to the differences in electric field (and associated E×B shear) as the most likely origin of the isotope dependence of the L-H power threshold in deuterium and hydrogen.