Neoclassical study of the isotope effect in density pedestals

The isotope mass scaling of the energy confinement time in tokamak plasmas typically differs from gyro-Bohm estimates. This phenomenon – known as the isotope effect – remains an open issue in plasma physics, with important implications for the extrapolation from present day, mostly deuterium (D), fusion experiments to future deuterium-tritium (D-T) reactors. Differences in mass scaling in L-mode and various H-mode regimes suggest that the isotope effect may, in large part, originate from the pedestal. In the pedestal, sharp gradients render local diffusive estimates invalid, and global effects due to orbit-width scale profile variations have to be taken into account, potentially leading to mass scalings different from gyro-Bohm. We calculate cross-field fluxes from a radially-global linearized drift-kinetic equation using the PERFECT code, to study isotope composition effects in density pedestals. We define dimensionless parameters from the ratios of density length scale, pedestal width and orbit width, and study global effects in terms of these parameters for different pedestal profiles and bulk species. Quantifying global effects by the relative difference between peak heat-flux values in global and local simulations, we find that this quantity saturates at an isotope-dependent value, but the dimensionless parameters do not capture all the isotope dependencies. We also consider D-T and H-D mixtures, and compare the calculated heat fluxes to fluxes calculated from single species simulations with artificial "DT" and "HD" species