The Role and Impacts of Isotope Mass on Burning Plasma Performance from DIII-D Similarity Experiments

The performance of burning plasmas is strongly influenced by the ion mass, M_I, which affects global energy and impurity particle confinement times, L-H power threshold, pedestal height, width, ELM characteristics, energetic particle modes, and RMP ELM-suppression. To understand the mechanisms behind these M_I dependencies and enable more accurate extrapolations towards D-T plasmas in ITER or an FPP, a set of systematic experiments and related modeling were organized and performed at DIII-D in similar hydrogen and deuterium plasmas. Several techniques for reducing P_LH in hydrogen plasmas were found: helium gas puffing, initiating L-H transitions at lower I_p and neoclassical toroidal viscosity rotation driven by non-resonant magnetic perturbations [1]. The H-mode pedestal width and pressure height increase while f_ELM decreases with M_I in dimensionally similar plasmas, and RMP ELM-suppression has so far not been achieved in hydrogen within the known deuterium access criteria [2]. Core turbulence amplitude surprisingly increases with M_I, counter to the lower observed transport, but the radial correlation length decreases [3]; CGYRO simulations indicate a shift to higher wavenumber instabilities at lower M_I. For a given injected power, Alfven eigenmode amplitudes and fast ion transport are generally higher for higher M_fast and M_I, consistent with TGLF-EP calculations and of potential concern for D-T plasmas [4].