Isotope effects on energy transport in the core of ASDEX-Upgrade tokamak plasmas: Turbulence measurements and model validation

Abstract

Design and operation of future tokamak fusion reactors using a deuterium-tritium 50:50 mix requires a solid under- standing of how energy confinement properties change with ion mass. This study looks at how turbulence and energy transport change in L-mode plasmas in the ASDEX Upgrade tokamak when changing ion species between hydrogen and deuterium. For this purpose, both experimental turbulence measurements and modeling are employed. Local mea- surements of ion-scale (with wavevector of fluctuations perpendicular to the B-field k⊥ <2 cm−1, k⊥ρs <0.2 , where ρs is the ion sound Larmor radius using the deuterium ion mass) electron temperature fluctuations have been performed in the outer core (normalized toroidal flux ρTor = 0.65 − 0.8) using a multi-channel correlation electron cyclotron emission diagnostic (CECE). Lower root-mean-square perpendicular fluctuation amplitudes and radial correlation lengths have been measured in hydrogen versus deuterium. Measurements of the cross-phase angle between a normal-incidence re- flectometer and an ECE signal were made to infer the cross-phase angle between density and temperature fluctuations. The magnitude of the cross-phase angle was found larger (more out-of-phase) in hydrogen than in deuterium. TRANSP power balance simulations show a larger ion heat flux in hydrogen where the electron-ion heat exchange term is found to play an important role. These experimental observations were used as the basis of a validation study of both quasi- linear gyrofluid TGLF-SAT2 and nonlinear gyrokinetic GENE codes. Linear solvers indicate that, at long wavelengths (k⊥ρs < 1), energy transport in the deuterium discharge is dominated by a mixed ion-temperature-gradient (ITG) and trapped-electron mode (TEM) turbulence while in hydrogen transport is exclusively and more strongly driven by ITG turbulence. The Ricci validation metric has been used to quantify the agreement between experiments and simulations taking into account both experimental and simulation uncertainties as well as up to five different observables accross different levels of the primacy hierarchy.