Multispecies density peaking in gyrokinetic turbulence simulations of low collisionality Alcator C-Mod plasmas

Abstract

1Peaked density profiles in low-collisionality AUG and JET H-mode plasmas are probably caused by a turbulently driven particle pinch, and Alcator C-Mod experiments confirmed that collisionality is a critical parameter. Density peaking in reactors could produce a number of important effects, some beneficial such as enhanced fusion power and transport of fuel ions from the edge to the core, while others are undesirable such as lower beta limits, reduced radiation from the plasma edge and consequently higher divertor heat loads. Fundamental understanding of the pinch will enable planning to optimize these impacts. We show that density peaking is predicted by nonlinear gyrokinetic turbulence simulations based on measured profile data from low collisionality H-mode plasma in Alcator C-Mod. Multiple ion species are included to determine whether hydrogenic density peaking has an isotope dependence or is influenced by typical levels of low-Z impurities, and whether impurity density peaking depends on the species. We find that the deuterium density profile is slightly more peaked than that of hydrogen, and that experimentally relevant levels of boron have no appreciable effect on hydrogenic density peaking. The ratio of density at r/a=0.44 to that at r/a=0.74 is 1.2 for the majority D and minority H ions (and for electrons), and increases with impurity Z: 1.1 for helium, 1.15 for boron, 1.3 for neon, 1.4 for argon, 1.5 for molybdenum. The ion temperature profile is varied to match better the predicted heat flux with the experimental transport analysis, but the resulting factor of two change in heat transport has only a weak effect on the predicted density peaking.

A new mechanism for increasing density peaking in tokamaks: improvement of the inward particle pinch with edge E × B shearing

“…Most of the previous studies have, however, been performed in the electrostatic limit (β → 0). Also more recent nonlinear simulation work falls into this regime [22]. An early extension to the electromagnetic regime is presented in Ref.…”

Fast H isotope and impurity mixing in ion-temperature-gradient turbulence

“…The simulations are performed with the non-linear δf (gradient-driven) gyrokinetic code GKW [19] in its flux-tube mode (local approximation). To compute the ion and electron particle diffusivity and pinch, four kinetic species are included in the simulation, following [25]: two main species, deuterium and electrons, and two trace species with zero density and zero density gradient but otherwise identical to the main species. Alternatively (and equivalently), four nontrace species could be used provided their respective density gradient combine to a total of R/L n = 3 for each species.…”

Control of the hydrogen:deuterium isotope mixture using pellets in JET

“…The deuteriumtritium ratio in the plasma core can gradually drift from premixed value due to a number of reasons. Firstly, gyrokinetic analysis and modelling have identified mechanisms for tokamak particle transport coefficient differentiation between hydrogenic isotopes [1,2,3,4], although in practice the resultant density profile separation is expected to be weak; the isotope dependence of pedestal transport is more uncertain however. Secondly, for engineering reasons neutral beams are likely to inject pure deuterium as planned on ITER.…”