No abstract

Trace impurity transport is studied with the flux-driven gyrokinetic GYSELA code [V. Grandgirard et al., Comp. Phys. Commun. 207, 35 (2016)]. A reduced and linearized multi-species collision operator has been recently implemented, so that both neoclassical and turbulent transport channels can be treated self-consistently on an equal footing. In the Pfirsch-Schlüter regime likely relevant for tungsten, the standard expression of the neoclassical impurity flux is shown to be recovered from gyrokinetics with the employed collision operator. Purely neoclassical simulations of deuterium plasma with trace impurities of helium, carbon and tungsten lead to impurity diffusion coefficients, inward pinch velocities due to density peaking, and thermo-diffusion terms which quantitatively agree with neoclassical predictions and NEO simulations [E. Belli et al., Plasma Phys. Control. Fusion 54, 015015 (2012)]. The thermal screening factor appears to be less than predicted analytically in the Pfirsch-Schlüter regime, which can be detrimental to fusion performance. Finally, self-consistent nonlinear simulations have revealed that the tungsten impurity flux is not the sum of turbulent and neoclassical fluxes computed separately, as usually assumed. The synergy mostly results from the turbulence-driven in-out poloidal asymmetry of tungsten density. This result puts forward the need for self-consistent simulations of impurity transport, i.e. including both turbulence and neoclassical physics, in view of quantitative predictions for ITER.

Poloidal asymmetries of the E × B plasma flow are known to play a role in neoclassical transport. One obvious reason is that an asymmetrical potential can produce a flux. Also the associated distribution function may correlate with the magnetic drift velocity to enhance the neoclassical flux. Finally, poloidal variation of the electric potential can produce poloidal asymmetries of the density of an impurity, which in turn may enhance its neoclassical flux. According to conventional neoclassical theory, the level of poloidal asymmetry of the electric potential is expected to be very small. Conversely, poloidal flow asymmetries can be driven by small scale turbulence via non linear coupling, and therefore change this result. In the present work, a general framework for the generation of axisymmetric structures of potential by turbulence is presented. Zonal flows, geodesic acoustic modes and convective cells are described by a single model. This is done by solving the gyrokinetic equation coupled to the quasineutrality equation. This calculation provides a predictive calculation of the frequency spectrum of flows. It also shows that the dominant mechanism comes from zonal flow compression at intermediate frequencies, while ballooning of the turbulence Reynolds stress appears to be the main drive at low frequency.

Turbulent transport is a key physics process for confining magnetic fusion plasma. Recent theoretical and experimental studies of existing fusion experimental devices revealed the existence of cross-scale interactions between small (electron)-scale and large (ion)-scale turbulence. Since conventional turbulent transport modelling lacks cross-scale interactions, it should be clarified whether cross-scale interactions are needed to be considered in future experiments on burning plasma, whose high electron temperature is sustained with fusion-born alpha particle heating. Here, we present supercomputer simulations showing that electron-scale turbulence in high electron temperature plasma can affect the turbulent transport of not only electrons but also fuels and ash. Electron-scale turbulence disturbs the trajectories of resonant electrons responsible for ion-scale micro-instability and suppresses large-scale turbulent fluctuations. Simultaneously, ion-scale turbulent eddies also suppress electron-scale turbulence. These results indicate a mutually exclusive nature of turbulence with disparate scales. We demonstrate the possibility of reduced heat flux via cross-scale interactions.