Quasilinear carbon transport in an impurity hole plasma in LHD

Abstract

Comprehensive electrostatic gyrokinetic linear stability calculations for ion-scale microinstabilities in an LHD plasma with an ion-internal transport barrier (ITB) and carbon “impurity hole” are used to make quasilinear estimates of particle flux to explore whether microturbulence can explain the observed outward carbon fluxes that flow “up” the impurity density gradient. The ion temperature is not stationary in the ion-ITB phase of the simulated discharge, during which the core carbon density decreases continuously. To fully sample these varying conditions, the calculations are carried out at three radial locations and four times. The plasma parameter inputs are based on experimentally measured profiles of electron and ion temperature, as well as electron and carbon density. The spectroscopic line-average ratio of hydrogen and helium densities is used to set the density of these species. Three ion species (H,He,C) and the electrons are treated kinetically, including collisions. Electron instability drive does enhance the growth rate significantly, but the most unstable modes have characteristics of ion temperature gradient modes in all cases. As the carbon density gradient is scanned between the measured value and zero, the quasilinear carbon flux is invariably inward when the carbon density profile is hollow, so turbulent transport due to the instabilities considered here does not explain the observed outward flux of impurities in impurity hole plasmas. The stiffness of the quasilinear ion heat flux is found to be 1.7–2.3, which is lower than several estimates in tokamaks.

Observation of a novel reduced-turbulence regime with boron powder injection in a stellarator

“…Another possibility is turbulence being reduced by the increased effective charge Z ef f due to B injection, also referred to as plasma dilution. Indeed, an increase Z ef f has also been reported to have a stabilizing effect on ITG turbulence [22,23,24]. More likely, the combination of the two above mentioned effects might occur in our case, similarly to what reported in Ref.…”

Parallel impurity dynamics in the TJ-II stellarator

“…The input parameters are the ion and impurity rotation frequencies ω i , ω z ≡ ω, the friction coefficient ν = R 0 ν * zi (see equation (8)) and the magnetic geometry. Note that equations (C.1) and (C.2) depend only on the relative density variation n z / n z and not on the absolute value of the impurity density.…”

“…Recent theory and modeling efforts have investigated both turbulent [8] and extended neoclassical mechanisms [9] in an attempt to identify the physics underlying these anomalies. However, the observation of outward convection of impurities in the 'impurity hole' regime of the Large Helical Device has not found an explanation in these efforts so far.…”

Effects of collisions on impurity transport driven by electrostatic modes

“…There is a large body of theoretical research on the neoclassical transport of impurities in stellarators (Helander et al 2017;Velasco et al 2017;Calvo et al 2018), which we shall not describe in detail. Far less has been done to study turbulent particle transport -either of impurities or of the bulk ions and electrons -and most of these studies rely on quasi-linear transport theory (Mikkelsen et al 2014). Recently, however, there have been direct numerical simulations of turbulence in impure stellarator plasmas using gyrokinetic codes (Nunami et al 2020).…”