Ionization balance of impurities in turbulent scrape-off layer plasmas I: local ionization-recombination equilibrium

Abstract

In magnetized fusion devices, cross field impurity transport is often dominated by turbulence, in particular in the scrape-off layer. In these outer regions of the plasma, fluctuations of plasma parameters can be comparable to mean values, and the way ionization and recombination sources are treated in transport codes becomes questionnable. In fact, sources are calculated using the mean density and temperature values, with no account of fluctuations. In this work we investigate the modeling uncertainties introduced by this approximation, both qualitatively and quantitatively for the local ionization equilibrium. As a first step transport effects are neglected, and their role will be discussed in a companion paper. We show that temperature fluctuations shift the ionization balance towards lower temperatures, essentially because of the very steep temperature dependence of the ionization rate coefficients near the threshold. To reach this conclusion, a thorough analysis of the time scales involved is carried out, in order to devise a proper way of averaging over fluctuations. The effects are found to be substantial only for fairly large relative fluctuation levels for temperature, that is of the order of a few tens of percents.

Progress in edge plasma turbulence modelling—hierarchy of models from 2D transport application to 3D fluid simulations in realistic tokamak geometry

“…The impact of the excursion of plasma parameters around their average due to fluctuation can be taken into account to compute 'fluctuation dressed' cross-section for instance, see [23,24]. However, to use these expressions in practice in a transport code, one needs at least to estimate the level of fluctuations in the plasma.…”

Assessment of the effects of scrape-off layer fluctuations on first wall sputtering with the TOKAM-2D turbulence code

“…The average sputtering yields are plotted in figure 2(a), for D on Be and fluctuation levels R ranging from 0 (no fluctuations) to 80%. The effect of fluctuations is reminiscent of that observed on ionization rate coefficients in [22]: significant sputtering occurs at mean temperatures lower than expected because of positive temperature fluctuations. These results are in accordance with figure 1(b) which show that the fluctuation amplification factor F is large at near threshold energies.…”

“…All these effects come on top of unavoidable sputtering by energetic neutrals ('charge exchange' neutrals), which are not dealt with here (see [21] for a discussion on the effects of fluctuations on the latter). Finally, it should be noted that similar effects have been studied for the ionization balance of impurities or neutrals [22,23].…”

Self‐consistent coupling of the three‐dimensional fluid turbulence code TOKAM3X and the kinetic neutrals code EIRENE

“…Several groups have implemented neutral models in their code, either in 2D or 3D codes: a simplified kinetic model in GBS and a fluid model in BOUT++ . Moreover, it has been pointed out that additional non‐linearities introduced by neutral particle physics lead to additional closure issues in transport codes, which are difficult to address quantitatively in mean‐field models. Building on the experience gained during the development of Soledge2D‐EIRENE, the TOKAM3X code has been fully coupled to the EIRENE Monte Carlo solver .…”