Impact of neutral density fluctuations on gas puff imaging diagnostics

Abstract

A three-dimensional turbulence simulation of the SOL and edge regions of a toroidally limited tokamak is carried out. The simulation couples self-consistently the drift-reduced two-fluid Braginskii equations to a kinetic equation for neutral atoms. A diagnostic neutral gas puff on the low-field side midplane is included and the impact of neutral density fluctuations on light emission investigated. We find that neutral density fluctuations affect the emission. In particular, at a radial distance from the gas puff smaller than the neutral mean free path, neutral density fluctuations are anti-correlated with plasma density, electron temperature, and fluctuations. It follows that the neutral fluctuations reduce the emission in most of the observed region and, therefore, have to be taken into account when interpreting the amplitude of the emission. On the other hand, higher order statistical moments (skewness, kurtosis) and turbulence characteristics (such as correlation length, or the autocorrelation time) are not significantly affected by the neutral fluctuations. At distances from the gas puff larger than the neutral mean free path, a non-local shadowing effect influences the neutral density fluctuations. There, the fluctuations are correlated with the neutral density fluctuations, and the high-order statistical moments and measurements of other turbulence properties are strongly affected by the neutral density fluctuations.

Influence of molecular dissociation on blob-induced atom density perturbations

“…It is found that densities of thermal molecules and atoms which do not originate from dissociative processes behave as reported in [7,8], i.e., the neutral density decreases locally due to interactions with the electrons in the filament. The density of atoms from dissociated molecules, however, displays an opposite behaviour if the reaction rate for dissociation of molecules is bigger than that for ionization of atoms, which is the case in the SOL region for the results presented here.…”

“…is always negative, resulting in the anti-correlation between the atom density and the electron density as observed in [7,8]. For the source of deuterium atoms in (5), however, the sign is determined by the ratio of the absolute value of the two terms…”

“…Ignoring the fluctuations result in a significant difference when calculating reaction rates for blobs due to the correlation between ∆n e and ∆T e . Recently, the error resulting from assuming a static neutral density when calculating emission rates have been studied as well [7,8]. Both papers draw attention to the fact that the neutral density perturbation anti-correlates with that of the electron density, as the two are coupled trough the ionization process A + e → A + + 2e ,…”

Comparison between mirror Langmuir probe and gas-puff imaging measurements of intermittent fluctuations in the Alcator C-Mod scrape-off layer

“…The PDF of these signals furthermore are close to a normal distribution, with low moments of skewness, excess kurtosis and no elevated tails. It is plausible that ionization, where hot plasma filaments locally decrease the puffed gas density, is the main cause of this phenomenon and therefore should be accounted for in such an attempt to reproduce the emission from measurements of n e and T e (Thrysøe et al 2016;Wersal & Ricci 2017).…”

A reduced model of neutral-plasma interactions in the edge and scrape-off-layer: Verification comparisons with kinetic Monte Carlo simulations

“…In the 1D model, turbulence structures interact with neutrals through poloidal (y) averages, i.e., the neutrals "average over" the turbulence by virtue of their relatively long MFPs. For example, the neutral shadow [25] cast by a plasma blob may show up as a negative fluctuation in the radial neutral density profile, provided the blob shows up in the SOL plasma profile. This admittedly limited description may suffice for modeling the self-consistent evolution of neutral and plasma profiles.…”