Kinetic modeling of neutral transport for a continuum gyrokinetic code

TL;DR

This paper introduces a novel coupling of a continuum gyrokinetic turbulence model with a 6D kinetic neutral model using the Gkeyll code, providing insights into neutral effects on plasma edge behavior.

Contribution

It is the first to couple a full-f gyrokinetic turbulence model with a 6D kinetic neutral model, verified against analytical and Monte Carlo benchmarks.

Findings

Neutral interactions reduce density fluctuation levels.

Neutral effects increase auto-correlation times.

Density profiles become flatter, resembling experimental shoulders.

Abstract

We present the first-of-its-kind coupling of a continuum full-f gyrokinetic turbulence model with a 6D continuum model for kinetic neutrals, carried out using the Gkeyll code. Our objective is to improve the first-principles understanding of the role of neutrals in plasma fueling, detachment, and their interaction with edge plasma profiles and turbulence statistics. Our model includes only atomic hydrogen and incorporates electron-impact ionization, charge exchange, and wall recycling. These features have been successfully verified with analytical predictions and benchmarked with the DEGAS2 Monte Carlo neutral code. We carry out simulations for a scrape-off layer (SOL) with simplified geometry and NSTX parameters. We compare these results to a baseline simulation without neutrals and find that neutral interactions reduce the normalized density fluctuation levels and associated skewness and kurtosis, while increasing auto-correlation times. A flatter density profile is also observed, similar to the SOL density shoulder formation in experimental scenarios with high fueling.