Effects of neutral transport on plasma scrape-off layer turbulence in gyrokinetic simulations*

Interactions between the plasma and neutral particles play a crucial role in determining the exhaust characteristics of tokamak plasmas. We present the first 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 blobby transport and edge plasma profiles. Previous gyrokinetic simulations with kinetic neutrals included only adiabatic electrons. We include both gyrokinetic ions and electrons in our simulations, allowing us to explore the effect of neutrals on density profiles. Our model incorporates electron-impact ionization, charge exchange, and wall recycling boundary conditions. Ionization and charge exchange interactions are coupled as sources and sinks to the gyrokinetic species, which avoids the statistical noise convergence properties associated with hybrid continuum-Monte Carlo codes. The model has been successfully verified with analytical predictions and benchmarked with the DEGAS2 Monte Carlo neutral code. We have carried out simulations for a scrape-off layer (SOL) in simplified geometry with NSTX parameters. Compared to simulations without neutrals, these tests exhibit a reduction in density gradients and fluctuation levels, increased skewness and kurtosis of the PDFs, increased autocorrelation times, and a decrease in the heat flux to the divertor. These results point towards a complex interaction between neutral effects and plasma turbulence that cannot be accurately captured by transport codes used to model exhaust characteristics in future devices and highlight the importance of coupling first-principles turbulence models with neutrals in predictive SOL modeling.