# Total Fluid Pressure Imbalance in the Scrape-Off Layer of Tokamak Plasmas

## TL;DR

This study uses kinetic simulations to investigate pressure imbalances in the scrape-off layer of tokamak plasmas, revealing significant contributions from ion viscosity and neutral sources but still not fully explaining the observed pressure deviations.

## Contribution

It provides a detailed analysis of the pressure imbalance in the SOL, highlighting the roles of ion viscosity and neutral sources in kinetic plasma simulations.

## Key findings

- Ion viscosity and neutral sources significantly affect pressure balance.
- Observed pressure imbalance cannot be fully explained by these effects alone.
- Simulation results show large deviations from simple pressure balance assumptions.

## Abstract

Simulations using the fully kinetic neoclassical code XGCa were undertaken to explore the impact of kinetic effects on scrape-off layer (SOL) physics in DIII-D H-mode plasmas. XGCa is a total-f, gyrokinetic code which self-consistently calculates the axisymmetric electrostatic potential and plasma dynamics, and includes modules for Monte Carlo neutral transport. Previously presented XGCa results showed several noteworthy features, including large variations of ion density and pressure along field lines in the SOL, experimentally relevant levels of SOL parallel ion flow (Mach number~0.5), skewed ion distributions near the sheath entrance leading to subsonic flow there, and elevated sheath potentials [R.M. Churchill, Nucl. Mater. & Energy, submitted]. In this paper, we explore in detail the question of pressure balance in the SOL, as it was observed in the simulation that there was a large deviation from a simple total pressure balance (the sum of ion and electron static pressure plus ion inertia). It will be shown that both the contributions from the ion viscosity (driven by ion temperature anisotropy) and neutral source terms can be substantial, and should be retained in the parallel momentum equation in the SOL, but still falls short of accounting for the observed fluid pressure imbalance in the XGCa simulation results.