DEGAS 2 model validation study: comparison of measured and modeled helium and deuterium line emission arising from an external gas puff on Alcator C-Mod

TL;DR

This study validates the DEGAS 2 neutral transport model against measurements of helium and deuterium line emission from a gas puff on Alcator C-Mod, revealing good agreement for deuterium but significant discrepancies for helium.

Contribution

It provides the first detailed comparison of DEGAS 2 simulations with experimental neutral emission measurements in a tokamak boundary plasma.

Findings

Good agreement for deuterium line emission profiles.

Helium emission is underpredicted by a factor of three.

Discrepancies may be due to local cooling and turbulence effects.

Abstract

The ability to accurately model and predict neutral transport in the boundary plasma is important for tokamak operation. Nevertheless, validation of neutral transport models can be challenging due to the difficulty in measuring neutral particle distributions. Taking advantage of the localized neutral gas puff associated with the Gas Puff Imaging (GPI) diagnostic on the Alcator C-Mod, a validation study of the neutral transport code DEGAS 2 has been performed for helium and deuterium neutrals. Absolutely calibrated measurements of helium and hydrogen line emission are compared with simulated emission from DEGAS 2, accounting for the measured gas flow rates and employing a realistic geometry. Good agreement in peak brightness and profile shape is found for a deuterium puff case. However, helium line emission measurements are found to be lower by a factor of three than that predicted in the steady state DEGAS 2 simulations for a wide range in plasma density. Discrepancies in the light emission profile shape are evident as well. As possible causes for this discrepancy, two effects are discussed. First is the effect of local cooling due to gas puff. Second is the role of time-dependent turbulence effects on neutral penetration.