Plasma-Parameter Dependence of Ro-Vibrational Temperatures for $\mathrm{H}_2$ in LHD Divertor

TL;DR

This study analyzes how plasma parameters influence the ro-vibrational temperatures of hydrogen molecules in the LHD divertor, revealing a strong correlation with electron density and the importance of electron-impact processes.

Contribution

It introduces a two-temperature model to describe non-thermal hydrogen molecule populations and links temperature variations to plasma parameters using Bayesian inference.

Findings

Lower rotational temperature depends linearly on electron density.

Higher temperatures and mixture coefficient increase with electron density.

Population distribution is significantly affected by electron-impact processes.

Abstract

We analyzed a thousand visible spectra of Fulcher-$\alpha$ band measured for divertor plasmas in Large Helical Device. With a coronal model and Baysian inference, the population distribution of hydrogen molecule in the electronical ground state were estimated. The non-thermal population distribution was recovered with a two-temperature model, which has two sets of rotational and vibrational temperatures, as well as their mixture coefficient. The lower rotational temperature significantly changes according to the plasma parameters. Its nearly linear dependence on the electron density was found, which is consistent with previous works. The lower vibrational temperature also shows a small density dependence, as reported by a previous work. On the other hand, the higher rotational and vibrational temperatures as well as the mixture coefficient only show slight changes over the broad range of plasma parameters. These population parameters show a significant correlation; with higher electron density, all the temperatures and the fraction of the higher-temperature component increase simultaneously. This suggests that the electron-impact plays an important role to determine the population distribution.