Inference of the degree of dissociation of weakly collisional hydrogen plasmas using collisional-radiative models

TL;DR

This paper introduces a new analysis technique for inferring the degree of dissociation in weakly collisional hydrogen plasmas, utilizing collisional-radiative models and experimental data to improve plasma diagnostics.

Contribution

The work develops and tests a novel method combining collisional-radiative modeling with experimental spectroscopy to accurately determine hydrogen plasma dissociation degrees.

Findings

Successfully inferred dissociation degrees from experimental data.

Enhanced collisional-radiative models account for bi-Maxwellian electrons and radiation trapping.

Validated technique with hydrogen plasma experiments in a large chamber.

Abstract

A new analysis technique for the inference of degree of dissociation of weakly collisional hydrogen plasmas was developed and tested with an experiment. Neutrals in low temperature plasmas are critical to a wide range of plasma-based technologies, and thus, it is important to investigate their properties. The most important physical parameter for neutrals in hydrogen plasmas is the degree of dissociation, and the technique for its inference was developed in this work. To improve the accuracy of the analysis, the collisional-radiative models for hydrogen atom and molecule were constructed and modified to handle bi-Maxwellian electron energy distribution and radiation trapping effect. An additional analysis of Fulcher-alpha transitions was conducted to obtain gas temperature and ground vibrational temperature from rotational-vibrational distribution of excited molecules. The technique involves multiple steps to generate calculated excited state distributions from experimental data, which are then compared to measured distributions to infer the most probable degree of dissociation. To test and verify the analysis technique, experiments were conducted. Hydrogen plasmas were generated in a large cylindrical chamber named MAXIMUS, by DC discharge with a hot cathode. Various diagnostics including the optical emission and absorption spectroscopy, and the Langmuir probe measurement were performed to obtain spectra and electron parameters. The degrees of dissociation for the generated plasmas were inferred, based on the measured data and the analysis technique. Several problems and implications were discussed and improvement strategies were outlined. Overall, the analysis technique developed in this work is expected to play a valuable role in future diagnostic and analytical studies of hydrogen plasmas.