Inference of degree of dissociation for weakly collisional DC hydrogen plasmas with collisional-radiative models

Abstract

A new analysis technique for the inference of degree of dissociation of weekly collisional DC hydrogen plasmas is developed and tested with an experiment. There are neutrals, namely hydrogen atoms and molecule, present in hydrogen plasmas. These neutrals are utilized in many technological fields, and thus, it is important to investigate the information of neutrals in hydrogen plasmas. The most important physical parameters about hydrogen neutrals is the degree of dissociation. Therefore, the technique to infer the degree of dissociation is newly developed in this work. To improve the accuracy of the analysis technique, the collisional-radiative models for hydrogen atom and molecule are newly constructed and modified so that they can handle bi-Maxwellian electron energy distribution and radiation trapping effect. Also, there is an additional analysis for the Fulcher-alpha transitions in order to obtain gas temperature and ground vibrational temperature from rotational-vibrational distribution of an excited molecule. Various steps are involved in the technique to produce calculated excited state distribution from the information obtained from the experiment, and the calculated distribution is compared to the measured distribution to infer the most reasonable degree of dissociation of the measured plasma. In order to test and verify the analysis technique, an experiment is conducted. Hydrogen plasmas are generated in a large cylindrical chamber named MAXIMUS, with a hot cathode, and the gas pressure ranged from 3~6mTorr. Various diagnostics including the optical absorption & emission spectroscopy, and the Langmuir probe measurement are performed to obtain spectra and electron parameters. With the measured data and the analysis technique, the degrees of dissociation for the generated plasmas are inferred to be around 1%, and increase with the gas pressure. There are some problems found for the analysis technique, due to lack of accurate atomic data and imperfect optical measurement, and possible causes are discussed.