Interpretation of Stark broadening measurements on a spatially integrated plasma spectral line

TL;DR

This paper investigates how Stark broadening measurements of hydrogen spectral lines in thermal plasmas are affected by spatial integration, revealing that the measured electron density reflects the maximum electron density profile rather than the local value.

Contribution

A Python simulation code was developed to model Stark broadening in spatially integrated plasma spectra under LTE assumptions, clarifying the interpretation of electron density measurements.

Findings

Measured electron density is 70-80% of maximum electron density.

Stark broadening is governed by the maximum electron density profile.

Spatial integration affects the interpretation of spectral line broadening.

Abstract

In thermal plasma spectroscopy, Stark broadening measurement of hydrogen spectral lines is considered to be a good and reliable measurement for electron density. Unlike intensity based measurements, Stark broadening measurements can pose a problem of interpretation when the light collected is the result of a spatial integration. Indeed, when assuming no self-absorption of the emission lines, intensities simply add up but broadenings do not. In order to better understand the results of Stark broadening measurements on our thermal plasma which has an unneglectable thickness, a Python code has been developed based on local thermodynamic equilibrium (LTE) assumption and calculated plasma composition and properties. This code generates a simulated pseudo experimental (PE) H$\alpha$ spectral line resulting from an integration over the plasma thickness in a selected direction for a given temperature profile. The electron density was obtained using the Stark broadening of the PE spectral line for different temperature profiles. It resulted that this measurement is governed by the maximum electron density profile up until the temperature maximum exceeds that of the maximum electron density. The electron density obtained by broadening measurement is 70% to 80% of the maximum electron density.