Application of a collisional radiative model to atomic hydrogen for diagnostic purposes

Abstract

Optical emission spectroscopy is one of the standard diagnostic methods to determine plasma parameters. In hydrogen plasmas often the intensity of the Balmer lines is recorded. To analyze such measurements population models for the hydrogen atom are needed. The flexible package Yacora is used to construct a new collisional radiative model for low pressure, low temperature hydrogen plasmas. This model includes six possible excitation channels: effective excitation of H, recombination of H + , dissociative excitation of H 2 , dissociative recombination of H 2 + , dissociative recombination of H 3 + and mutual neutralization of H - and H x + . The model is applied to a uniform ECR plasma with high dissociation degree and low ionization degree, i.e. electron collision excitation from H is the dominant excitation channel. The cross sections for this channel taken from literature showed a non-physical discontinuity at electron energies close to the threshold energy. This discontinuity has been removed by using a smoothing procedure. For known plasma parameters the deviation between measured and calculated population densities decreases significantly by using the smoothed data. Furthermore, the agreement of the electron density deduced from the ratio H β / H γ with results of other diagnostic methods is enhanced dramatically.