Transport mechanisms of metastable and resonance atoms in a gas discharge plasma

Abstract

Atoms in electronically excited states are of significant importance in a large number of different gas discharges. The spatio-temporal distribution particularly of the lower excited states, the metastable and resonance ones, influences the overall behavior of the plasma because of their role in the ionization and energy budget. This article is a review of the theoretical and experimental studies on the spatial formation and temporal evolution of metastable and resonance atoms in weakly ionized low-temperature plasmas. Therefore, the transport mechanisms due to collisional diffusion and resonance radiation are compared step by step. The differences in formation of spatio-temporal structures of metastable and resonance atoms in plasmas are attributed to these different transport mechanisms. The analysis is performed by obtaining solutions of the diffusion and radiation transport equations. Solutions of stationary and non-stationary problems by decomposition over the eigenfunctions of the corresponding operators showed that there is, on the one hand, an effective suppression of the highest diffusion modes and, on the other hand, a survival of the highest radiation modes. The role of the highest modes is illustrated by examples. In addition, the differences in the Green functions for the diffusion and radiation transport operators are discussed. Numerical methods for the simultaneous solution of the balance equations for metastable and resonance atoms are proposed. The radiation transport calculations consider large absorption coefficients according to the Lorentz contour of a spectral line. Measurements of the distributions of metastable and resonance atoms are reviewed for a larger number of discharge conditions, i.e. in the positive column plasma, afterglow plasma, constricted pulsed discharge, stratified discharge, magnetron discharge, and in a discharge with a cathode spot.

Plasma heating power dissipation in low temperature hydrogen plasmas

“…where E νe is the average radiative energy of all radiative states, r νe is the total excitation rate to radiative states (excluding recombination), E νr is the average radiative energy emitted subsequent to recombination, r rec is the total volumetric recombination rate and O ν describes less significant destructive processes of photons such as photo-ionization and -excitation 20,21 . The chemical potential ∇ • S µ can be divided into ionization potential and chemical potential associated to dissociation and excitation energies of vibrational levels and metastable electronic states, i.e.…”

“…where E inz is the ionization energy, ∆E µc is the average change of chemical potential in discrete processes, r µc is the corresponding total volumetric reaction rate and O µ describes less significant destructive processes of chemical potential, e.g. collisional quenching 22,23 or electron impact de-excitation 21,24 of metastable states.…”

“…P heat = P J,kH + P J,em = P J,ν + P J,µ + P J,kp + P J,kn . (21) Therefore, the ratios of the total photon emission, ionization and chemical potential powers to the plasma heating power can be expressed as P J,ν P heat = P J,ν P J,ν + P J,µ + P J,kp + P J,kn…”

Influence of Penning effect on the plasma features in a non-equilibrium atmospheric pressure plasma jet

“…For an indepth discussion of the transport mechanism of metastable particles in a discharge, the readers can consult Ref. 24.…”

Argon metastable and resonant level densities in Ar and Ar/Cl2 discharges used for the processing of bulk niobium

“…The escape factor in its most accurate form is evaluated from the integral of the photon transmission coefficient over the plasma volume and is dependent on the distribution of emitters and absorbers along the observation column. [26][27][28] A more complete investigation of the photon escape factor and its application to low temperature plasmas can be seen in the work by Zhu et al 29 There are, however, several approximations that can be used in measurements with various levels of accuracy. 22,26,[30][31][32] For this work, we follow the case of a homogeneous distribution of emitters and absorbers proposed by Mewe, 30,31…”