Afterglow of Argon Plasmas with H₂, O₂, N₂, and CO₂Admixtures Observed by Thomson Scattering

Abstract

This study assess the decay of a power-pulsed microwave plasma in argon mixtures using time resolved Thomson scattering. In argon at intermediate pressure, i.e. tens of mbar, the electron temperature decays very fast after power off and approaches the heavy particle temperature, while the electron density decays slower, in the order of tens of microseconds. Argon with small admixtures of O 2 or H 2 exhibits a similar behavior. However, strikingly different is the result for small additions of N 2 or CO 2 . For up to 100 ms after the power is switched off, the electron temperature does not decrease to the gas temperature but remains at values as high as 0.8 eV. Nitrogen and carbon dioxide exhibit relatively large cross-sections to excite vibrational states by electron collisions. Knowing that, the observed post-heating can be attributed to superelastic energy transfer from vibrational excited N 2 (X) and CO 2 (X) molecules to electrons.

Influence of Nitrogen Admixture on Plasma Characteristics in a dc Argon Glow Discharge and in Afterglow

“…At that, even at t = 30 ms the electron temperature is rather high Te ≈ 6590 eV. It is worth noting that the calculated Te values agree with those measured in the afterglow of a pulsed microwave discharge in the Ar + 1%N2 mixture [3].…”

“…At that, even at t = 30 ms the electron temperature is rather high T e ≈ 6590 eV. It is worth noting that the calculated T e values agree with those measured in the afterglow of a pulsed microwave discharge in the Ar + 1%N 2 mixture [3]. The high degree of the vibrational excitation of nitrogen molecules leads to high effective electron temperatures in the afterglow plasma (see Table 1).…”

“…As a consequence, there was a strong coupling between the degree of the vibrational excitation of nitrogen and the electron temperature. The EEDF and the effective electron temperature in pure nitrogen afterglow plasma were studied in a large number of papers (see [40,41] and references therein), though there are a few works, in which similar studies were performed in gas mixtures of a rare gas with nitrogen [3,26,[42][43][44].…”

“…The estimated vibrational temperature was about 4000-5000 K, and the electron temperature, calculated by the measured EEDFs, was varied in the range 4000-6000 K (depending on the gas pressure and the discharge current). In [3], the electron temperature in the afterglow of Ar:N 2 power-pulsed microwave plasma was measured. Measurements were performed in a quartz tube with an inner radius of 0.3 cm in mixtures with N 2 percentage of 1-20% at a total gas pressure of 8-30 Torr.…”

“…Over the past decades, many experimental and theoretical works have been carried out in which the effect of nitrogen admixture on the characteristics of an electric discharge in argon has been investigated. Various types of discharges have been studied: microwave [1][2][3], RF (magnetron, inductive, and capacitive) [4][5][6][7][8][9], surface [10], barrier [11][12][13], dc [14][15][16][17][18][19][20][21][22][23][24], and pulse-periodic [25][26][27] discharges. It is shown that nitrogen impurity can significantly change the electrical and spectral characteristics of the discharge, the plasma ion composition, and the population of argon energy levels.…”