Atmospheric pressure plasmas in argon are of particular interest due to the production of highly excited and reactive species enabling numerous plasma-aided applications. In this contribution, we report on absolute optical emission and absorption spectroscopy of a radio frequency (RF) driven capacitively coupled argon glow discharge operated in a parallel-plate configuration. This enabled the study of all key parameters including electron density and temperature, gas temperature, and absolute densities of atoms in highly electronically excited states. Space and time-averaged electron density and temperature were determined from the measurement of the absolute intensity of the electron-atom bremsstrahlung in the visible range. Considering the non-Maxwellian electron energy distribution function, an electron temperature (T e) of 2.1 eV and an electron density (n e) of 1.1 × 10 19 m - 3 were obtained. The time-averaged and spatially resolved absolute densities of atoms in the metastable (1 s 5 and 1 s 3) and resonant (1 s 4 and 1 s 2) states of argon in the pure Ar and Ar/He mixture were obtained by broadband absorption spectroscopy. The 1 s 5 metastable atoms had the largest density near the sheath region with a maximum value of 8 × 10 17 m - 3, while all other 1s states had densities of at most 2 × 10 17 m - 3. The dominant production and loss mechanisms of these atoms were discussed, in particular, the role of radiation trapping. We conclude with comparison of the plasma properties of the argon RF glow discharges with the more common He equivalent and highlight their differences.
Bibliographical noteFunding Information:
This work was supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences General Plasma Science program under Award Nos. AT4010100 and DE-SC-0020232 and by the NSF Grant under Award No. PHYS 1903151. G.N. gratefully acknowledges the Kermit and Ione Ebeltoft Interdisciplinary Doctoral Fellowship for 2018–2019 and the Doctoral Dissertation Fellowship for 2019–2020.
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