Excitation mechanisms in the argon-ion spectrum at near laser conditions and temperatures and densities in a hollow cathode argon-arc discharge

Abstract

For a hollow cathode, low-pressure, magnetically-confined argon-arc discharge, an investigation was made of spectral line-intensities of the argon II spectrum to ascertain the excitation mechanisms in this spectrum, especially for the 4 p group (excitation energy 19.2–20.0 eV). It appears that, for n e values of 1 × 10 19 to 2 × 10 19 m -3, stepwise excitation from the ion ground levels through short-lived intermediate levels (16.3–16.8 eV) contributes at least 70% of the 4 p argon II group population. Direct excitation from the ion ground levels is dominant for considerably smaller n e values. Cascade radiation plays a minor role in populating the 4 p group. The stepwise excitation may lead to laser action in this type of discharge in the same way as it does in the common argon ion laser. The proposed mechanism is not cited in the literature with any certainty as being responsible for laser action. The electron temperature T e was determined from absolute line-intensity measurements of the argon I and II spectra. For argon I, the method is based on cross-section data for the 4 p group, for argon II on Corona equilibrium formulae. For a well-confined plasma, T e (0) on the axis was 30 × 10 3 to 40 × 10 3 °K at an axial distance of 0·6 m from the cathode. The value of the electron density n e was calculated from phase-shift measurements with microwaves, accounting for the non-linear relation between n e and the refractive index μ. The calculation was performed with the help of a method described in a previous paper. The influence of the slice breadth, interacting with the microwaves and of diffraction are discussed. It appeared that n e (0) on the axis is 1 × 10 19 to 2 × 10 19 m -3 for a confined plasma. Ion temperatures were determined from Doppler-width measurements of an argon II line with a Fabry-Perot interferometer. The values T i varied for different discharge conditions from 1 × 10 3 to 30 × 10 3 °K. The temperatures of the neutral particles, determined with the help of the width of an argon I line, varied from 500 to 4 × 10 3 °K for similar conditions. The ionization degree is between 0 and 80 per cent.