Spatially and Temporally Resolved Electron and Atom Concentrations in an Afterglow Gas Discharge

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Abstract
The use of a modified laser interferometer to measure the temporal and spatial behavior of a medium density (1013–1014 electrons/cc) afterglow plasma is described. In the work described here, the plasma to be studied is located inside the laser cavity. The presence of the plasma changes the laser frequency (within the Doppler‐broadened transition) and this frequency shift is determined by coupling an external cavity to the laser. This modification of the interferometer preserves the high sensitivity of using the transverse modes in the external cavity and has a spatial resolution in the medium being studied of less than 1 mm2. Two optical wavelengths, λ0=0.6328 μ and λ0=1.1523 μ, and two microwave wavelengths, λ=4, 8 mm, are used in this study and excellent agreement is obtained between measurements. It is shown that there is a strong migration of the gas atoms away from the discharge vessel axis due to the localized heating during the active discharge. Electron loss processes in such plasmas are discussed. Because of the excellent spatial resolution, the rate of loss of electrons by diffusion can be measured directly, and hence the electron temperature can be estimated. Electron temperatures obtained in this manner agree to within a factor of two with that obtained by the double‐probe technique.