Investigation of the D.C. Glow Discharge Using Optogalvanic Spectroscopy.

Optogalvanic effects are employed in the study of D.C. glow discharges to discern important discharge characteristics. A model that predicts the absolute size of the 594.5 nm optogalvanic effect in a Ne positive column is developed. This model is based on applying a linear perturbation to the key rate equations that govern the discharge. The predictions of this model are in good agreement with experimental measurements. Accurate spatially resolved electric field measurements are made in the cathode fall region of He and Ne glow discharges using optogalvanic detection of Rydberg atoms. Because of their large size and small separation between states of opposite parity, Rydberg atoms are particularly sensitive to local electric fields. This technique is demonstrated with both a single and two step excitation of the Rydberg states. Two step excitation provides a pinpoint method of measuring fields that are not uniform along the path of a single laser beam. Gas density measurements are also made in the cathode fall using optogalvanic detection. Because of symmetric charge exchange, over one half of the discharge input power is converted into ground state atom translational motion. This can result in gas temperatures in the cathode fall significantly above ambient temperature. The ion charge density, the average ion energy and velocity, the ion and electron current densities, the local ionization rate, and the size of an avalanche due to an electron leaving the cathode can all be derived from the information contained in the field and gas density measurements. These experimentally derived quantities are compared to the results of computer simulations of electron avalanches through the cathode fall. The Monte Carlo code makes use of the null collision technique for nonuniform fields. The results of the simulations unambiguously demonstrate the non-hydrodynamic nature of the cathode fall.