Excitation and breakdown of Ar at very high ratios of electric field to gas density

Abstract

Spatial distributions of 811-nm emission from the 2${p}_{9}$ and 2${p}_{7}$ (Paschen notation) levels of Ar have been measured for electrical discharges in Ar at very high ratios of electric field to gas density (E/n) and low nd, where d is the electrode separation. Normalization of the lowest-E/n data to published electron excitation coefficients yields absolute excitation coefficients for 270E/n43 000 Td (1 Td=${10}^{\mathrm{\ensuremath{-}}21}$ V ${\mathrm{m}}^{2}$) and for 6.4\ifmmode\times\else\texttimes\fi{}${10}^{19}$nd3.5\ifmmode\times\else\texttimes\fi{}${10}^{21}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}2}$. Direct and cascade excitation of 811-nm emisison by electrons calculated using a ``single-beam'' nonequilibrium electron model is an order of magnitude too small to account for the observed emission at the higher E/n.A model which includes Ar excitation and ionization by ${\mathrm{Ar}}^{+}$ and by fast Ar (10--200 eV) is developed to explain the observations. The fast atoms are produced by charge-transfer collisions of ${\mathrm{Ar}}^{+}$ with Ar. The estimated excitation by ions is negligible and has the wrong spatial dependence. Using the very limited published cross-section data for 811-nm excitation by fast Ar, the model yields spatial dependencies of emission which agree with experiment, but which are too small by factors ranging from 2.5 at 43 kTd to 10 at 6.3 kTd. This variation in the 811-nm emission with E/n is used to obtain energy-dependent excitation cross sections for fast atoms. The good fit of theory to the experimental spatial dependence near the cathode at the higher E/n shows the importance of ionization of Ar by fast Ar atoms. Excitation by backscattered secondary electrons is an important source of 811-nm emission near the anode. Electrical-breakdown and discharge-maintanance voltages from various experiments, including ours, are compared with the predictions of the model. These analyses show that ionization by ions and fast atoms dominates that by electrons from E/n>15 kTd. The estimated ionization by electrons backscattered from the anode provides sufficient feedback to explain much of the electrical-breakdown data and our discharge-maintenance data. Other breakdown data require either a large yield of ionization by backscattered electrons or a very large ion-induced electron yield at the cathode.