Interaction of relativistic electron beams with neutral argon.

Abstract

Relativistic electron beams are frequently used to pump gas lasers. However underst and ing of population of excited levels by fast electrons is little known. Recently unusual light emission has been observed in REB produced plasmas. To underst and these studies of argon ion, electron impact excitation and ionization cross sections and deposition efficiencies for ArII experiencing collisions with relativistic electrons were generated. Calculations of cross sections were made using the Born approximation modified for low energies. A consistent set of energies and oscillator line strengths were generated from a simulation by Robert Cowan. Deposition efficiencies were produced from the Peterson and Green integral equation based upon CSDA calculations of energy apportionment. For most transitions the values of the efficiencies are roughly constant above a beam energy of about 1 keV, although in a few cases the efficiency continues to increase up to beam electron energies of 10 keV. Inner shells contribute up to 12% of the efficiency for electrons above 10 keV. A model of an electron beam interacting with argon was developed with rate equations written for excited levels of ArI and ArII coupled with equations for energy group electron densities. These coupled differential equations were solved for populations of excited levels of argon. Intensities of spectral lines were calculated, from predicted population densities and Einstein coefficients, and compared to experimental results. The thermal plasma generated persists beyond the beam pulse width time. Low energy 4s and 4p levels of ArII are generated by thermal electrons and have similar time profiles to the plasma density. However high energy 4p$\\prime\\prime$ and 4p$\\prime$ levels which can be reached by direct ionization of ArI to 4p levels of ArII have the same temporal profile as the beam current. They are populated predominantly by direct ionization and exceed levels populated by thermal electrons. The 4p$\\prime\\prime$ levels decay rapidly to 4s$\\prime\\prime$ levels which have very high beam deposition efficiencies for being pumped back to the 4p$\\prime\\prime$ levels. In this way certain transitions between energetic excited levels of ArII are pumped by the electron beam. Without direct ionization, these transitions would be negligible compared to lower energy transitions.