Electron-Beam Gun System for Electron-Capture Rate-Coefficient Measurement

Abstract

<p>A simple, pulsed, relativistic electron-beam gun has been built to measure electron attachment rates. The high voltage pulse (100KV) is provided by a spiral generator initially charged to 10KV. The electron-beam is formed by a lead (Pb) cathode vacuum-diode. The resulting electron-beam pulse has an energy of 90KeV per electron, a peak current density at the anode center of 175 A/cm², and a duration of 12 ns (FWHM).</p> <p>The electron-beam is injected through a 26-μm thick aluminum foil into a gas mixture located in an electric field. By placing suitable screens in front of the beam, the injected electron-beam current-density is reduced to 1-2 A/cm². This results in an electron density in the plasma (produced by the primary electrons injected in an atmospheric-pressure gas mixture) which is typically 10¹³ cm⁻³. The electric field applied across the plasma is maintained constant (within 0.2%) by means of a storage capacitor (1μF). Rate coefficients of electron-capture reactions are determined by observing the temporal evolution of the induced discharge current pulse.</p> <p>The system was used to measure electron dissociative-attachment rate coefficient for HCl as a function of the reduced field E/N, in both Ar-HCl and Ar-N₂-HCl mixtures. These coefficients are 6.45±0.95 x 10⁻¹¹ cm³ s⁻¹ in Ar-HCl (99:1) and 2. 59±.25 x 10⁻¹⁰cm³ s⁻¹ in Ar-N₂-HCl (35: 64:1), for E/N 10⁻²⁰V.m². From these measurements, and from the use of a numerical solution of the Boltzmann equation for electrons, we have calibrated the relative electron dissociative-attachment cross-section measured by Abouaf and Teillet-Billy. The peak values that give the best fit to our data are 9.4 x 10⁻¹⁹ cm² for Ar-HCl and 18.0 x 10⁻¹⁹ cm² for Ar-N₂-HCl.</p>