Electron production in low pressure gas ionized by an intense proton beam

Electron density measurements from previous ion-beam-induced gas ionization experiments [F. C. Young el al., Phys, Plasmas 1, 1700 (1994)] are re-anal yzed and compared with a recent theoretical model [B. V. Oliver et al., Phy s. Plasmas 3, 3267 (1996)]. Ionization is produced by a 1 MeV, 3.5 kA, 55 n s pulse-duration, proton beam, injected into He, Ne, or Ar gas in the 1 Tor r pressure regime. Theoretical and numerical analysis indicates that, after an initial electron population is produced by ion beam impact, ionization is dominated by the background plasma electrons and is proportional to the beam stopping power. The predicted electron density agrees with the measure d electron densities within the factor of 2 uncertainty in the measurement. However, in the case of Ar, the theoretically predicted electron densities are systematically greater than the measured values. The assumptions of a Maxwellian distribution for the background electrons and neglect of beam en ergy loss to discrete excitation and inner shell ionization in the model eq uations are considered as explanations for the discrepancy. (C) 1999 Americ an Institute of Physics. [S1070-664X(99)00202-5].