Air spectra induced by high-current relativistic electron beams

Single-shot photographic spectra from 3300 to 6200 A have been obtained from 6.5-MeV electron beams propagating in air at 0.2-Torr pressure. Spectra at this pressure exhibit the effects of the beam self-magnetic field compressing the plasma into a hot central core. The core is spectrally characterized by nitrogen- and oxygen-ion emissions with an obvious continuum. The core temperature is estimated to be 39,000 K from time-integrated ion-line ratios. A time-integrated electron density of 1 x 1016 cm-3 is estimated from H-β line broadening in the core. Spectra taken at 2, 10, and 50 Torr are totally dominated by nitrogen molecular emission. Vibrational temperatures of a few thousand degrees kelvin are estimated from time-integrated spectra, with the temperature decreasing with increasing pressure