Investigation of the Afterglow in Helium-Xenon Mixtures at High Pressures

The behaviour of high pressure helium afterglows has been measured by means of conductivity and light intensity decay measurements. The plasma was created in a pulsed discharge of typically 30 usee length and then investigated by means of time resolved spectroscopy and probing pulse diagnostics under conditions which are of interest for noble gas afterglow MHD generators, i. e. a pressure range of 1—4 atm static pressure, the temperature varying from 600 — 970 °K. A marked increase in plasma lifetime was found when xenon was seeded with a rate of 10 to 103 to helium. This effect is ascribed partly to the influence of Penning ionization and partly to the fact, that slowly recombining atomic xenon ions dominate the recombination behaviour at the lower xenon seed rates during the investigated time interval of 150 /usee. The pressure dependent effective recombination coefficient is found to be (0.95 + 1.95 • 103 P0) 108 cm3 sec1 for pure helium and (0.2 + 5.1 -IO4 P0) 108 cm3 sec1 for a xenon seed rate of 3-104 (P„ = reduced pressure in Torr).