Flow Properties of a Partially Ionized Free Jet Expansion

Diagnostic studies were carried out in an induction-heated low-density supersonic plasma jet. The neutral number densities were obtained from the calibrated intensity of the 4609-Å argon II line excited by a 25-kV electron beam, and the velocities and heavy particle temperatures were determined by measuring the Doppler shift and Doppler broadening with a Fabry-Perot interferometer in two linearly independent directions, following a technique developed by Muntz. The contribution to the 4609-Å line due to the plasma self-emission was found to be negligible. The density, impact pressure, and velocity along the free jet center line all followed the isentropic source flow model up to the Mach disc. The temperature, however, showed an unexpected rise above the isentropic prediction. Ion number densities and electron temperatures were measured with a double Langmuir probe. The fraction of ionization was frozen at about 0.1% along the free jet axis. Calculations of the fraction of the energy of recombination absorbed by the electrons and the fraction lost by radiation from an electron energy balance showed good agreement with an a priori estimate based on collisional-radiative recombination.