Laser-Sustained Plasma in a Sonic Nozzle Probed by Fast Atom Beam

Abstract

The plasma sustained by an infrared laser, upstream of a sonic nozzle, is used for the generation of a supersonic freejet. Through a skimmer, a fast atom beam is extracted from the axial part of the freejet final stage, inside the zone of silence. In these conditions, the kinetic properties achieved at the end of the expansion are conserved in the atom beam and reflected in its velocity distribution. As the expansion of the freejet is closely described by the classical isentropic laws, at least on the axis, an equivalent stagnation temperature can be deduced from the time-of-flight velocity distribution of the atom beam. In the same way, the axial velocity at the nozzle throat can be obtained from the mean velocity of the atom beam. Also, the temperature and velocity fields of the plasma are calculated with fluid dynamic models, in the flowfield of the sonic nozzle. Two different solving procedures, including the laser beam energy absorption process, are presented. The values of equivalent stagnation temperature and the axial velocity at the nozzle throat, directly issued from these calculations, are found to be in reasonable agreement with the experimental results.