Generating conditions of a laser-sustained argon plasma jet

Abstract

An argon plasma jet is produced from a continuous optical discharge at gas pressures in the range 2<or=P0<or=8 bar and effective laser power in the range 165<or=P<or=360 W. Plasma production is characterized by measurement of the threshold of the stability region (in terms of laser power-gas pressure). Study of the threshold law given by a simple model allows the core temperature of the plasma to be deduced. Temperatures are also measured by time-of-flight analysis of the neutral beam extracted from the plasma jet. They are compared with averaged temperatures deduced from the nozzle flow rate. From this comparison, a temperature profile over the nozzle radius can be calculated as can the heat flux through the nozzle. The kinetic energy yield of the continuous optical discharge can then be estimated. Values as high as 60% of the input laser power are obtained.

Properties of an argon plasma free jet generated from a continuous optical discharge

“…10 The two beams are focused just ahead of the sonic nozzle, inside the stagnation reservoir of the nozzle beam generator. A plasma is maintained by continuous radiation ͑power used here: 150ϽP Ͻ400 W͒ in argon at pressure P 0 р8 bars.…”

“…This lens can be continuously moved axially during operation by a worm-gear driven from outside the vacuum chamber; thus the distance between the laser waist and the nozzle throat can be adjusted accurately ͑this distance is typically of 1 mm͒. This allows the plasma to be moved, during the experiments, 10,14 in regions where it is more or less affected by the gas flow through the nozzle. In spite of a rather complicated optical path ͑seven copper mirrors for a 7 m path-length͒, the alignment remains accurate and stable over one or two weeks within 0.1 to 0.2 mm accuracy with respect to the nozzle axis.…”

“…The time-of-flight system is of a conventional type, as largely described previously. 10,17 The optical device for the local spectral analysis of the light emitted by the plasma free jet is described in Fig. 2.…”

“…where K abs is the usual Kramers coefficient 10,31 and I(x) is the laser power density per unit area of the laser beam cross section. As the calculation deals only with the plasma region which is downstream of the plasma core, not only the laser divergence is taken into account, but also the absorption of the laser intensity by the upstream plasma.…”

“…This behavior appears through the time-of-flight measurements carried out on the atom beam skimmed from the plasma free jet. 10 On the contrary, the electronic excitation is mainly produced by the electron collisions because the cross sections governing this process are much larger than for the collisions between heavy particles. Then are expected strong deviations from equilibrium between the kinetic energy distributions of heavy particles and electrons, as well as the kinetic and internal energy distributions of the heavy particles.…”