Diagnostic of vacuum subsonic and supersonic plasma flows with enthalpy probe, schlieren and high speed camera methods

Abstract

Summary form only given. HF plasma has been used for numerous applications. HF plasma of subsonic flow is used for densification and spheroidization of powders, thermal protection deposition and liquid and gaseous waste destruction. As for HF plasma supersonic flow, in spite of its less frequent use, its diagnostic is crucial in order to improve processes involving this type of plasma. Recently, HF plasma of supersonic flow has been used for the synthesis of nanometric powders and since many years, this plasma simulates ionized gas that space crafts encounter during their re-entry in the atmosphere. Thus, in this study, three diagnostic tools are used to acquire complementary information on HF vacuum plasma of subsonic and supersonic flows. The plasma jet is generated by a PL-35 induction plasma torch operating with two convergent-divergent Laval nozzles giving velocities around Mach 1.5 and Mach 3.0 and lower velocity when a subsonic nozzle is used. The plasma plate power and chamber pressure are fixed at 20 kW and under 60 torrs respectively. The plasma is made of argon and also of argon and 4% hydrogen. The first diagnostic tool is the enthalpy probe which measures enthalpy and stagnation pressure of the plasma and afterward, local plasma velocity and temperature are calculated. Completed axial and radial temperature and velocity profiles have been measured. The second tool, the Schlieren photography, is an optic system based on density gradient detection inside the plasma jet. Actually, because of the low chamber pressure and consequently the low plasma density, Schlieren photography of vacuum supersonic plasma jet is quite difficult, so this study will described the application limits of the Schlieren photography. The last tool is a high speed camera that gives relevant photographs about the stability and behaviour of the plasma jet. Photographs with different exposure times from 10 /spl mu/s to 1 ms reveal that supersonic plasma jet is stable. Also, subsonic plasma jet photographs taken with an exposure time of 100 pm show that plasma jet stability and specific energy are affected by the central gas flow and the chamber pressure. All this diagnostic of plasma jet behaviour is evaluated against plasma condition parameters in order to improve diagnostic and applications involving HF plasma.