Characterization of supersonic low pressure plasma jets

Low Pressure Plasma Spraying (LPPS) processes use a DC plasma jet expanding at low pressure for fast deposition of dense coatings in a controlled atmosphere. The LPPS technology is widely used industrially in particular in the aeronautics and medical industries among others. Unlike atmospheric pressure plasma jets, which have been extensively studied experimentally and theoretically, the interest in low pressure DC plasma jets only occurred recently. However, the process development has been mainly based on empirical methods and the basics of the physical mechanisms that govern them still remain to be investigated. Further improvement of the processes requires, in particular, the knowledge of physical properties of the plasma jet such as the temperature, flow velocity and plasma density. Low pressure plasma jets present unconventional properties such as low collisionality, large dimensions and supersonic flow. Therefore specific diagnostics have to be adapted to these conditions. In this study, argon plasma jets at pressures between 2 and 100 mbar are investigated. Imaging has been used to allow a qualitative description of the plasma jet topology for different pressures and torch parameters. Low pressure plasma jets are most of the time supersonic, compressible and in an aerodynamic non-equilibrium, which results in visible successive compression and expansion zones corresponding to a variation of the local pressure, temperature and density. Imaging, combined with pressure measurements inside the plasma torch, has evidenced three different types of flow regimes with respect to the chamber pressure. For chamber pressures below 45 mbar, the flow is under-expanded and is characterized by an exit pressure higher than the chamber pressure. For pressures above 45 mbar, the plasma jet is over-expanded, in this case the exit pressure is lower than the chamber pressure. When the exit pressure is equal to the chamber pressure, the plasma jet is in the so-called design pressure regime. A diagnostic tool, extensively applied on atmospheric plasma jets, the enthalpy probe system, has been modified in order to allow gas sampling from the plasma jet at low pressures. A shock wave appears in front of the probe when it is immersed in a supersonic plasma jet, making the interpretation of enthalpy measurements more difficult.The free-stream properties, like the Mach number, temperature and free-stream enthalpy have to be inferred from stagnation measurements. Two interpretations of enthalpy probe measurements are described in this study. The first method uses the energy conservation equation and LTE assumptions with calorically perfect gas and neglecting the aerodynamic non-equilibrium, whereas the second method, uses a complementary measurement of the static pressure just after the shock using a specially developed tool: the Post Shock Static Pressure Probe (PSSPP). This allows the use of the conservation equations to determine the free-stream properties of the p