This paper describes the investigation of low pressure supersonic plasma jets as found in LPPS processes. The main objective is to develop and validate a two-dimensional axisymmetric mathematical model representing such flows. Due to the supersonic nature of the jet, insertion of a measurement probe leads to the appearance of a detached shock in front of the probe. Consequently plasma values are measured behind the probe-induced shock, namely the stagnation enthalpy ( h o ), the stagnation pressure ( p o ) and the static pressure ( p ). The first two values are taken from enthalpy probe measurements while the third value comes from a new technique. Combining these measurements, a new interpretation method enables the calculation of the free-stream supersonic plasma jet properties. The mathematical model is validated using the enthalpy probe measurements and the free-stream properties from the new interpretation method. Results show that the model does not predict a static pressure as large as the new interpretation method. The principal cause for this discrepancy is attributed to the LTE assumption which is questionable for a 40 mbar plasma jet. The modelling effort reported here confirms the need to develop more detailed mathematical models for low pressure supersonic plasma jets in the future.

A numerical model of an argon jet exiting a LPPS torch has been developed and validated against enthalpy probe measurements for a slightly overexpanded jet at a chamber pressure of 100 mbar. Visualization of the jet using a CCD camera shows the presence of a small Mach reflection in the first compression-expansion cell with only oblique shock waves in the second cell. This jet topology is also observed in the model results. The images of the enthalpy probe on the axis of the plasma jet reveal that the shock layer, or shock-probe distance, varies according to the axial location of the probe. Shock-probe distance can be as large as 3 mm and should be considered when mapping plasma jets. Paper includes a German-language abstract.

This paper investigates the effect of chamber pressure on plasma jet expansion characteristics. It presents images of the plasma jet corresponding to different chamber pressures and torch parameters and correlates them with enthalpy probe and pressure measurements recorded in different areas of the torch nozzle. A transition from an over-expanded to an under-expanded flow regime, as evidenced by a change in jet topology, is shown to be a function of chamber pressure. This transition pressure strongly depends on torch parameters and is characterized by an estimation of a rarefaction parameter based on nozzle exit and chamber pressure. At low chamber pressures, a progressive change from a continuum to a transition flow regime is shown by the thickening of the shock structures. Paper includes a German-language abstract.

This paper describes an experimental investigation of plasma jet properties of a DC torch operated at low pressure (below 10 mbar). A modified enthalpy probe system is described, which allows gas sampling from the plasma jet at pressures down to the mbar range. Measurements of the specific enthalpy, temperature and velocity throughout the jet for different pressures are presented and discussed. In the pressure range investigated, the jet flow is supersonic and compressible theory is used to infer the velocity from the dynamic pressure measured at the probe tip. In addition, optical emission spectroscopy of the plasma jet is used to evidence the differences of these low-pressure plasmas with respect to common, atmospheric pressure thermal jets. These preliminary measurements are the starting points towards a better understanding of plasma jets at low operating pressures in view of new process development and optimisation.

Plasma spray coating has achieved outstanding technological and commercial progress. However the underlying fundamentals still require a better understanding to overcome some limitations coming from, in particular, the instabilities of the arc and the strong erosion of the electrodes. In this paper we present experimental investigations of the fluctuating behavior of a Sulzer Metco F4 gun operated at atmosphere. The temporal evolution of the torch voltage and current, and of the plasma jet emission have been measured, hi addition, an optical fiber inserted inside the gun allows to measure fluctuations of the arc emission directly. Depending on the external parameters, different modes of operation have been identified. In the "restrike" mode which prevails for spraying-relevant operation conditions, detailed analysis of the voltage signals and corresponding arc and jet light emission reveals different categories of voltage drops corresponding to arc interruptions or reconnections. Spectral analysis of the different fluctuation signals shows clearly-defined peaks in the frequency range 3-30 kHz which are attributed to the arc motion and restrike inside the torch. The dependence of these peaks on operation conditions in terms of gas flows and composition, and gas injection geometry is presented. In addition a study of the effect of electrode aging on the torch fluctuations is reported.