Experimental Study of a Double Arc Nitrogen Plasma: Static and Dynamic Behavior

In the frame of studies devoted to the reentry simulations, we have devised and tested an original double arc plasma torch which provides a long-time and highly stable low-pressure supersonic nitrogen plasma jet. The arcs are investigated at both anode levels by means of spectroscopic diagnostics. They are not attached on the anode walls at single and local roots as it is generally the case at atmospheric pressure. In the first arc chamber, the plasma is totally dissociated and ionized, and the measured electron density and temperature are close to those derived from a local-thermal-equilibrium calculation. The electron density and temperature are also measured within the second arc chamber and at the torch exit where the plasma flow is abruptly released. The results allow to examine the validity of the thermodynamic equilibrium criteria in nonstandard conditions as those encountered in the arc chamber plasma. In the purpose of estimating both plasma jet specific enthalpy and torch energetic efficiency, Mach number measurements are also performed on the jet axis by using a pitot probe and they are compared to fluent numerical simulation results. The dynamic behavior of the plasma is analyzed by means of classical tools such as fast Fourier transform, correlation functions and Wigner distribution. The temporal series exhibit two main characteristic frequencies: At low frequency, the electric generators yield very stable and reproducible 150-Hz oscillations, whereas the firing of the second arc gives birth to a sharp 6.7-kHz peak which is ascribed to the generation of acoustic waves in the region of the first anode attachment. The value of this frequency that depends neither on the supplied power nor on the mass flow rate is interpreted by assuming that the arc chamber acts as a Helmholtz oscillator