Radiative emission analysis of an expanding hydrogen arc plasma—I. Arc region diagnostics through axial emission

Abstract

A numerical and experimental investigation of the axial emission of 1 and 5 kW radiatively-cooled hydrogen arcjet thrusters is presented. The numerical study consists of an integration of the 1-D radiative transfer equation along the arcjet centerline to predict the axial emission spectral intensity. The complete visible spectrum is modeled, taking into account graybody thermal emission from the cathode and plasma radiation from hydrogen atoms and free electrons. A collisional-radiative model is used to determine the atomic hydrogen line emission. The modeling results suggest the feasibility of determining the cathode temperature and the arc region electron number density from the measured axial emission spectrum. Experimental measurements of the axial emission spectra were performed at several operating conditions. The cathode temperature is found to be in the neighborhood of the tungsten cathode melting point and increases with arcjet power at a constant mass flow rate, suggesting an increase in the current density at the arc attachment point. The measured arc region electron number density is also found to increase with power, confirming an increasing arc current density with arcjet power. Relatively flat radial profiles of electron number density measured in the arc indicate the importance of diffusional transport processes.