Nonequilibrium radiation from a recombining nitrogen plasma

International audiencePrevious experiments performed at Stanford University studied the properties of a recombining non-equilibrium nitrogen plasma at atmospheric pressure. They used an inductively coupled plasma torch to create an equilibrium plasma and then imposed rapid cooling within a water-cooled tube in order to force rapid recombination and generate non-equilibrium distributions of ground and excited species. Emission spectroscopy was used to measure the plasma properties. Comparisons with numerical simulations showed that CFD codes did not correctly predict the temperature drop and the radiative flux leaving the plasma. This paper discusses a re-analysis of these experiments using new temperature measurements performed using Raman spectroscopy. A key result is that the measured temperature drop due to the imposed cooling is even higher than that measured by prior emission spectroscopy measurements. In addition to the Raman measurements, optical emission measurements were also performed and used to measure the atomic nitrogen density. From these measurements, we show that a large portion of the plasma enthalpy is stored in non-recombined nitrogen atoms. This is an important consideration for CFD codes looking to model atmospheric reentry dynamics