Non-Intrusive Characterization of the Ionized Flow Produced by the Nozzle of a Hypersonic Wind Tunnel

The supersonic expansion of an ionized gas through a converging-diverging nozzle is characterized by strong departure from equilibrium conditions. In this work, a complete plasma characterization has been carried out on the Alta High-Enthalpy Arc-Heated Tunnel (HEAT), as a preliminary activity of a test campaign focused on the effect of the MHD interaction with the plasma of the shock layer. After a standard aerothermodynamic characterization of the hypersonic flow carried out through pressure and heat flux measurements, two different plasmas diagnostic systems have been utilised to characterize the system: emission spectroscopy in the HEAT plenum chamber and microwaves at the exit of the nozzle. The plasma in the arc heater plenum chamber has been investigated by means of optical emission spectroscopy techniques relying on the high pressure and, thus, on the PLTE conditions before the expansion in the nozzle. Electron excitation temperature has been evaluated from continuum and Boltzmann plots; electron number density has been derived from H-alpha Stark broadening and continuum. Conversely, the plasma cools down throughout the expansion in the nozzle and electrons develop non equilibrium distribution, as demonstrated by the calculations done in order to simulate the process. Microwaves absorption diagnostic technique shows a very weak dependence on electron energy distribution, and therefore was chosen to probe the electron number density at the end of the nozzle. Probing the plasma at the same time both at the beginning and at the end of the nozzle leads to the correlation of the measures to the results of a quasi one-dimensional model of the plasma expansion code based on the state-to-state kinetics, that consider a kinetic equation for each internal state of the heavy particles, self consistently coupled with the Boltzmann equation of free electrons.