Nonequilibrium Effects in Precursor Electrons Ahead of Shock Waves

Precursor electrons ahead of the shock wave are measured with a Langmuir probe using the hypervelocity shock tube of the Japan Aerospace Exploration Agency. The current-voltage curve of the probe is obtained with rapid scanning of the applied voltage, and the distributions of electron number density and temperature are obtained. The experimental results are compared with the analytically calculated distributions of the electron properties, including the effect of shock-tube radius and spectrum modeling of vacuum ultraviolet (VUV) radiation from the shock layer. The experimental results of the distribution of number density show larger values than the analytical results obtained with the assumption of thermochemical equilibrium in the shock layer for radiation calculations. The energy source for the production of precursor electrons is VUV radiation from the shock layer, and this VUV spectrum consists of atomic lines. In a nonequilibrium calculation with a two-temperature model, the electronic excitation temperature in the nonequilibrium region is lower than the equilibrium temperature. This means that the VUV radiation is predicted to be lower than in the equilibrium case, and the two-temperature model cannot be used to reproduce the precursor effect. For the simulation of the precursor effect, the implementation of a nonequilibrium model that produces highly excited atoms associated with VUV radiation is important.