Departures from Local Thermal Equilibrium in a Magnetically-Driven Shock Wave

An experimental study is reported of the time-resolved spectrum of the plasma produced by the magnetically-driven shock wave in a T-tube. It is shown that the population densities of the excited levels of helium ions in the plasma depart from the values they would have in local thermal equilibrium. On the other hand if local thermal equilibrium is assumed and the electron temperature estimated for the intensity ratio of He II (4686Å) to He I (5876Å) then in the conditions of the experiment an error of a factor of two in the electron temperature arises. The reason for the departure from local thermal equilibrium was shown to be the rapid rate of change of the plasma conditions (especially the electron temperature). This is so great that the ionization and recombination rates, being slower, cause certain population densities to lag behind the electron density and electron temperature. These rates were found to be consistent with the rates calculated by Bates et al. in their collisional-radiative model for the ionization and recombination of hydrogen-like ions.