Validity of the Local Thermal Equilibrium Assumption in Electromagnetic Shock Tubes

Abstract

In order to check the assumption of local thermal equilibrium in a transient plasma with kTe ∼ 4 eV and Ne ∼ 1017 cm−3, the electron temperature has been determined by measuring both the spectral line intensity ratio of He II 4686 Å to He I 5876 Å, and the laser light scattered by the plasma. The former method is strongly dependent on the local thermal equilibrium assumption, whereas, the latter method is independent of this assumption. These measurements were made on incident and reflected shock waves produced in an electromagnetic T-tube which had been prefilled with helium or hydrogen to a pressure of 0.5−1.0 Torr. Within the experimental uncertainty of ± 15%, the two electron temperatures agreed, indicating that the assumption of local thermal equilibrium used in the spectroscopic temperature determination is a reasonable approximation. Validity criteria for local thermal equilibrium given by Griem are examined for these conditions.