Electron density measurement behind a shock wave by means of H(beta) line profile matching

Abstract

Transient spectroscopic measurement has been performed in a free-piston double-diaphragm shock tube in order to evaluate the electron density behind a strong shock wave, using nitrogen as the test gas. Transient spectra from behind the shock wave are taken by an ICCD camera with a gate width of 100 ns. The laser schlieren diagnostics is used to detect the shock arrival, and to accurately correlate observed spectra with the distance from the shock front. The electron density is measured by means of a lime profile matching technique, using the H@ line primarily broadened by the Stark effect. Using this measurement system, electron density distribution is obtained with high spatial accuracy of f0.6 mm at a shock velocity of 12 km/s. Experimental results show that, especially at high shock velocities, the measured electron density increases more quickly behind the shock front than expected by the thermal and chemical nonequilibrium model widely used, although it approaches the calculated values far behind the shock front. Several problems in the conventional ionization model are forecasted for high shock velocities.