Electron Density Measurements Behind Strong Shock Waves by H-β Profile Matching

摘要

Emission spectroscopy was performed in a free-piston double-diaphragm shock tube to measure the electron density behind a strong shock wave, using nitrogen as the test gas. Time-frozen spectra from behind the shock wave were taken by an image-intensified charge-coupled device camera with a gate width of 100 ns. The laser schlieren diagnostics was used to detect the shock arrival and to correlate observed spectra with the distance from the shock front accurately. The electron density was measured by means of a line profile matching technique, using the H-β line broadened by the Stark effect. Using this measurement system, the electron density distribution was obtained with a high spatial resolution of +-0.6 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 predicted by the thermal and chemical nonequilibrium models widely used. Several drawbacks in the conventional ionization model at high shock velocities are pointed out.