Ion Density Profiles behind Shock Waves in Air

Abstract

Experimental results are reported for the ionization rate behind shock waves of Mach numbers between 7.2 and 9.2. The results were obtained using a hollow, total collector probe in combination with a 38-mm-diam, stainless steel shock tube designed to minimize the impurity level in the test gas. Good agreement was obtained with a theoretical result based on Lin and Teare's rate equations. Equilibrium ion densities in good agreement with theory were measured behind shocks of Mach number 7.2-12.8, with initial pressures between 0.5 and 40-mm Hg. The effects on the measurements of deviations from ideal shock tube flow were determined to be small. Ion densities measured range from 108 to 10 12 cm~3. At the higher ion densities there are large space charge effects inside the probe. Under these circumstances, proper operation of the probe still could be obtained provided the collector potential was not made too high. This implies that the mobility of the NO ions collected must be rather large. ON density profiles behind shock waves in air have been the subject of various recent investigations. Lin, Neal, and Fyfe1 made measurements in a low-density shock tube using microwave techniques and a magnetic induction device, in the range of shock Mach numbers Ms between 14 and 20. A theoretical analysis of their results based on chemical rate equations was given by Lin and Teare.2 Experimental results for even higher shock Mach numbers were reported by Wilson,3 who found that for shock Mach numbers above 27, electron densities become high enough so that electron impact reactions become dominant. At the lower temperature end experimental data have been obtained by Thompson, 4 who used a low-density shock tube in combination with microwave techniques, with infrared emission measurements, and with a hollow, total collector probe described by de Boer.5 The present investigation was undertaken in order to extend the range of measured profiles to still lower temperatures. At these temperatures, both the ion density and the ionization relaxation rate become much smaller, and it is impractical to use a low-density shock tube because the test time required is too long. A 38-mm-diam shock tube was used in combination with the hollow, total collector probe of Ref. 5. This probe has a very high sensitivity and can be used without difficulty in the regime of interest. The main problem to be overcome was the influence of impurities on the ionization rates. This problem could be solved by using great care in the experimental procedures.