Precursor electron density measurement ahead of strong shock waves

In order to understand the fast ionization process behind the strong shock wave observed in the previous shock-tube experiment, we measure the transient plasma properties in the precursor region by means of the triple-probe method with the high signal/noise ratio and time resolution. Using nitrogen as the test gas, the electron density in the precursor is observed to increase very quickly as the shock wave approaches, while the electron temperature is almost uniform ahead of the shock wave. The electron density is found to depend strongly upon the shock velocity, while the electron temperature is less sensitive to the shock speed. At the shock velocity of 12.4 km/s, the electron density amounts to 10(19) m(-3), and the electron temperature is 7000 K at 20 mm ahead of the shock wave. Such a large number of electrons are considered to allow the much more vigorous electron-impact ionization even immediately behind the shock wave than expected by the conventional ionization models behind the shock wave.