Experimental study on fast ionization wave in supersonic flow

Results of a supersonic flow ionization experiment are described in this paper. In the experiment, the Mach 3 indraft supersonic wind tunnel and the air were used. As an ionization method, the first ionization wave (FIW) was used because FIW is expected to produce a large number of electrons and as a result, to enhance an electrical conductivity of the boundary layer on a wall of the wind tunnel. The main objective of the experiment was to investigate discharge formation inside the boundary layer by FIW. A repetitive power supply was designed, built-up, and tested. The power supply has shown to provide a peak voltage of ±32 kV, a pulse width of 100 ns, a rapid voltage rise time of 0.6 kV/ns and a repetition rate of about 1 kHz. When the high voltage pulse was applied, a bright light emission from the air was observed near the wall, and it extended downstream over a long distance. The propagation of FIW along the flow was examined using two capacitive voltage probes. As a result, the propagation of FIW was confirmed, and its propagation velocity was found 4.7 × 10 ⁶ m/s. Therefore, the light bright emission near the wall was ascribed to excitation and ionization of air molecules due to the propagation of FIW. The reduced electrical field induced by the FIW propagation was also estimated, and it was relatively high (=3.0 × 10^-19 V.m ²) at the peak. At the same time, this high electric field duration time was found short (=3.5 ns). Furthermore, the positive polarity voltage applied to the high voltage electrode was found to provide higher reduced electric field and shorter duration time of high electric field than the negative one for gas number density less than 3.8 × 10²⁴m ^-3. From these results, it is suggested that FIW has a possibility to become a useful ionization method for the supersonic flow.