An experimental investigation of ionization of supersonic air by a corona discharge

A technique is developed to ionize supersonic air flow in a shock tube by a weak corona discharge. The driven section of the tube is open to the atmosphere. The shock propagates in the driven section at about 1 km/s by means of an area contraction near the downstream end of the driven section. Air is ionized by a unipolar corona discharge device comprising of a sharp-edged wedge as a high-field electrode inserted in a 41.25 mm (2") ID tube that forms a low-field electrode. The device requires less than 0.5 W. A ring probe, downstream of the corona discharge device, collects charges (ions and electrons) from the air; its output voltage is thus a measure of the degree of ionization. The degree of ionization is varied by varying the corona discharge voltage and the flow speed.; Corona generation was initially demonstrated with bench tests. These indicated an increased degree of ionization with an air flow created by a fan, relative to that in static air. Tests in a shock tube, with subsonic and supersonic air flow, and with a negative corona, provided results with the same ionization behavior. The operating range of discharge voltages is relatively small for a positive corona; hence, shock tube tests were confined to a negative corona.; Shock tube test results indicate that, on a 10 millisecond time scale, corona generated ionization was convected downstream to the probe location in a supersonic flow. An anomalous shock tube result is a probe signal, without a corona discharge, that is similar, but weaker, to the signal with a discharge.; Although the tests were done with shock speeds up to about 1 km/s and with atmospheric air ahead of shock wave, the technique of ionization and plasma measurement can be extended to higher speeds and lower pressures. It is also suggested that a supersonic wind tunnel with a Langmuir probe be used for this type of work.