MICROWAVE RESONANT CAVITY MEASUREMENT OF SHOCK PRODUCED ELECTRON PRECURSORS,

Microwave resonant cavities operated in the TM sub 010 mode and constituting a part of the driven section of a pressure driven shock tube are used in the investigation. With argon as the driven gas at a density of approximately 0.001 to 0.02 atmospheres and hydrogen as the driver gas, the shock Mach number is varied over a range between 10 and 13.5. With the cavities operated in the S-Band region, electron densities of the order of 5 x 10 to the 7th power to 1 x 10 to the 10th power elcubic cm can be measured. Using two cavities in tandem, tuned for the same resonant frequency and separated by a known distance, a measure of the precursor velocity is obtained. Several experiments are performed to determine the nature of the electron precursor, i.e., whether the precursor is due to large scale diffusion of electrons from behind the incident shock, or to radiation effects. It was found that electron precursor densities as high as 10 to the 8th power elcubic cm were measured as far as 12 meters in front of the shock, the electron precursor velocity is equal to the shock velocity, and the electron precursor in front of the shock is generated by radiation emitted from the ionized gas behind the incident shock. Author