SSD - TR - 67 - 95 TR - 100 l ( 2240 - 20 ) - 11 1 * 1 , 00 00 * o CO ^ ? Microwave Breakdown of the Reentry Boundary Layer

Measurements of the microwave power required for breakdown have been made in a simulated reentry boundary layer flow. By establishing a fully developed laminar pipe flow between cold walls, fed by a subsonic arc jet, we have simulated the appropriate boundary layer temperature, density, electron concentration, and chemical composition profiles over an altitude range from 80 to 180 kft. Correlation with existing theoretical models and comparison with recent hot-air breakdown measurements in shock tubes are made. The effects of chemical composition of hot air and of typical ablation products and quenchants upon breakdown are briefly explored. It is found that significant reduction in breakdown power occurs in hot air flows and that present theoretical models do not predict the high altitude breakdown power within a factor of three or more.