Investigation of a Laser-Supported Directed-Energy 'Air Spike' in Hypersonic Flow

摘要

Preliminary experiments to demonstrate the laser-supported DEAS concept in Mach 6.2 flow (real air) and Mach 7.8 flow (ideal air) were conducted in the IEAv 0.3-m Hypersonic Shock Tunnel. A CO_2 TEA laser was used to drive the air ignition upstream of a hemisphere cylinder installed in the modified shock-tunnel test section. It was observed that, for high-enthalpy reservoir conditions, laser-induced air ignition could be established consistently in spite of the low static pressures present in the hypersonic airflow. A piezoelectric pressure transducer, installed in the model, indicated a drop in the impact pressure that coincided with the duration of the luminosity generated by the laser-induced air breakdown. Time-lapse-type photographs have shown a conical flow structure superimposed to the bow shock wave standing in front of the hemisphere cylinder. The dynamics of the formation of the conical flow structure was revealed through the use of a high-speed CCD camera. The frames show the initial bow shock in front of the hemisphere, followed by a conical flow structure at the moment of the laser-induced air ignition, which transforms back into the bow shock when the ignition is extinguished. These preliminary results seem to indicate that the laser-induced air breakdown was indeed generating a directed-energy air spike, and this effect reduced the impact pressure on the model. Low-enthalpy, ideal air, runs were also performed, but the laser-supported air ignition could not be established either consistently or completely.