Regimes of Laser Plasmas - MW Field Interaction

Abstract

The possibility of laser spark initiation of microwave discharge in quiescent air in wide range of air pressure has been investigated. For the first time the stable electrodeless MW discharge initiated by laser spark produced on the second harmonics of Nd:YAG laser with 15ns-pulse duration in air under the atmospheric pressure is realized. The MW breakdown thresholds under the variation of radiated laser energy and air pressure are measured. Stable breakdown at reduced MW field intensity is registered under a laser beam action. The effect of MW breakdown onset/stabilization in the presence of sub-breakdown intensity of laser beam is eliminated. The initiation ability of laser spark over MW pulse delay depends substantially upon air pressure and changes from 220μs under the normal conditions to more than 100ms under 70Torr. MW discharge development in the direction of sub-breakdown laser beam, but across MW beam is observed. This effect is analogues to laser triggering of DC spark. The possibility of Ruby laser spark initiation of 1μs pulse duration MW discharge in quiescent air under the atmospheric pressure was also demonstrated. The critical delays for MW radiation coupling with decaying laser plasma, exceeding 300μs were determined. The additional energy input due to MW energy deposition was recorded. Experiments with 400ps laser pulse duration on the first and second harmonics of Nd:YAG laser have shown that energy for laser spark creation is more than order of value less in comparison with nano-second pulse duration. The breakdown levels were attained under substantially lower levels of the applied energy – less than 1mJ for 532nm radiation and about 3mJ for 1064nm radiation. The shock waves, arising after picosecond-laser breakdown in air under the normal conditions, are registered by means of the Shlieren system. The effectiveness of transformations of laser pulse energy in an energy, inputted in the gas can be estimated as 0.3±0.1. Numerical investigation of laser plasma decay is carried out and regimes of MW filed interaction with decaying laser plasma are analyzed. Copyright © 2006 by Yu. F. Kolesnichenko. Published by the AIAA, Inc. with permission Introduction Beamed energy technique is attracting growing interest during last years. Progress in development of MW and laser facilities is opening new areas for their application and aerodynamics can also try to derive benefit from such scientific and technological progress. Laboratory experiments with beamed energy deposition in supersonic flow for flowfield modification have begun about 15 years ago. Since that time there were obtained results – experimental and theoretical – which revealed areas of effective energy deposition in front of moving supersonic body for drag reduction and local flow control [1-4]. Both MW and laser generation sources were tested in experiments [57]. Direct experimental evidence of high efficiency of MW plasmoids (filaments) for drag reduction was revealed in our investigations of MW discharge interaction with supersonic blunt bodies [5, 8-11]. Elongated in the direction of flow MW plasmoid about 17mm length and 4mm in diameter was created on the symmetry axis in front of detached shock wave of blunt body. Experimentally obtained drag reduction efficiency for AD body 30mm in diameter under M=2.1 flow conditions turned out about 30. This means that having deposited of about 30mJ in a specific MW plasmoid saved was 1J in body’s drag during interaction of plasmoid with AD body. Taking into account that only 10% of the absorbed energy are converting directly to gas heating, i.e. 3mJ, estimation of physical efficiency rises up to about 300. But the effect is very sensitive to the filament position and orientation. If MW plasmoid (filament) is disposed exactly on the axis of symmetry of a body then drag reduction is maximal. Shifting the filament off this position or its formation under some angle to the axis leads to sufficient diminishing of the effect magnitude down to even changing of its sign [8,9]. Exactly the demand of precise positioning of energy deposition region underlies the fact that application of MW for distant creation of discharge domains meets some difficulties. The most important among them – multi-peak structure of a focused MW radiation, which leads in the case of breakdown MW field to origination of number of plasmoids. As a rule, only one plasmoid is working effectively, others are either useless or even harmful. Multi-plasmoid structure of MW discharge leads also to a problem in managing of useful MW plasmoid position, as owing to inter-plasmoid interaction the discharge structure demonstrates some jitter (variation) in consequent realizations. So, there appears a need in some kind of stimulation of MW plasmoid directly at a defined point. 1 American Institute of Aeronautics and Astronautics 44th AIAA Aerospace Sciences Meeting and Exhibit 9 12 January 2006, Reno, Nevada AIAA 2006-792 Copyright © 2006 by Yu. Kolesnichenko. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.