Investigation Of HPM Air Breakdown Near Output Window Of The Antenna

High power microwave (HPM) has reached the GW level, and one of the most problems to be studied is the air breakdown in the propagation of the high power microwave pulses through the atmosphere. As the increasing of the microwave power and pulse duration, the air breakdown near the output window of the antenna is a key issue in the application and experiment of the HPM. We need to have a deep understanding of the physical process and the breakdown threshold when HPM propagates in the air. The present paper gives the investigation of the air breakdown near the output window of the antenna, including the followings.1. The electric field near the output window of the antenna has been studied, it is showed that the output window of the antenna is most likely to have air breakdown. The basic theory about the transmission of HPM in the air has been introduced, as well as the relations of the breakdown threshold versus the microwave frequency, duration, and the pressure of the air.2. The microwave air breakdown could be enhanced with metal pins in the waveguide. The electric field strengthens by two metal pins which are parallel to the polarization of the electric field in the middle of the waveguide. The method has been confirmed by the simulation using electromagnetism software, which can be employed to design the experiment. The detailed measurements have been carried out. In addition, the theoretical model has been obtained, and the enhancement factor of the electric field has been derived. The experimental results are in good agreement with the theoretical ones. The microwave air breakdown threshold is 62kV/cm for of 1 atm, and 43kV/cm for 0.6 atm when the microwave frequency is 2.86 GHz and pulse duration is about 180ns.3. In order to determine the HPM breakdown threshold in gas, one can change the aperture of the horn antenna. The method for obtaining the microwave breakdown threshold by enhancing electric field in the waveguide is conventional, however; it is necessary to consider a new method to determine the HPM breakdown threshold in gas, which is to change the aperture of the horn antenna to change the electric field near the inner medium window. This method has been verified by the simulation using the electromagnetism software, and the detailed process and experiment have been carried out. The typical results have been analyzed as well as the experimental data. The different effects of microwave transmission in air and SF₆ have been explained. Finally the breakdown threshold has been given under certain conditions, and the experimental results are in good agreement with the theoretical ones. The L-band microwave air breakdown threshold is 33kV/cm for 0.39 atm, 47kV/cm for 0.6 atm, 58kV/cm for 0.85 atm, 63kV/cm for 0.95 atm, and 72kV/cm for 1 atm when the frequency is 1.57 GHz and pulse duration is about 30ns. The breakdown threshold in SF₆ is about 2.5 times larger than air.4. A novel radiation system with a high-speed air spout has been proposed, aiming at preventing the HPM breakdown near the output window of the antenna in air. The cases of air near the output window of the antenna have been analyzed under the conditions of with and without the air spout. The pressure will increase to 2 atm within 400mm away from the output window of antenna when the antenna (with cover) is moving at a speed of 0.8 mach and the spurt pressure is 2 atm, and increase to 3.5 atm within 600mm away from the output window of the antenna when the spurt pressure is 3 atm, and the microwave air breakdown threshold will increase to 110kV/cm and 160kV/cm on the L-band. Compared with the data mentioned above, the novel radiation system is proved to be effective to prevent the air breakdown.