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Today, radio waves are mainly used for transmitting intelligence and information. However, the threat of the lack of energy resources, especially for electrical energy, is increasing as a result of the population explosion and rapid industrialization over the globe. Therefore, considering that the energy problem on our mother planet Earth, and the crisis of the Earth's environment have become urgent issues for mankind, we need to re-examine the use of radio waves for transmission of clean electrical energy from one place to another, especially from space to the ground, without wires.
The idea of radio power transmission was first conceived by Tesla about a century ago. However, the first practical use of radio waves was for transmitting intelligence and information, and not for transmitting electrical power per se. At the close of World War II, engineers and scientists re-examined the original Tesla idea of transmitting electric power to a distant place via radio, as high-power microwave technology became available. These efforts in 1960's resulted in the idea of the Solar Power Satellite (SPS) which was proposed by P. Glaser in 1968. The NASA/DOE concept of the SPS was extensively developed in the late 1970's.
The SPS is a gigant solar power station (5km*20km, 50,000 tons) with solar cells (Si or GaAlAs) in geostationary orbit (36,000km above). It generates 5 to 10 GW power and a microwave is used for an energy transmission (Microwave Power Transmission ; MPT). A frequancy is 2.45GHz. Among many technological key issues which must be overcome before the SPS realization, microwave power transmission (MPT) is one of the most important key research issues. The problem contains not only the technological development of microwave power transmission with high efficiency and high safety, but also scientific analysis of microwave impact onto the space plasma environment.
Toward the development of the MPT technology, we carried out several MPT experiments in Japan. We had two MPT rocket experiments. One is called MINIX ( Microwave Ionosphere Nonlinear Interaction eXperiment ) rocket experiment which was carried out in 1983. It is the first microwave energy transmission experiment in the ionosphere in the world. The other is called ISY-METS ( International Space Year - Microwave Energy Transmission in Space ) rocket experiment. which was carried out in 1993 [4]. We adopted active phased array system in the ISY-METS. The objectives of the rocket experiments are twofold. One is to develop and verify an energy transmission technology via microwave beam in space. The other is to investigate the nonlinear plasma effects caused by the microwave energy beam through the space plasma as well as the counter effects onto the microwave beam. In the ISY-METS experiment an attempt of concentrating the microwave beam at one point. As a result, we can investigate the plasma effects in more detail than in the MINIX..
We carried out two MPT experiments on the ground. One is MPT experiment from the ground to a fuel-free airplane in 1992, which is called MILAX ( Microwave Lifted Airplane eXperiment ) airplane experiment. The other is groung-to-ground MPT experiment which was carried out in 1994 and in 1995 by Kyoto Univ., Kobe Univ. and Kansai Electronic Power Company. We focused a rectenna array and analyzed data concerning the rectenna array in the MPT experiment.
We carried out an open experiment of microwave power transmission with automatically target-chasing system which was carried out in October 1, 1996. It was carried out in METLAB which is an anechoic chamber in RASC of Kyoto Univ.. We developed a new microwave transmitter with retro-directive system . The microwave beam automatically chased a target by a pilot signal. We also developed a new rectenna array.
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