Efficient Electrostatic-Accelerator Free Electron Masers for Atmospheric Power Beaming

From:
Y.Pinhasi, I.M.Yakover, A.L.Eichenbaum, and A.Gover, "Efficient Electrostatic - Accelerator Free Electron Masers for Atmospheric Power Beaming", IEEE Trans. on Plasma Science,vol.24, no.3, pp.1050-1057, 1996


The concept and schemes of microwave power transmission through the atmosphere were proposed and developed by Brown. In 1964 he succesfully flew a small helicopter using microwave power beamed from a magnetron operating at 2.45 GHz. Modification of these schemes based on high-power millimeter wave sources are under intensive consideration. In this paper we shall address the problem of providing electric power to loitering high-altitude platforms requiring average power levels of 50 to 200 kW.

The radiation is transmitted through the atmosphere to a flying platform, where it is received and converted into the electric power by a rectifying receiving antenna (rectenna).

Realization of this concept for microwave power beaming with practical antennas to an unmanned aerial vehicle (UAV) flying at an altitude of 20 km (this sltitude is most fitting for commercial applications, such as communication) becomes possible by use of millimeter waves for power beaming. Utilizing suitable frequencies at millimeter waves for atmospheric beaming, the sizes of the rectenna and of the transmitting antenna become practical. For this application a new type of 35 GHz rectenna based on thin-film technology was designed, providing a rather high rectenna conversion efficiency (of the order of 50%).

In the selection of suitable microwave sources, one should consider the availability of high-power mm wave souces operating continuously at millimeter wavelengths. High conversion efficiency, long operation life-time, and very high reliability are required from a suitable mm wave source for power beaming applications. Among the new very high power mm wave sources are gyrotrons and free-electron masers.

Recently developed gyrotrons can provide 0.5 MW power for 2 s at 110 GHz. Development programs call for 1 MW CW gyrotrons at 280 GHz. Millimeter wave gyrotrons operating at 200 kW CW power are commercially available, and at the present time would probably be a preferable choice for moderate power transmission applications. There are, however, a number of limitations that should be pointed out as related to the use of gyrotrons and among these are the following.

  1. The need to use super-conducting magnets in order to provide the high-magnetic fields that are required at mm wavelengths.
  2. The electron beam is generated at very high currents, moderate voltage, and at considerable current densities at the cathode of a magnetron onjection gun. These lead to possible lifetime degradation.
  3. It is hard to realize an efficient depressed collector in a gyrotron, where a large transverse energy spread is generated in the spent electron beam.
  4. After many years of development, gyrotrons are approaching the upper limit of their power and frequency capabilities; significant duther improvement in their performances may be difficult.
It is thought that the newly evolving technology of EA-FEM may alleviate some of these limitations. A forerunner in the development of this high-power technology is a major European program at the FOM Institute for Plasma Phisics, Netherlands, where a 1 MW CW FEM is being developed for operation in the 150 to 300 GHz band. This FEM is designed for use as a source for plasma heating in tokamak fusion reactors. It utilizes a Pierce gun for generating a low emittance e-beam, and employes an efficiency enhancement. We propose the elctrostatic-accelerator free electron mazer as an appropriate future source of high-power mm-waves radiation for power beaming applications. The features of EA-FEM's characterized as very high average power devices with high-energy conversion efficiency were recognized as fitting for various high-power applications, including plasma heating and power beaming. We wish to point out that such devices can operate also with quite compact accelerators (0.5-1 MeV voltage) at power levels sufficient for energy transmission applications.