planetary radar), and the klystrons used in the electron storage rings of Stanford's linear accelerator in the USA operate at 500 kWe. Military klystrons have produced over 1 MWe CW and over 100 MWe pulsed. The ‘reference' design of the satellite power system used klystrons designed for a modest 70 kWe. (c) Virtual-cathode oscillator (vircator). These are very simple devices, but operate at low frequencies and broad bandwidths, making them impractical for a phased-array power antenna. They have delivered 20 GWe at 1 GHz (pulsed). (d) Gyratron. Gyratrons are most effective at millimeter wavelengths (over 100 GHz) and can operate either CW or pulsed. They have attained peak (pulsed) power of 7 GWe. (e) Free Electron Laser (FEL). FELs are under intensive development by the US SDI program as ground-based beam weapons. With cyclic-induction accelerators, such devices could conceivably become small enough for space basing. They too are most effective at millimeter-wave frequencies, and have attained pulsed power levels over 1 GWe. (f) Beam Plasma Devices. The most recent development in microwave power generation is a device that uses the interaction between a relativistic electron beam and a low-density (a millionth-atmosphere) plasma. Suitable only for pulse-mode operation, they have developed power levels in the microwave frequencies (3 GHz) up to 100 MWe. (g) Field-Effect Transistors (FETs). Although gallium-arsenide FETs are very low power devices, temperature-limited to a few tens of watts each, they are very inexpensive and can be used literally by the millions to generate substantial CW microwave power. Their main disadvantage is the very large area they would require. Whereas it is too early to tell which of these microwave generators is likely to be best for the space-based central-station system, it is likely that current intensive efforts to develop military HPM for disabling satellites will be successful. System planning can proceed, however, based on conventional klystron or magnetron technology, with high confidence that a feasible and practical microwave power transmission system can be developed by the mid-to-late 1990s. Once the microwave beam has been generated, transmission of microwave power in space involves no problems other than the normal diffraction of any phased-array beam. Since the efficiency of transmission depends roughly inversely on the product of the beam wavelength and range, it is possible to increase the transmission distance without increasing antenna sizes simply by reducing the wavelength. This makes obvious the advantage of using submillimeter waves instead of the more conventional 10-12 cm microwaves, and the dramatic gains offered by laser transmission at 1-10 //m wavelengths. For example, the diffraction-limited transmission efficiency of a 12- cm (2.45 GHz) beam over a distance of 20 km using a 4-m transmitting antenna and a 10-m receiving antenna would be only a few per cent; but at 300 GHz (0.1 cm wavelength) the transmission efficiency jumps to 95%. Reconversion of microwave power at the receiver is also a mature technology, using half-wave dipoles. Such dipoles are inexpensive and efficient; typically well over 90% for the combined collection/rectification process investigated most intensively for the satellite power system study, and even higher if the load demand is for AC rather than DC power. Rectifying antennas (rectennas) with 92% efficiency able to convert 1 kW per kg of mass were reported as early as a decade ago, and lifetime of the simple solid-state devices is expected to exceed that of the spacecraft. In summary, microwave transmisson of power from a space-based central-station
RkJQdWJsaXNoZXIy MTU5NjU0Mg==