Experimental homopolar generators have been demonstrated by Westinghouse Electric in the USA (15 MJ) and by the National University at Canberra, Australia (500 MJ at 800 V). The other family of likely prospects are variations on synchronous alternators, particularly the compensated pulsed alternator or ‘compulsator', which is best suited for pulsed power loads such as beam or electromagnetic-gun weapons would demand. Conventional alternators have been used successfully to store billions of joules for pulsed power devices at the National Magnet Laboratory at MIT and the Princeton University Plasma Physics Laboratry in the USA, CERN in Switzerland, and the UKAEA tokamak laboratory in Culham. A 2500 MW compulsator at the University of Texas was designed to produce pulse widths of 300-2000 zzs at 6-15 kV. Although compulsator power density is limited to perhaps 3-4 GW per square meter of rotor surface, it performs all three primary power-supply functions: energy storage, generation of electricity, and pulse power conditioning. The homopolar generator, which has far greater energy storage potential, performs only the first two functions. An interesting future prospect for high-efficiency energy storage may be offered by the current rapid developments in high-temperature superconductors, which might be used to fabricate storage rings to maintain very high currents indefinitely. This prospect, however, cannot be considered at this time for planning purposes because it is still in the very early research stage. The main problem with short-period, high-power (‘burst' mode) power supply is the delivery mechanism. Both microwave (or submillimeter-wave) and tether transmission systems would need to be designed for the full power rating (i.e. 1 GWe), even though it would only be a short-term demand. This might add considerably to system cost and mass. Laser transmission would be able to handle the burst-mode power, but the investment needed to upgrade the laser source and optics for the 1 GWe power rating would be prohibitive. Moreover, the customer would have to be able to receive, transform, and utilize that level of power, which is beyond current state-of-the-art (unless he simply uses a ‘fighting mirror' to redirect the power station's beam as a weapon). Another alternative would be for the customer to install his weapon source onboard the central-station satellite, which would both defeat the rationale for the central-station concept and bring serious problems of classification and military control into an otherwise commercial enterprise. The burst-mode power transmission issue, therefore, must be addressed in order that the central-station concept be able to satisfy a major potential customer demand (see subsequent discussion on microwave power transmission). Power Transmission in Space The four mechanisms for power transmission in space are tethers, microwave beams, submillimeter-wave beams, and lasers. Tethers are limited to co-orbital flight at relatively close spacing (e.g. less than 100 km), and lasers involve relatively inefficient conversion from and to electricity (they are most useful when the customer demand is for laser, rather than electric, power). The most practical transmission mechanisms appear to be microwaves (about 10 cm wavelength), for which there exists a sound technology base and a large body of applicable experience, and submillimeter waves (wavelengths less than 0.1 cm), which allow much smaller transmitting and receiving antennas and offer much better diffraction-limited beam coherence than microwave transmission. There is little doubt that both microwave and submillimeter-wave transmission technologies will be sufficiently mature for use in space-based centralstation powerplants by the mid-to-late 1990s.
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