thermonuclear fusion is much more challenging. Proponents of controlled thermonuclear fusion must not only create/demonstrate/manufacture (in large quantities) the fusion reactors but they must also learn how to extract the useful energy from essentially within the zone of fusion. The challenges are immense; the payoffs are also immense (73). But, very likely, the number of options for transducing the “controlled" thermonuclear power to useful heat and electricity are more limited than for the space solar power and very likely to be more expensive per unit cost ($/kg) of needed hardware. Preliminary engineering estimates indicate that approximately 35,000 tons/GW of very high technology equipment (specialty steels, superconducting coils, lithium liquid films for energy extraction, recovery systems for radioactive byproducts, etc.) must be provided (74). The massive, sealed systems must operate for many years (as must successful nuclear reactors) surrounding a very hostile plasma undergoing thermonuclear fusion. In contrast the SPS systems are open, readily accessible during operation for repair, and operate (both in space and on Earth) under relatively benign conditions. The 35,000 tons/GW of high technology mass surrounding the thermonuclear reactor does not compare favorably to the 500 tons/GW of power reception mass in a SPS microwave system. The tonnage per GW of supporting facilities for a thermonuclear power system will likely be at least the scale of a coal fired power plant and possibly much larger. Simpler low power, fusion systems (Riggetron) have been proposed that might circumvent many of the technological challenges. Periodic reworking of the matter of the containment vessel will offer the opportunity to extract valuable radio-induced elements. Reactor rebuilding will introduce a base-level cost proportional to energy output. There are always major engineering justifications for high power, compact energy systems (Bussard, pers. comm). Possible hazards associated with long-term human exposure to the low power fringe regions of microwave beams must be examined extensively. The power can be beamed to locations remote from people and most organisms. For example, offshore platforms could likely service many metropolitan areas. Non-diffraction limited beams could sharply reduce side-lobe power densities. It can not be overemphasized that the microwave beams and their direct effects can be rapidly switched off. There are no long-term direct effects from widely dispersed products such as CO2 or potential hazards from dispersed or contained waste radioactives or ash. Extraction of flow energy from the Gulf Stream by means of underwater parachutes appears to be the only competitor to SPS in terms of efficient use of capital mass. However, on a planetary (industrial) scale very little power is collectible (21). Three NASA sponsored studies have investigated various aspects of construction of the SPS reference system from lunar materials. It was found that even without redesign of the reference system, over 90% of the system mass in space could be made from lunar materials. Fabrication of components in space seemed to offer advantages over Earth-based fabrication and shipment to space. Obvious factors such as avoiding packing and unpacking components were also complemented by production possibilities suited to construction of large area, fragile components in the vacuum and zero-gravity of space. Many aspects of terrestrial versus space fabrication have been discussed in the literature (66-70). Space systems constructed of materials from off Earth would be intrinsically conservative of terrestrial resources. They would provide a net gain of useful energy. In fact they could have no more than 10% loss of energy to the biosphere prior to end use. The other systems in Table 6 (except 3) would be only a few percent to 60% efficient in production of electric power. There would be no large, continuing of
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