rectenna size are shown in Figure IV.A.2-4. The rectenna size is that required for a 90% flux intercept efficiency, or collection efficiency. The ground facility, which is the area fenced off for the rectenna system, was arbitrarily chosen to intercept all power density levels out to .05 mw/ cm2. This density level example is 200 times more stringent than the 10 mw/ cm2 radiation standard set by the United States; however, it is 5 times greater than the .01 mw/cm2 USSR standard. The exact power density standard to use is not known at this time and should be one objective of a microwave radiation study. However, it is felt that the SPS density limit will be somewhere between the present USA and USSR standards. Since the radiation will be continuous, the SPS standard may be closer to the USSR limit. The facility required for a 1 km transmit array covers 80,000 acres - a large land area. In summary, the 1 km transmit array, together with a 10 dB Gaussian taper illumination for a 5GW system, was chosen as the model configuration for three reasons: (1) the system operates just below the 23 mw/cm2 threshold level expected for nonlinear ionosphere interactions, (2) the power density at the transmit array is at the 21 kw/m2 limit due to thermal constraints for the waveguides and klystrons, and (3) a size/cost tradeoff for the rectenna and transmit array has a broad minima at a 1.0 km array diameter. IV.A.2(b) IONOSPHERIC EFFECTS The microwave beam/ionospheric interactions can be divided into two categories: the ionospheric effects on the beam and the beam effects on the ionosphere. Previous work by Raytheon (ref. 1) indicates the ionospheric effects on the beam, such as phase dispersions, beam displacement, and power absorption, will be minimal. However, the beam perturbations to the ionosphere must be considered when sizing the SPS power. As the microwave power density increases above a threshold level, nonlinear interactions between charged particles in the ionoshpere and the power beam begin to occur. This threshold level has been postulated to be approximately 23 mw/cm2 for a 2.45 GHz frequency, which is close to the peak density expected for a 5GW system. When the microwave power approaches the threshold level, the ionosphere will be perturbed as shown in Figure IV.A.2-5. In the "F" layer there will be heating and expansion with a corresponding reduction in the electron/ion density. A "hole" will be punctured in the "F" layer due to the reduction in electron/ion concentration. The neutral particles however will not be appreciably affected by the heating, and their concentration will remain the same. These particles, i.e., the electrons, ions, and neutrons are moving through the ionosphere at some velocity - estimated to be 50 meters/ second (ref. 4). Thus, as these particles sweep pass the "hole" region, they will slowly diffuse back until the normal equilibrium density level is again reached. Rough calculations indicate the "hole" will be closed within five minutes, or about 15 km, after the particles leave the heated region. In the "D" region the microwave heating slows the electron/ion recombination rate and the density may actually increase. There will be no
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