The study of electromagnetic wave interactions with plasmas is motivated by its relevance to ionospheric modification research (1), laser-fusion plasma heating (2), and investigations of diverse astrophysical phenomena (3). In particular, this study finds application in the environmental impact assessment of ionosphere-microwave beam interactions associated with the microwave power transmission system of the solar power satellite concept (4). The microwave power beam responsible for transmitting energy from space to the ground-based rectenna is designed to operate at 2.45 GHz. This frequency represents a compromise between the ionosphere-microwave interactions, which are more easily excited at lower frequencies, and the increased scattering losses from tropospheric hydrometers such as rain and hail, which occur at higher frequencies. Although no major changes in this operating frequency are anticipated, small changes may be made to reduce interference effects on other electromagnetic systems. To avoid nonlinear ionospheric interactions, the maximum power flux density in the SPS microwave beam was originally predicted to be 23 mW/cm2. Therefore, to deliver 5 GW of power, the downcoming beam would have to be at least 5 km in diameter. The downcoming microwave power beam is directed onto the ground- based rectenna by an upgoing pilot beam. This retrodirective system is designed to operate at a frequency close to that of the power beam and is centered in the rectenna. Thus, the upgoing pilot beam propagates through the same ionospheric plasma as the downcoming power beam. Any disturbances generated in the ionosphere could lead to degradation of the performance of the pilot beam. The 23 mW/cm2 limit for SPS power density to avoid nonlinear interaction of the microwave beam and the ionosphere came from a predicted (5) threshold for thermal runaway in the lower ionosphere. The SPS heating of the electrons would cause their temperatures to run away — increase by about an order of magnitude where a new loss mode would be excited that provides an efficient quenching mechanism to limit the temperature rise. Observational evidence and the use of additional loss mechanisms have changed the picture considerably. Experimental results have led to a reduction of the expected saturation temperature for the microwave heating of the ionosphere so that 23 mW/cm2 is no longer a valid theoretical limit for SPS power density and the question becomes, at what power densities is the ionosphere modified in a way that produces unacceptable communication effects, system effects and/or environmental impacts? The limit can at least be doubled pending further tests. 2. MICROWAVE BEAM EFFECTS ON THE IONOSPHERE The microwave beam affects the ionosphere in three ways: 11] it resistively heats the free electrons in the ionosphere, [2] it interacts with the electrons collectively, and [3] it generates spectacular instabilities when the modifying wave resonates with the ionospheric plasma. The latter is not directly possible for the SPS since its frequency, 2.45 GHz, is orders of magnitude higher than the maximum ionospheric plasma frequencies (~ 15 MHz). The first two cases are discussed in this section with some added notes about other instabilities, electric breakdown of the atmosphere and atypical ionospheric conditions.
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