Outside the atmosphere the total solar energy for the mean distance between the sun and the earth is (10 percent in ultraviolet, 40 percent visible, and 50 percent infrared) . Assume that the absorptivity, , equals 0.9 through- out; then equals . The following sections use Equation (F-4) to estimate the size, and hence the weight, of body and collector radiators. F. 2. 1 COLLECTOR The PPM tube has a multistage reflex collector with electrodes radiating at 900°C and emissivity 0. 92. The temperature is chosen conservatively because of the insulation needed to separate the depressed stages. Radiation from the rear electrodes is absorbed and reradiated by those in front. For the weight estimate each square centimeter is assumed to weigh 1 gram. The high-power tube has a radiating tungsten envelope (reference 4-3, Addendum E) at 1400°C with emissivity 0. 55 and thickness 0. 002 cm. The weight estimate is treated as a sphere; density 3 of tungsten equals 19.2 g/cm . The view factor F is unity since the collector "sees" open space. Using Equation (E-3) and the power estimates of Appendix D and Table 2-1 gives the following: Notice that the multistage depressed collector weighs more per watt radiated. The length required to spread the beam is estimated to be: The last two estimates are based on beam-spread curves (reference 25 of Section 4) allowing for residual angular velocity due to cathode flux and increased charge density in a beam of 25 percent its initial energy.
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