4.1.9 parameters versus power level The 2.45 GHz tube including the passive thermal radiator was scaled to power levels from 2 to 20 kW. The optimum power appears to be in the 5 to 10 kW region. The summary data are shown in Figure 4-13. Higher power levels could be obtained only with a great increase in specific weight or by the use of liquid cooling. The high temperature of operation, 300°C or higher, rules out the use of normal coolant such as water or oil. Liquid mercury could be considered, but it is heavy and undesirable for a long life tube. Heat pipes are another alternative for cooling. They have an extraordinarily high thermal conductivity, but they are limited in maximum power density to only a few tens of watts per square centimeter, and would use liquid metals which would reduce the reliability of the system by risk of leakage and widespread arcing. Discussion of the two basic contributors to specific weight and cost is important to understand why the near minima occur as shown. a. The core elements (cathode, anode, vanes, and magnets) weight and cost are functions of size which in turn is a function of wavelength. For a fixed wavelength or frequency = 2. 45 GHz the weight and cost of the core elements are essentially established so that specific weight and cost would increase as power level decreases. b. The thermal control system weight and cost, for a fixed core size, increases non-linearly with increasing power to be dissipated. The sum of these two contributors results in minimum specific weight and cost occurring as shown in Figure 4-13. Figure 4-13. Amplitron Weight and Cost versus Power
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