2.1.2.2 Thermal Evaluation Preliminary thermal analysis of the MPTS centered about studies that would indicate the sensitivity of temperature level and thermal gradient on antenna size, power level, microwave converter selection, and distribution. 2.1.2.2.1 Temperature Level - Structural temperature levels, material and antenna size combine to place limitations on the power that can be transmitted by the antenna. Figure 2-5 shows the limit power level for antenna diameters between 0.7 and 1.4 km. Aluminum, epoxy and polyimide are shown as representative materials. Aluminum and graphite/epoxy lose their strength characteristics at approximately 450°K. This limits system power levels for 1 km diameter antenna to 17 gw with a 90% efficient microwave converter and to 4 gw with a 70% efficient converter. Limit power levels can be significantly increased with the use of polyimide composite materials. 2.1.2.2. 2 Thermal Gradient - Figure 2-6 presents the thermal gradients between primary structural caps for distances of 40 and 90 meters. The trend indicates that to limit tip deflections to less than 1 arc-min, the average distance between caps should be somewhat less than 40 meters. This would keep temperature gradients below 4°C. The worst case thermal gradients occur when the antenna microwave surface shades the structure from the sun. 2.1.3 Design Options and Groundrules for Task 2 Concept Definition Task 1 resulted in recommendations that a frequency of 2.45 GHz be selected and four configurations of slotted waveguide transmitting arrays be studied in Task 2. These configurations involve combinations of amplitrons with aluminum structure and array, amplitrons with graphite composite structure and array, and a klystron with the same two materials. Task 1 also showed that a 5 gw ground output power level would be a reasonable choice for all Task 2 study vehicles. An antenna diameter of 1 km was selected based on the relative insensitivity of this parameter to overall system cost and performance. Figure 2-7 summarizes the guidelines for Task 2 study.
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