The klystron and amplitron designs require solutions to special space problems before the tubes can be utilized. Some of these problems are: 1. High Temperature Operation — High temperature tube operation is new. There may be high voltage breakdown problems because of the high temperature of the positive electrode, as studied by others. Perhaps this is related to gas, in which event one would expect improved conditions with seasoning. 2. Eclipses — Problems may arise when the satellite goes through eclipse. Large temperature changes will occur in relatively short intervals. 3. Thermal — Range of temperatures for normal heat pipe operation. 4. Mechanical — Distortion of tube parts, parts failure rate. 5. Electrical — Detuning of resonant cavities; restoration time for normal operation. 6. Vacuum — What will the vacuum be in the space environment? 7. Maintenance — Who are the tube-systems engineers who will get the electronics working? How does one replace tubes or components of tubes while system is operational (at a relatively high temperature)? 2.3.2 DC-DC Efficiency The dc-dc efficiency of the MPTS is one of the key sizing determinants of the SPS. Figure 2.3-4 shows the microwave energy conversion efficiency chain as presented in a NASA in-house study report. Based on technical discussions with Varian about klystrons and amplitron projected efficiencies and as quoted in the NASA in-house study, the 87-percent efficiency goal was assumed. This goal of 87 percent requires significantly more advanced technology development since a tube with these efficiency, weight, and power performances do not exist. Modern day computer-aided design techniques are being employed and significant progress in efficiency improvement is being made. Using the efficiencies for atmospheric losses, collection losses, and rectenna losses, the major uncertainty in the efficiency chain is the efficiency of the transmitting array. Using klystrons as an example, Figure 2.3-5 shows the efficiency breakdown of the transmitting antenna. To obtain these numbers, the 10-meter by 10-meter panels were laid out with klystrons to obtain feed line lengths, and Rockwell's adaptive phase circuitry was used to determine the component losses. It should be recognized that for different retrodirective phasing techniques the losses will vary; however, other techniques will suffer similar losses. The net computed efficiency for the Rockwell array design is 95.6 percent. Using this antenna efficiency, the overall dc-dc efficiency of 65 percent can be realized. Under the assumptions that the major efficiency contributors as presented in other technical studies are valid, the overall 65 percent efficiency can then be established as a realistic design goal.