approximately 50 percent of the waveguide structure is part of the "a" waveguide dimension, 50 percent of the weight can possibly be saved by this technique. The operational success and performance of the RCR is not known at this time (it is currently in test); however, Rockwell is investigating these and other techniques of weight savings and efficiency improvements for the SPS microwave system. 2.3.1 DC-RF Conversion Devices One of the key tradeoffs in the dc-RF conversion devices is the use of klystrons or amplitrons or a combination of both. Some of the key issues associated with these devices are: 1. Efficiency 2. Noise power 3. Failure modes and rates 4. Gain 5. Phase stability 6. VSWR effects 7. Stability 8. Startup/shutdown characteristics 9. Cooling and temperature interfaces 10. Cost 11. Maintenance Since a large amount of study effort was dedicated to amplitron concepts in previous Raytheon studies, Rockwell concentrated more of the study efforts on higher gain devices such as the klystron. Rockwell's studies indicated that the basic 6.25-kw amplitron when fed in series poses a difficult problem in maintaining the phase control of the 1.5-million amplitrons. Using klystrons with power levels of 50 kw, the phase control problem is simplified; however, the heat dissipation problem must be resolved. An example of a 50-kw PPM/PM klystron as designed by Varian Associates is shown in Figure 2.3-3. Estimates of mechanical dimensions, heat flow, and temperatures are shown on the figure. The klystron example uses sumarium cobalt magnets of present technology for electronic beam focusing. The PPM/PM klystron is just one of the different focusing schemes that can be used in a high-efficiency klystron. Single-pole and multiple-pole EM (electro-magnet) techniques are variations. The EM technique offers the highest efficiency potential; however, it is the heaviest of the three techniques. Table 2.3-1 shows the relative weights for the various focusing schemes.