( is proportional to through the skin depth of copper. ) A temperature change from 20 to 300°C increases the heat losses by a factor of 1. 5, reducing the net efficiency of the PPM tube (with depressed collector) from 76 to 72 percent and that of the EM tube from 72 to 71 percent. However, the weight of the body radiator is reduced by more than 93 percent. The PPM tube has a lower circuit efficiency because the lower bunching level produces less rf output voltage for a given output-gap voltage, while the latter determines the heat lost. 4. 2. 3 KLYSTRON EFFICIENCY WITH SOLENOIDAL FOCUSING For high efficiency, full advantage can be gained from narrowband operation at the single specified frequency. For example, a 50 kW water-cooled klystron has demonstrated up to 74 percent conversion efficiency (the Varian VKS-7773)(4,11), at 2.45 GHz, using confined-flow focusing and two second-harmonic bunching cavities. While the Varian tube uses a 0. 5-microperveance beam, a reduction of per- veance should further increase electronic efficiency. (12) Accounting also for the fall of circuit efficiency, as in Appendix E (Equation (E-12) and Table E-l)), a conversion efficiency of 80 percent should be attainable with a microperveance of 0. 2, and a cavity temperature of 300°C. A permanent magnet of the size needed for a 2. 45 GHz tube would weigh at least eight times as much as a solenoid, even if using samarium cobalt instead of the conventional alnico which can halve the weight. The additional power consumed by a solenoid is relatively small with more than 16 kW output. Figure 4-25 shows the resulting efficiency with a nominal solenoid power of 1 kW. (Appendix E details the calculations used to derive the output power. ) The power summary (Figure 4-22) compares the PPM- and EM-focused tubes; the net efficiency of the PPM tube is 15 percent less before collector depression. Note that average efficiencies may be a few percent lower because of the time-varying supply voltage (see Paragraph 4. 2. 5); the design parameters correspond only to maximum power. To further increase efficiency, voltage bias across or just before the output gap has been proposed:(13) however, no recent studies have been published. Voltage bias is not recommended for the following reasons:
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