For an undepressed collector the simplest design is an envelope of tungsten or graphite foil as patented by Lien (22) (see also Reference 3, Appendix E). The vacuum window is convenient for initial tests, but is unnecessary when the tube is opened to space. Although the radiant emissivity is only 0.55, the radiating temperature may be 1400°C or higher, restricted primarily by sublimation of the envelope material. A molybdenum reflector backed by a heat shield then protects the body of the tube, which has a separate radiator to ’’see past” this shield. The tube drawing (as shown in Figures 4-28(a) and 4-28(b) shows the size required for a 43 kW (or 48 kW) tube; the dimensions and weight are calculated in Appendix C for tubes of output power 16 kW and 43 kW. Since existing high-power collectors are generally liquid- or air-cooled, additional development of this design may be needed. The Varian VKS-7773 klystron(4,11) appears to be a suitable test vehicle. For increased efficiency with several depressed stages the reflex collector eliminates backstreaming to the drift tunnel by reversing the electron motion before collection. A magnetic refocusing section (16) allows the beam to spread in a linearly decreasing axial magnetic field between output gap and collector, and reduces both charge density and radial velocities. In addition, the power density can be reduced enough to allow the electrodes to radiate directly; flat pyrolytic graphite electrodes backed by copper may be suitable. Of course, the recovered power leads to savings of area and weight, but at the expense of a more complex design. 4.2.5 VARIATIONS OF SUPPLY VOLTAGES While the tubes are designed here for a maximum supply voltage, in practice the voltage and available power from solar cells, for example, may fall by 5. 3 percent of the maximum in an annual cycle, and by a further 6. 6 percent over a 25-year lifetime. Three alternative control schemes proposed for the klystron are as follows: a. No regulation. Here the current is controlled by the electron gun under space-charge-limited operation. This produces the largest variation in output power but is the simplest and most stable method. b. Constant current. This requires a gridded gun, which is undesirable in a long-life tube.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==