160, 000 40 kW tubes or 32, 000 200 kW tubes. It would be a significant technological advance to successfully apply tubes of these power levels in space. 4. 2. 1 PERIODIC PERMANENT MAGNETIC FOCUSING The focusing system for the electron beam constitutes a major portion of the weight of an electron tube; for low power space applications periodic permanent magnets give a distinct saving in comparison with either a single permanent mag- net or a solenoid. Samarium cobalt magnets are particularly attractive, (4) requiring much less magnetic material for a given focusing field. PPM focusing has been used in satellite traveling-wave tubes of up to 125 kW peak pulsed and 10 kW average output power at S-band, (5) as well as in an X-band broadband klystron. This study evaluates the klystrons1 capabilities in more detail. For calculation of the efficiency and the cooling requirements a nominal beam power of 10 kW is chosen. The magnetic period must be as small as the magnetic material can supply in order to focus the beam sufficiently well, but must be at least twice the length of one klystron cavity to allow space for pole pieces around the cavities. Accordingly a magnet period of one-half the free space wavelength corresponds to quarter-wavelength cavities. According to preliminary computations on the spreading of ideal electron beam bunches, a beam perveance as low as is necessary to reduce the radial space-charge forces sufficiently for good transmission. The outline design is described in paragraph 4. 2.4 and the tube layout is illustrated in Figure 4-28(a). Under low-level modulation the focusing and stability of the beam may be described analytically, (7,8) but a large signal computer model is necessary to predict beam interception and efficiency under saturated rf drive. A program developed by Shared Applications, Inc. , uses the "deformable-disc model" (8.9) in which sixteen or more electron ’’discs” per period are followed in phase through the bunching cavities. The calculation is relativistically accurate and treats the rippling, spreading and interception of the beam under large-signal rf conditions. The plotted trajectories (Figures 4-17 to 4-19) show the results. In Figure 4-17 the three drift lengths contain respectively three, three, and two magnetic focusing periods. As the beam forms bunches it expands radially (Figure 4-18), and as a result 1. 3 percent (130 watts) of the beam power is computed to be intercepted in
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