to the tube axis), so that a "stepped-height” transition is necessary from the iris in the output cavity. ' The following design note numbers refer to column 4 of Figure 4-32: a. The cavities represent only a small fraction of the weight (see paragraph 4. 2. 9), so that the tube can use as many as are needed for high efficiency; an existing Varian design (VKS-7773) (4,11) used eight, of which two are tuned to the second harmonic of the drive frequency. The drift lengths can be optimized using existing designs and further large-signal calculations. b. Relativistic effects reduce the focusing field required by about 4 percent in the high-power tube (see Appendix H). The field used must be large enough to give 98 percent beam transmission or better at saturation in the solenoid tube; in the PPM tube the peak field is restricted by beam stability. The cathode field is chosen to give balanced flow. (See also Reference 8, Appendices G and H.) c. Cathode current density is highest at the edge, but is restricted to or less in a tube of long life. The gun convergence of 45 to 1 is, how- ever, well within current technology (2) . The gun outline drawn follows Branch and Mihran, allowing a length of 30 beam radii for convergence and a gun diameter three times the cathode diameter. d. For maximum gap coupling the beam should be as large as transmission allows. e. The normalized tube radius, , equals frequency x tube radius/beam speed. A low value has advantages: 1. The output-gap interaction is efficient (12) 2. The buncher gaps have almost uniform fields and thus produce low radial rf velocities. f. At this stage an outline is sufficient for the cavities; further design should use the values from existing tubes at this frequency, while field computations may be useful to increase and . The short cylindrical cavity in the PPM tube enables the magnetic period also to be short. In the confined-flow tube toroidal cavities may give higher efficiency. (21) At the output a gap length of 1.3 tube radii
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