The choice of beam and coupling parameters for broad-band klystrons

The quantity M2G0(R/Q) for the output cavity is a useful figure of merit for a broad-band power klystron since it is proportional to the product of bandwidth and efficiency. In this expression M is the effective gap-coupling factor, G0 is the d.c. beam conductance and (R/Q) is the ratio of the shunt impedance of the cavity to its Q-factor. The quantity has been evaluated for a commonly used shape of cavity at various values of beam perveance and axial magnetic field strength. It is found that, for any particular magnetic field strength, there is a perveance for which the figure of merit is a maximum. In practice, an upper limit may be set to the perveance that can be used by the debunching forces in the beam which, as the perveance is increased, reduce the maximum spacing permissible between successive cavities until the point is reached where the design is no longer realizable.