An investigation into the design of a gridless low-voltage reflex klystron

The paper describes an investigation to determine the limit of lowvoltage operation of reflex klystrons employing gridless apertures. The problems involved include the design of suitable electron guns and the determination of the maximum useful current. Initial calculations indicated that outputs of several hundred milliwatts should be obtainable from a 3 cm oscillator with a 500-volt 30 mA electron gun if the effective current was assumed to be half the total current and space-charge effects in the reflector space were neglected. Practical values fall far short of this. Some measurements of unbunched effective current and the current-density distribution across the waist have enabled a considerable part of this discrepancy to be removed. The remaining discrepancy is thought to be due mainly to the effects of bunching on the effective current, of space charge on the bunching factor, and of phase aberrations. Further work is required to obtain more information on these points. The measurements of effective current indicate that a maximum useful mean current density exists in the region of 0.5 amp/cm2, beyond which the effective current decreases. The effect of non-uniformity of the beam shape on the beam coupling factor and beam damping is considered in an Appendix. Expressions for the correction factors have been calculated for the case where the current-density distribution follows a Gaussian form, and these are presented, together with some graphs covering a range of the relevant parameters. Graphs giving the beam coupling and beam-damping factors are also included for convenience.