Debunching of Electron Beams Constrained by Strong Magnetic Fields

To obtain a clearer insight into the design of velocity modulated tubes, a more complete understanding of the bunching process is required. A theory, to be useful in the design of high power tubes, should include the effects of space charge and large signals. Although such a theory has not yet been developed, a small signal theory, due to Feenberg, including the effects of space charge, has been obtained for some useful physical configurations. This paper deals with the experimental confirmation of this theory and the determination of its range of validity. Numerous experiments were devised to check the various aspects of the theory, and results of these are presented. Tests were made on a conventional two-cavity klystron with a cylindrical drift-tube and gridless gaps at the cavities. Transverse motion of the electrons was constrained by a large axial magnetic field. The extrapolation of the theory beyond its obvious range of validity is shown to be accurate even for large values of the bunching parameter such as to include the first peak of the output current.