An experimental investigation of velocity-modulated electron beams

One of the earliest and simplest forms of velocity-modulation device is the 2-cavity klystron described by the Varian brothers in 1939. The theory of such a device has been studied by many authors, but the detailed comparison of the theory with practical results is generally difficult, owing to the high operating frequency. Moreover, the distribution of the r.f. current along the electron beam cannot be investigated, since a normal klystron has its output cavity at a fixed position, so that the drift length cannot be altered. Electrical measurements can more easily be made at a lower frequency. To study in detail the operation of a klystron, a valve was constructed in which the drift length could be varied. By using the comparatively low frequency of 300 M/s, electrical measuring difficulties are reduced and it is sufficient to use a relatively low beam-current density which facilitates the formation of the electron beam. In the first part of the paper, kinematic and space-charge wave theories of klystron operation are reviewed and the results of numerical calculations are discussed. The second part describes the experimental equipment used and compares the practical results with those obtained theoretically.