Extended interaction output circuits

Extended interaction output circuits for klystrons offer many advantages over conventional circuits. Most important are increased gain-bandwidth product, greater power handling ability and increased efficiency. Possible sacrifices are increased length and finite stability margin. In the past year, theoretical and experimental studies have been undertaken at Varian Associates to evaluate incorporation of extended interaction into practical devices. Experimental tubes used are production-type X-band 36 kv, 70 kw pulse amplifier klystron bodies. These have been fitted with two-gap coupled cavity output resonators of different phase velocities (lengths). Extensive comparisons between the above tubes and similar production tubes have been made showing clear areas of superiority of the extended interaction circuits. By varying phase velocity and loading, increases up to 40% in output power or increases up to 70% in bandwidth have been demonstrated. Self-oscillation could not be induced near usable operating parameters. Calculation and measurement of small signal negative beam loading agreed well and gave assurance of safety from self-oscillation. Small signal gain and beam loading were found to agree with the space-charge theory of Wessel-Berg. As was found by Priest and Leidigh, behavior of the circuit at saturation levels was considerably different from small signal or single cavity klystron theory. Attempts to explain this discrepancy have resulted in a computer ballistic analysis with low level space-charge corrections. Many of the details of saturation are explained from the resulting knowledge of electron velocity spread and change in dc velocity with application of ac modulation.