An investigation of saturated RF-currents in high perveance hollow beams

A number of computer studies of the bunching process in electron beams have become known during recent years. It was the purpose of the work described by this paper to investigate experimentally saturated RF-currents for various bunching conditions in a 40 KV hollow beam of perveance and to compare the results with published computed data. The test tube, a six cavity L-band klystron, offered the advantage of a number of cavities tunable over a wide band and separated by drift regions of different length. The perveance could be adjusted by modulating the anode of a magnetron injection gun. RF-beam currents for a variety of bunching conditions were calculated from induced voltages across low impedance gaps. Despite of the high perveance range of these experiments saturated RF-beam currents in strong magnetic focusing fields were in fair agreement with published computed data. At reduced magnetic field levels a deterioration of saturated currents was observed. It is assumed that this effect is related to transverse beam instabilities. For multi-cavity operation RF-voltages at the penultimate cavity were generally higher than predicted by Webber\´s narrow gap calculations. This reflects the effect of enhanced space charge forces in finite bunching gaps.