Dynamics of a magnetized relativistic electron beam in composite drift tubes with virtual cathodes

A computer simulation of the dynamics of a magnetized electron beam in composite drift tubes with multiple virtual cathodes is presented. For a single-cavity tube, the value of the limiting current is found to equal that given by an exact formula. For two-and three-cavity tubes, it is revealed that the values of the limiting currents are smaller than those reported previously and that the current diagram may have a different structure. These phenomena are attributed to Pierce instability. The efficiency of microwave generation as a function of injection current is examined. It is established that the efficiency is minimum if the current equals any of the limiting values refined.