Generation of trains of subnanosecond bunches by automodulation of IREB passing through a series of coaxial cavities

Automodulation of an intense relativistic electron beam was reexamined experimentally to obtain trains of subnanosecond electron bunches. Transmission line theory suggested that the round trip time for the electromagnetic wave in the cavity must be comparable with the rise time of the beam current to obtain high voltage at the gap. Single cavity experiments were carried out using the beam with current rise time of 6 ns. Cavities whose round trip time was less than 6 ns were utilized. The longer cavity resulted in the higher gap voltage and the higher current modulation. For high-frequency current modulation for a beam with slow rise time, we propose a series of cavities with decreasing lengths to improve the current rise time.