Sub-nanosecond pulse modulation of a triode electron gun at 25 MHz

Although the electron injection system of the infrared free electron laser FELIX permits a high electron bunch repetition frequency of 1 GHz, user experiments often require a lower frequency. At the same time, the pulses from the gun have to be sufficiently short to fit within the longitudinal acceptance of a 1-GHz prebuncher. The generation of 300 ps electron bunches with a 25 MHz repetition rate, produced by pulse modulation of a triode gun, is addressed in this paper. The modulating waveform was generated by superposition of a 1 GHz sinusoidal wave, a 25 MHz train of pulses with a duration of 2 ns and a DC bias voltage. The hardware for the adopted modulation method, which consists of two RF cavities and an adjustable delay line, is described and analysed. Measurements of the electron bunch length are presented.