Improvement of the PFN control system for the klystron pulse modulator at LEBRA

The short-wavelength free electron laser (FEL) requires a stable electron beam with long pulse duration, narrow energy spread, and high current density. The pulse forming network (PFN) for the klystron pulse modulator at LEBRA was designed to realize pulse flatness error within 0.05%. The PFN consists of 30 capacitors and adjustable inductors. A PFN adjustment was performed to suppress changes of the RF phase difference between the input and the output of the klystron. The flatness of the pulse voltage applied to the klystron has been adjusted to an error within 0.06% as a result of the phase flatness error within 0.3° achieved by the PFN adjustment. Furthermore, the time dependence of the energy spectrum in the pulse duration has also been suppressed.