Proposal for an Extra RF System Dedicated to Bunch Length Control and Synchrotron Movement Stabilization

A gain limitation for the F.E.L. installed in ACO appears as the charge density decreases in connection with the "anomalous lengthening" and the synchrotron movement instabilities which are affecting high current operation of the storage ring. An extra RF system is designed to cure both these effects. The basic component of the system is an extra cavity driven through a power RF amplifier by a signal whose spectrum is composed by the even harmonics of the revolution frequency and the nfo - fs side bands. The proper input is achieved from the signal induced by the beam on a radial pick-up electrode by means of a pulsed window amplifier. The amplifier equalizes the two components of the picked-up signal, one due to energy oscillation and the other to phase oscillation and thus realizes the suitable spectrum for the input. A feedback loop ensures the normalization of that signal. Synchrotron movement stabilization and bunch length control are expected to work for stored currents up to 100 mA.