Mutual influence of the coherent synchrotron oscillations of the Super-ACO positron beam on the FEL

Abstract

The operation of a free electron laser (FEL) on the Super-ACO storage ring at Orsay has been conditioned by the state of the positron beam in the ring. The circulating bunches of positrons interact with the vacuum chamber and the rf cavity, and can, under certain conditions, induce coherent synchrotron oscillations of the beam. With two bunches in the ring, the dipolar modes of oscillations are counteracted by a longitudinal feedback system, based on a method developed by Pedersen at CERN. Synchrotron oscillations (dipolar, quadrupolar and hexapolar modes) depend on the ring current, and are reproducible from one experiment to another. The threshold for the laser oscillation is strongly dependent on the coherent synchrotron oscillations. The gain reduction due to these longitudinal instabilities in the beam is analysed theoretically for the undulator and optical klystron case. For an undulator, the gain is lowered because of a modulation in the electronic density. For an optical klystron, there is an additional modification of the energy, and the gain can even become negative. Experimentally, the laser cannot start in presence of strong and stable hexapolar or quadrupolar modes of oscillations. Besides, when the FEL has started, it destroys the quadrupolar modes of oscillations. In addition, the stability and the temporal structure seem to depend strongly on the bunch oscillations.