Beam current limitations in the Jefferson Lab FEL: simulations and analysis of proposed beam breakup experiments

A series of beam experiments is being planned in the Jefferson Lab FEL driver accelerator in order to study multi-pass beam breakup instabilities in the machine and to test the predictions of the numerical code TDBBU. The tests are extensions of previously performed or proposed experiments, and will be considerably more sensitive with the present configuration. The experiments will include: a) observing the onset of instabilities by lowering the threshold current through manipulation of the beam energy, phase advance, and beam transfer matrices; b) measurements of beam transfer functions in the recirculating mode; and c) measurements of the single pass beam transfer functions to obtain direct measurements of the transverse shunt impedance of cavity modes with strong coupling to the beam. Simulations of the different experiments and studies of the sensitivities to the accelerator and beam parameters are presented.