L. Merminga, G. A. Krafft, C.W. Leemann, J. Sundelin, B.C. Yunn (TJNAF), J.J. Bisognano (SRC)
Recirculating linacs with superconducting cavities can potentially exhibit severe performance limitations due to multipass Beam Breakup (BBU), an instability which results from the interaction of the beam with Higher Order Mode (HOM) fields in the rf cavities. A two-dimensional simulation code, TDBBU , was developed at Jefferson Lab for the study of the instability and can be used to predict the threshold current for a given machine configuration. Previous experiments  designed to test the validity of TDBBU were not sensitive enough to unambiguously benchmark the code. Recently, a set of experimental studies was carried out at Jefferson Lab's IRFEL  which allowed direct comparisons with the code. Beam transfer function measurements between the beam excitation at the HOM frequency and the rf cavity field response, in the recirculating mode, allowed us to infer the threshold current. Single pass beam transfer function measurements allowed us to infer the shunt impedance of the HOMs. The rest of the relevant parameters were also determined experimentally. This paper describes the experimental techniques used in the measurements, presents comparisons between TDBBU and experimental data, and shows the functional dependence of the threshold current on accelerator and beam parameters. The threshold current for the present IRFEL accelerator is also derived from the data.
 G. A. Krafft and J. J. Bisognano, 'Two Dimensional Simulations of Multipass Beam Breakup,' Proceedings of Particle Accelerator Conference 1987, p.1356
 N. S. R. Sereno, 'Experimental Studies of Multipass Beam Breakup and Energy Recovery using the CEBAF Injector Linac,' University of Illinois Ph.D. Thesis 1994
 D. R. Douglas, 'The Jefferson Lab 1 kW IR FEL,' these proceedings
* Support provided by the US Department of Energy Contract No. DE-AC05-84ER40150
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