Beam breakup (BBU) instability experiments on the experimental test accelerator (ETA) and predictions for the advanced test accelerator (ATA)

In linear accelerators the maximum achievable beam current is often limited by the Beam Breakup (BBU) instability. The amplitude of this instability depends sensitively on two cavity parameters: the quality factor (Q) which characterizes the damping rate of an oscillator and the transverse shunt impedance (Z/Q) which is a measure of how well the beam couples to the cavity fields of the mode and, in turn, how the fields act back on the beam. It was found, in the experiments reported in this paper, that placing ferrite on the drive blades and back walls of the cavities greatly reduced Q, and that Q was reduced further by placing a reflector in the corner of the accelerator cavities. A summary of the quality factors and transverse shunt impedances for the ETA and ATA cavities determined from bench tests is given. The experimental data, detailed in this paper along with the procedures used, was compared with the results of a computer code which employs the single mode theory of Neil, Hall and Cooper to calculate the growth of the BBU instability. In addition to the parameters Q and Z/Q, each accelerating cavity is characterized, in this code, by the angular frequency of the dominant mode. Results of the comparison are given