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. This instability arises from the interaction of a transversely displaced beram with the dipole modes of the accelerating cavities. The amplitude of the instability depends sensitively on two cavity parameters; Q and Z/Q. Q is the well-known quality factor which characterizes the damping rate of an oscillator. Z/Q 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. Lowering the values of both these parameters reduces BBU growth. The tickler experiment on ETA has permitted an improved determination of Z/Q and Q for the ETA cavities. The implications for ATA BBU growth have been discussed.