Suppression of longitudinal coupled-bunch instability using energy storage cavity in B-factory RF-system

The longitudinal coupled bunch instability due to the fundamental accelerating mode can be suppressed by attaching an energy storage cavity to a normal conducting accelerating cavity. Since the stored EM-energy in this system is very large, a small frequency detuning is sufficient to compensate the reactive part of the beam loading voltage induced by extreme high beam currents in B-factory storage rings. In the KEK B-factory case, it is possible to use a normal conducting single cell accelerating cavity connected to an energy storage cavity of 1.2 m diameter by 1.9 m length. The stored energy in this system is 8.3 Joul, and the loaded Q is 7.2×10⁴. The effective impedance for the coupled bunch model m=-1) is reduced to as low as 18 kΩ/ring. Since the growth time of the instability is 22 msec, which is comparable to the radiation damping time, this type of instability will not occur