Signatures of microwaves instability

The microwave instability is probably one of the most well known manifestations of collective effects in accelerators. However the theory describing this instability for bunched beams is still under development. The result of the instability is a significant increase in longitudinal emittance of the beam, leading to a serious performance limitation in many machines. In this lecture we first discuss what is the phenomenon that one calls the microwave instability and then describe measurements of instability threshold, momentum spread and spectrum of the unstable bunch.