Bunch Lengthening and Microwave Instability

A single-bunch instability that leads to blow-up of bunch area and microwave signals (100 MHz to 3 GHz) has been observed in the pS1) and the ISR2). A similar instability may cause bunch lengthening in electron storage rings. Attempts to explain this as a high-frequency coasting-beam instability require e-folding rates faster than a synchrotron period, and wavelengths shorter than a bunch length. In this case, the usual Keil-Schnell coasting-beam criterion3) is used, but with local values of bunch current and momentum spread, as suggested by Boussard1). This yields |Z/n| ?? 13 ?? for the ISR, and values about five to ten times larger for the PS. The restricitons mentioned above, however, are not fulfilled near threshold, or for frequencies as low as 100 MHz.