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 Ω 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.