Bunch lengthening and microwave instability; 1

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 PS and the ISR. 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. A more direct approach is presented in which the basic idea is that the usual bunched-beam modes, dipole, quadrupole, sextupole, etc., become unstable at intensities sufficiently high for their coherent frequencies to cross. (11 refs).