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 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. In this case, the usual Keil-Schnell coasting-beam criterion is used, but with local values of bunch current and momentum spread, as suggested by Boussard. This yields Vertical bar Z/n Vertical bar approx. = 13 ..cap omega.. for the ISR, and values about five to ten times larger for the PS. The restrictions mentioned above, however, are not fulfilled near threshold, or for frequencies as low as 100 MHz. A direct approach, without coasting-beam approximations, is presented. 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. If Z(Omega) is known, the frequency shifts can be computed, and surprisingly, one finds thresholds near the coasting beam values, but with fewer assumptions. The lowest thresholds occur for wakefields that decay in about a bunch length. In general, lowering Q-values does not help, since the threshold depends on the area under the resonance curve. For very rapidly decaying wakes, the bunch is stable, in agreement with a conjecture of Hereward. Only one wavelength along the bunch is sufficient for instability.
- Research Organization:
- CERN, Geneva
- OSTI ID:
- 7285598
- Journal Information:
- IEEE Trans. Nucl. Sci.; (United States), Vol. NS-24:3; Conference: Particle accelerator conference, Chicago, IL, USA, 16 Mar 1977
- Country of Publication:
- United States
- Language:
- English
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