Bellows Wake Fields and Transverse Single Bunch Instabilities in the SSC
- Stanford Univ., CA (United States); SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
In this paper we study the transverse wake field due to the bellows in the SSC and its effect on the transverse mode coupling instability. For long bunches this instability has a small growth rate which depends critically on the real part of the transverse impedance at ω≤1/σr where σr is the bunchlength in seconds. The instability is caused by the coupling of mode 0, the rigid dipole mode, and mode '--1', the mode shifted by -ν8 from the betatron tune. Thus, the instability needs a large frequency shift as well as a large 'off diagonal' element to couple the modes. The frequency shift is induced by the imaginary part of the transverse impedance at ω≤1/σr while the off diagonal element is related to the real part of the impedance at ω≃1/σr. In addition, if the beam current is large enough, there can be a coating-beam-like instability with a growth rate larger than the synchotron frequency. By comparing the transverse and longitudinal instability thresholds, it is straightforward to see that the single bunch transverse instability is more restrictive than the single bunch longitudinal instability in the SSC. there have been calculations which suggest that bellows can be a large source of transverse impedance, and thus can drive transverse instabilities. Therefore, in this paper we will emphasize the bellows impedance but also include the beam position monitor impedance. There are certainly other sources of transverse impedance which will be omitted here. In the following sections we first discuss the instability thresholds and then show calculations of the bellows wake fields. To reduce the wake field effects, another design with shielded bellows is then considered. Next we discuss briefly the impedance of the beam position monitors. Finally, we show the effect of the bellows and beam position monitors on transverse mode coupling.
- Research Organization:
- SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 1447882
- Report Number(s):
- SLAC-PUB-3862
- Country of Publication:
- United States
- Language:
- English
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