There is now adequate evidence that low natural emittance and low coupling limits the lifetime even in high energy rings due to Touschek scattering. For new rings the main challenge is to increase the Touschek lifetime by increasing the momentum acceptance, rather than compromise the beam brightness or bunch length. Problems related to the lattice design as well as r.f. systems for obtaining the large acceptance were therefore addressed. Various methods of manipulating the bunch density were also discussed as a means of increasing lifetime in existing high brightness machines.

Large Momentum Acceptance Lattices

The general difficulties of optimizing low emittance lattices with large momentum acceptance, particularly with low periodicity racetrack configurations were mentioned. For the SLS, a techniques which involves cancelling all resonance driving terms up to 3rd order was described.

It was pointed out that non-zero chromaticity has a large effect on the dynamic aperture e.g. in SOLEIL reducing from ± 4 % (x=0) to ± 1.5 % (x=+1). At the ESRF a practical upper limit is found to the lifetime with an acceptance of about ± 2.5 %, due to the large tune spread induced by the large positive vertical chromaticity. A careful selection of working point and/or resonance compensation would therefore be required to enlarge the effective momentum acceptance in this case. It was pointed out however that high positive chromaticity is not used in all machines.

The need for large horizontal vacuum chamber apertures was also pointed out, to provide the betatron acceptance for particles that are Touschek scattered in dispersive regions. The SLS ± 32 mm chamber dimension limits the effective energy acceptance for Touschek scattered particles to between 8 % (h=0) and 4 % (h=hmax). The net result is a lifetime that limits with a voltage of 4 MV, equivalent to a ± 6 % energy acceptance. The need to position the septum strip sufficiently far out so as not to limit the dynamic aperture was also mentioned.

R.F. Systems for Large Momentum Acceptance

The r.f solution for the SLS is to adopt normal conducting ELETTRA-type cavities utilising temperature tuning of the higher order modes since this is a well-proven technique, compatible with the SLS time schedule. In the case of "unlucky" distributions of modes HOM frequency shifters can be utilised. If it proves necessary to increase the voltage in order to increase the Touschek lifetime, the plan is to install a single passive HOM-free super-conducting cavity. Detuned far from resonance such a cavity can provide an extra 2 MV. The possibility of using a higher frequency cavity is an option that may be considered.

Bunch Density Manipulations

At the ESRF an increase to 2/3 filling from the usual 1/3 improves the lifetime from 29 h to 44 h at 200 mA. A re-adjustment of the cavity temperatures is required because of higher order modes being excited. Other possibilities for increasing the lifetime that have been studied are the installation of a higher harmonic cavity and increasing the vertical coupling. In ELETTRA the lifetime can be made to vary between 7 h and 37 h at 250 mA depending on the coupling, vertical dispersion and excitation of longitudinal coupled bunch instabilities. Insertion device spectra taken under various conditions show that allowing a slight longitudinal excitation is a better way of increasing the lifetime compared to operating on the coupling resonance, a factor of two in lifetime being obtained for only a small reduction in peak radiation intensity. Another technique employed at BESSY I is to introduce noise via striplines to increase the vertical beam size by a factor two.

At the ESRF, measurements of coupling using a pair of X-ray pinhole cameras cast some doubt on the validity on the widely used single resonance formula to calculate the coupling based on the minimum tune separation. A large discrepancy is also observed in the case of the excitation of a single skew-quadrupole. The same system also shows that there are significant changes in coupling due to helical undulator gap changes.

prepared by C.J. Bocchetta and R.P. Walker