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From: Andy Wolski
Email: awolski@lbl.gov
Date: 17 Mar 2005
Time: 02:39:27 PM
I think that Winni Decking's suggestions - reducing the energy and retuning the arcs - are the most practical (conventional) ways to reduce the horizontal emittance in the damping rings. But for these rings, neither approach is particularly attractive. Reducing the energy will make the beam more sensitive to collective effects that could affect stability, and dropping from 5 GeV to 3 GeV is a significant reduction in energy. Retuning the arcs also might not be as easy as it sounds (because of the problem of maintaining good dynamic aperture), but it depends on the particular lattice design. Although the electron beam will be much smaller from the source than the positron beam, the electron damping ring will still need a pretty good dynamic aperture, and if you also reduce the energy, the incoming beam will be larger than the beam at higher energy (and with larger energy spread) because you have lost some adiabatic damping. Also, reducing the emittance by retuning the arcs is also likely to lead to a reduced momentum compaction, which will again have an adverse impact on beam stability. Also, a little more retuning in the arcs will be needed than one might initially expect, because of the I5 from the wiggler (the beta function could also be retuned in the wiggler...). In short, there are a lot of issues to consider. Although it would be possible (and maybe not too difficult) to design a ring for a smaller horizontal emittance, designing a ring that has flexibility in the horizontal emittance does present some difficulties, and will need some careful consideration - so far, we are still working on just the basic damping ring design. I don't see how it would be possible to operate e+e- and gg simultaneously (in two separate IPs?) if you require such different parameters from the damping rings.