11 May 2006
One of the more difficult decisions we made last fall was which damping ring option to choose for our baseline. No recommendation came out of the Snowmass Workshop and our damping ring working group continued to study the question through much of the fall. They recommended a 6 km circular damping ring for electrons and two 6 km rings, one stacked above the other, for positrons.
The Damping Ring group has documented their work in an impressive 300 page document entitled, “ Configuration Studies and Recommendations for the ILC Damping Rings” edited by A. Wolski (LBNL, Berkeley, CA), J. Gao (IHEP, Beijing, China, and S. Guiducci (LNF, Frascati, Italy) on behalf of the 47-member working group.
I commend this fine piece of work and recommend all those interested in the technical details of the ILC and the reasoning behind this particular design choice, to download and read this document. As a reminder, the job of the damping rings in the ILC is to sufficiently reduce the emittance of the beams by synchrotron radiation before the beams enter the main linac to be accelerated to 250 GeV. (see Director’s Corner, 23 November 2005) This is a challenging task because of such things as the long beam pulse, the large number of bunches, and the bandwidth of the injection / ejection kicker magnets.
Comparisons of damping rings options were performed in a “parametric study” by using a range of lattices that represented a variety of different configuration options. They studied beam dynamics, technical subsystems, costs, and operations issues for a number of configuration choices that included different circumferences, energies and lattice cell structures. They also considered different options for the various technical subsystems, such as the injection and ejection kicker magnets, and wigglers.
Based on their studies, the Damping Ring working group made a set of recommendations, highlighted by the following:
“The positron damping ring should consist of two (roughly circular) rings of approximately 6 km circumference in a single tunnel. Electron-cloud effects make a single ring of circumference 6 km or lower unattractive, unless significant progress can be made with mitigation techniques. Space-charge effects will be less problematic in a 6 km than in a 17 km ring, and achieving the required acceptance will be easier in a circular ring than in a dogbone ring.”
The document also presents the conclusion and recommendation for the electron ring to consist of a single 6 km. Finally, they detail the future studies which are needed and indicate the R&D that will be required.
This very nice piece of work and the detailed documentation will be invaluable in performing the design and cost work ahead, in focusing the R&D program, and in delineating the issues (e.g. electron cloud effects) that still need attention before we can be certain that this design choice is the best one for building an ILC. Nevertheless, we now have a well defined and documented configuration to form the basis of our design and cost studies this year.
-- Barry Barish