23 November 2005
Reaching the luminosity goals of the ILC will be an enormous challenge. It will require producing nanometer scale beam spots at the collision point and a crucial element in achieving this will be to 'damp' the beams that come from the electron and positron sources to have ultra-low emittance (a measure of the non parallelism). Once achieved, the beams must then be transported to the collision points without significant emittance growth or uncontrolled beam jitter (a problem I will discuss in a later column).
The scheme for achieving such low emittance is to use the effects of synchrotron radiation on the particles transported through damping rings, and this is then combined with acceleration in RF cavities to produce sufficiently reduced emittance beams. A particular challenge for the ILC damping rings is that the long ~1 msec beam pulse containing up to 3000 bunches infers a long damping ring length. The actual length of the rings is determined by the bandwidth of the input and extraction systems.
The TESLA design employed a 17km 'dogbone' damping ring that shared the same tunnel as the main linac (to save money) and had turnarounds that made the shape of the damping ring look something like a dog bone. Recent developments on fast kicker magnets have allowed us to consider alternative shorter length damping rings of 3 km and 6 km that can be external to the main linac tunnel. Although not a consideration in the present study and recommendation, this might allow consideration of the use of existing rings for potential sites at some of our accelerator laboratories.
The ILC Damping Ring Working Group is led by Jie Gao (IHEP), S. Guidicci (INFN) and A. Wolski (LBNL). The comparison of different alternatives is a very complex problem with different trade-offs for different possible damping ring configurations. Our ILC damping ring working group has been working very systematically to analyze the problem and compare alternative designs. Comparisons of costs and the magnitude of various limiting effects have been analyzed using different lattice configurations, especially for the 6 km external ring and the 17 km dogbone damping ring.
This working group was not ready to make a recommendation at the Snowmass meeting and requested more time to complete their analysis. From November 9-11 this working group converged at CERN with an impressive turnout of 34 of the 50 members and met for several days to formulate their recommendation. The group succeeded in reaching a consensus and the recommendations are summarized in a presentation by Andy Wolski to the GDE Executive Committee and in the Baseline Configuration Summary Document. Both can be found on the damping ring section of the BCD, and these will be followed later by a more complete document.
Last week, the GDE Executive Committee accepted the recommendation of the Damping Ring Working Group to use one 6km ring for the electrons and 2x6km stacked rings for the positrons for the ILC. They make a strong recommendation for more R&D on electron cloud effects, which could limit the performance of this system, and note that further R&D will be required for the injection/extraction kickers for a damping ring with 6 km circumference. Electron Cloud effects have been observed in present day storage rings and due to the uncertainty in the ability to mitigate these effects, they make the important recommendation to carry the 17 km dogbone as an alternative to the baseline.
I would like to end by applauding the very complete and systematic approach that went into this recommendation and the excellent collaborative nature displayed by this working group.