Director's Corner

13 October 2005

Barry Barish

As I discussed in this column last week, the starting point for us in determining the baseline configuration for the ILC is a set of top-level physics parameters agreed to by the global community and contained in a report created by an international ICFA / ILCSC subcommittee dated September 30, 2003 and entitled, 'Parameters for the Linear Collider.' This report gives us the basic parameters lsuch as luminosity, energy reach and other crucial physics parameters. The report gives the maximum energy of the ILC as 500 GeV in the center-of-mass, and states that the machine should have the future option of extending the energy to ~1 TeV.

Nan Phinney

We created Global Group 1 for Snowmass, in order to translate this physics parameter set into overall machine parameters for the baseline ILC. This global group was to be run by Tor Raubenheimer, Nick Walker and Kaoru Yokoya, but Tor hurt his leg in an unfortunate accident and was not able to participate in the Snowmass meeting. Fortunately, Nan Phinney was able to very ably step in for Tor during the workshop.

The strategy developed by this group was not to develop a single parameter set, but rather to determine a range of values for the key parameters that could achieve the basic goals. Four self-consistent parameter sets were studied, in which key parameters are traded off against each other in order to achieve the desired luminosity goal. The combination of these sets then give a parameter space where the machine could achieve the goals (in this case L = 2 10³⁴ cm²sec^-1). Each parameter set tends to push the achievable limits of one or more specific subsystems. The four sets consisted of a nominal set, a set with reduced bunch charge by a factor of two, a set with increased vertical beam size from emittance growth by a factor of two, and finally a set with lower power by reducing the number of bunches by a factor of two.

One of the main motivations for creating a parameter space, rather than a 'point solution,' is that the final operating point for the machine should optimize somewhere within this parameter space and the various subsystems need to be designed to accommodate, as much as possible, the most demanding parameters within this space. We intend to use such a parametric approach broadly in optimizing the ILC design, and in this case in particular, this space and variation of parameters gives flexibility in machine operations by being able to vary crucial machine parameters. In other cases, the parametric approach will allow us to optimize cost to performance.

To see more of the work of Global Group 1 at Snowmass and also the summary of Question 2 of the list of decisions developed at Snowmass, I refer you to the summaries on our ILC webpages. The tentative conclusions from Snowmass were that "there are no compelling arguments to modify the originally proposed 500 GeV values, and the sub-systems should still design to the most demanding parameters. This will be again reviewed as and when new information from the working groups (especially the damping rings) is made available."

As we understand more about the impact on the designs of various subsystems, regarding performance, cost or reliability we may well modify and iterate these basic machine parameter sets.