23 February 2006
"Independent of the results from the first few years of running there are several reasons for an energy upgrade. Examples include higher sensitivities for anomalous gauge boson couplings, measurement of the Higgs boson self coupling, the coupling of the Higgs to the top quark, production thresholds for new massive particles or exploration of extra spatial dimensions. Consequently, the energy of the machine has to be upgradeable.
The strong likelihood that there will be new physics in the 500 – 1000 GeV range means that the upgrad eability of the LC to about 1 TeV is the highest priority step beyond the baseline."
I am quoting above from the document entitled, "Parameters for the Linear Collider" written by a subcommittee of the International Linear Collider Steering Committee and dated September 2003. This important report defines the top level science requirements that we are using in designing the ILC. Consequently, as we develop the design of the ILC, we are absolutely committed to the upgrade-ability of the machine to 1 TeV, and we have been seeking a well conceived upgrade path.
Possible upgrade scenarios to 1 TeV were discussed at length during the Snowmass Workshop last summer, but no conclusions or consensus emerged. As we moved toward a baseline configuration this past fall, the GDE Executive Committee formed five groups to analyze some key unresolved issues from Snowmass and to write white papers to help determine what to include in the Baseline Configuration Document. In the case of the energy upgrade, the white paper analyzed four different strategies. Three of them used somewhat more than 20 km of tunnel with cryomodules for the baseline 500 GeV machine and then expanded to more than 40 km with additional cryomodules to upgrade to 1 TeV. The fourth used a more sparce arrangement over 40+ km for 500 GeV and missing cryomodules would be installed at a later date.
These alternatives all require a significant investment in a full 40+ km tunnel from the beginning, but with different strategies for cryomodules implementation for the 500 GeV machine. At this point, however, the GDE Executive Committee feels that it is very difficult to justify the large extra expense of an unused tunnel. Therefore, we decided that the BCD should only include what is needed to insure that upgrading to 1 TeV is feasible. We concluded that we will need more work to come up with a well thought out and justified upgrade strategy. To quote the BCD, we decided, "the first phase (500 GeV centre-of-mass) will be constructed using a tunnel long enough to support a gradient of 31.5 MV/m operational gradient (~2x10 km assuming a 0.75 fill factor). A second phase (phase 2) upgrade to 1 TeV centre-of-mass will then require extending the tunnel (away from the IR) an additional ~2x9.3km assuming cavities capable of 36 MV/m operational gradient."
Despite the fact that the BCD does not include the construction of the extra tunnel, it does contain beam delivery systems and main dump systems that will be configured for the 1 TeV option from the start. As we work out strategies to realistically enable the upgrade to 1 TeV, we will revise the plans for the tunnels and cryomodules implementation. I should also emphasize that prospective sites must be chosen with the TeV machine in mind; specifically the total required land and power must be available.Through the work on the reference design this year and then a technical design for the ILC, we will have much better information on the costs, tradeoffs and viability of different upgrade paths. I am confident that using this deliberate approach, we will be able to come up with an attractive plan for the upgrade option that we can defend at the time we propose construction of the ILC.