1 December 2005
One Tunnel or Two?
Should we build the ILC using one underground tunnel? Or should we build it using two adjacent tunnels, one containing the support instrumentation and the other containing the accelerator?
These sound like innocuous questions, but they present one of the more complex and difficult problems we have been facing in forming the baseline for next year's reference design. There are a mix of issues that need to be considered, including safety, access, reliability and cost. From our present knowledge of these factors, we have recommended the use of parallel twin bored tunnels for the 'strawman' baseline, which will be discussed and finalized at Frascati next week.
Today, I want to highlight some of the considerations that went into our two tunnel recommendation, and the consequences it will have for our work next year. What are the issues? A single tunnel having a diameter of 5.2 m was proposed for TESLA, and it contained access shafts and surface halls at regular intervals (every 5km) for cryogenics and modulators housing. An alternate concept using two tunnels has been proposed, in which the RF sources and other support instrumentation are located in a separate tunnel that runs parallel and nearby to the beamline tunnel. The motivation for proposing two tunnels is that the instrumentation in the second tunnel would not be subject to radiation and could be accessed for repairs while the machine is running. The main argument against two tunnels is cost. As we understand the costs better over the next year, this may well become the critical argument.
The white paper concurs with the recommendation from Snowmass and proposes that we proceed with the two tunnel option. The committee analyzed several variants of what could go in each tunnel, depending on whether the sites are shallow or deep. They also list a number of justifications for their recommendation including reliability and safety, as well as lesser radiation damage, easier maintenance, etc.
We have accepted this recommendation for the 'strawman' baseline that will be considered at our GDE meeting in Frascati next week. This recommendation, if validated, will imply that the reference design will be carried out based on two tunnels. Site dependent considerations, however, could affect the exact configurations. Due to the rather substantial extra cost estimated for two tunnels (presently estimated to be ~$300M), we propose to carry out a limited secondary design next year for the one tunnel option. This study will prepare us to assess and possibly revert to the one tunnel option, if that becomes necessary due to cost considerations. I want to emphasize, however, that this alternative tunnel option will only be possible, if we have done the work to develop a design for a single tunnel plan that is viable in terms of safety, reliability and availability for science.