International Linear Collider

The International Linear Collider (ILC) is a proposed linear particle accelerator. It is planned to have a collision energy of 500 GeV initially, with the possibility for a later upgrade to 1000 GeV (1 TeV). Although early proposed locations for the ILC were Japan, Europe (CERN) and the USA (Fermilab), the Kitakami highland, in the Iwate prefecture of northern Japan, has been the focus of ILC design efforts since 2013. The Japanese government is willing to contribute half of the costs, according to the coordinator of study for detectors at the ILC.

The ILC would collide electrons with positrons. It will be between 30 km and 50 km (19–31 mi) long, more than 10 times as long as the 50 GeV Stanford Linear Accelerator, the longest existing linear particle accelerator. The proposal is based on previous similar proposals from Europe, the U.S., and Japan.

Studies for an alternative project, the Compact Linear Collider (CLIC) are also underway, which would operate at higher energies (up to 3 TeV) in a machine of length similar to the ILC. These two projects, CLIC and the ILC, have been unified under the Linear Collider Collaboration.

There are two basic shapes of accelerators. Linear accelerators ("linacs") accelerate elementary particles along a straight path. Circular accelerators ("synchrotrons"), such as the Tevatron, the LEP, and the Large Hadron Collider (LHC), use circular paths. Circular geometry has significant advantages at energies up to and including tens of GeV: With a circular design, particles can be effectively accelerated over longer distances. Also, only a fraction of the particles brought onto a collision course actually collide. In a linear accelerator, the remaining particles are lost; in a ring accelerator, they keep circulating and are available for future collisions. The disadvantage of circular accelerators is that charged particles moving along bent paths will necessarily emit electromagnetic radiation known as synchrotron radiation. Energy loss through synchrotron radiation is inversely proportional to the fourth power of the mass of the particles in question. That is why it makes sense to build circular accelerators for heavy particles—hadron colliders such as the LHC for protons or, alternatively, for lead nuclei. An electron–positron collider of the same size would never be able to achieve the same collision energies. In fact, energies at the LEP, which used to occupy the tunnel now given over to the LHC, were limited to 209 GeV by energy loss via synchrotron radiation.

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