The International Linear Collider (ILC) is a proposed linear particle accelerator. It is planned to have a collision energy of 500 GeV initially, and, if approved after the project has published its Technical Design Report, planned for 2012, could be completed in the late 2010s.[1] A later upgrade to 1000 GeV is possible. The host country for the accelerator has not yet been chosen. Studies for a competing project called CLIC are also underway; it seems unlikely that both machines will be built.
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 was previously known by various names in different regions; see below.
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There are two basic shapes of accelerators. Linear accelerators ("linacs") accelerate elementary particles along a straight path. Circular accelerators, 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 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 a hadron collider such as the LHC for protons or, alternatively, for lead nuclei, as a ring collider, 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 209GeV by energy loss via synchrotron radiation.
Even if the effective collision energy at the LHC will be higher than the ILC collision energy (14,000 GeV for the LHC[2] vs. ~500 GeV for the ILC), measurements could be made more accurately at the ILC. Collisions between electrons and positrons are much simpler to analyze than collisions between many quarks, antiquarks and gluons. As such, one of the roles of the ILC would be making precision measurements of the properties of particles discovered at the LHC.
It is widely expected that effects of physics beyond that described in the current Standard Model will be detected by experiments at the LHC and ILC. In addition, particles and interactions described by the Standard Model are expected to be discovered and measured. At the ILC physicists hope to be able to:
In August 2004, the International Technology Recommendation Panel (ITRP) recommended a Superconducting RF technology for the accelerator. After this decision the three existing linear collider projects – the Next Linear Collider (NLC), the Global Linear Collider (GLC) and Teraelectronvolt Energy Superconducting Linear Accelerator (TESLA) – joined their efforts into one single project (the ILC). Physicists are now working on the detailed design of the accelerator. Steps ahead include obtaining funding for the accelerator, and choosing a site. In August 2007, the Reference Design Report for the ILC was released.[3]
In March 2005, the International Committee for Future Accelerators (ICFA) announced the appointment of Dr. Barry Barish as the Director of the Global Design Effort. Barry Barish was director of the LIGO Laboratory from 1997 to 2005.
The electron source for the ILC will use 2-nanosecond laser light pulses to eject electrons from a photocathode, a technique allowing for up to 80% of the electrons to be polarized; the electrons then will be accelerated to 5 GeV in a 250-meter linac stage. Synchrotron radiation from high energy electrons will produce electron-positron pairs on a titanium-alloy target, with as much as 60% polarization; the positrons from these collisions will be collected and accelerated to 5 GeV in a separate linac.
To compact the 5 GeV electron and positron bunches to a sufficiently small size to be usefully collided, they will circulate for 0.2 seconds in a pair of damping rings, 7 km in circumference, in which they will be reduced in size to a few mm in length and less than 100 μm diameter.
From the damping rings the particle bunches will be sent to the Superconducting RF main linacs, each 12 km long, where they will be accelerated to 250 GeV. At this energy each beam will have an average power of about 10 megawatts. Five bunch trains will be produced and accelerated per second.
To maintain a sufficient luminosity to produce results in a reasonable time frame after acceleration the bunches will be focused to a few nm in height and a few hundred nm in width. The focused bunches then will be collided inside one of two large particle detectors.
The Draft Reference Design Report estimates the cost of building the ILC, excluding R&D, prototyping, land acquisition, underground easement costs, detectors, contingencies, and inflation, at US$6.65 billion.[4] From formal project approval, completion of the accelerator complex and detectors is expected to require seven years. The host country would be required to pay $1.8 billion for site-specific costs like digging tunnels and shafts and supplying water and electricity.
U.S. Secretary of Energy Steven Chu estimates the total cost to be US$25 billion. ILC Director Barish says this is likely to be an overestimate. Other Department of Energy officials have estimated a $20 billion total. [5]
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