CLIC preparations go up a notch
The Compact Linear Collider gears up for post-LHC physics with an international workshop.
A schematic diagram of CLIC.
In June CERN gained a new building: number 2010. And as chance would have it, this is more than just a number to its new residents. By the year 2010, teams working at the new CLIC Experimental Area, along with the already established CLIC Test Facility Three (CTF3), hope to have demonstrated the feasibility of the Compact Linear Collider and, depending on results from the LHC, embark on its final design and proposal. A workshop on 16t-18 October brought people from all around the world to CERN to exchange ideas and hear how the ambitious project is progressing.
CLIC is a project that aims to extend lepton collider technology to multi-TeV energy physics, colliding leptons with a centre-of-mass-energy up to 3TeV, more than ten times the energy of the LEP. This is only possible in a linear collider, where no energy is lost through synchrotron radiation due to the particles’ circular motion. The CLIC energy range will allow us to unravel the TeV physics, to be unveiled by the LHC. But to accelerate particles to these energies over a distance of 42km, the beams must gain a huge amount of energy per metre, requiring the supply of very high instantaneous power per metre.
The CTF3 is working on producing a beam that will demonstrate the feasibility of producing such peak power. "To do this a novel system will be employed where a low energy and frequency but very high intensity ‘drive’ beam will run alongside the main beam, providing it with power", explains Hans Braun, deputy project leader. The low frequency beam is compressed on transfer to the main beam, allowing for very high acceleration and luminosity. This use of a drive beam also allows the main RF power source to be in a separate, remote location. The previous facility, CTF2, showed that power transfer from drive beam to main beam is possible but, unlike CTF3, it was not scalable to the power ultimately needed. The new experimental area, a 42m long building connected to the 150m long CTF3 site, will test the beam once it is has been made.
The workshop, which discussed the physics, the detector and the machine, was the first of its kind and attracted more than 200 participants from 49 institutes in 19 different countries. One of its main aims was to communicate CLIC progress and new acceleration and frequency parameters that were changed at the end of last year. "We’ve reduced the acceleration field from 150 MW/m to 100 MW/m, and the frequency from 30GHz to 12GHz," explains Hans. "This should not only relax the technical requirements, but also improve the power and cost efficiency of the scheme."
Such frequencies are in fact already in use in SLAC in the USA and KEK in Japan. Utilising these links and fostering further collaboration with institutes from all over the world was another of the workshop’s important goals. Working groups were able to pool their wide expertise, focussing on a range of themes including beam dynamics, acceleration structures, instrumentation, RF structures and drive beam technology.
Did you know?
In a linear collider the beam must travel, as the name suggests, in a straight line. But when that line is 42km long, things are not quite so simple. The curvature of the Earth means that if a tunnel is built straight at a constant depth, it will itself be curved. Therefore a ‘laser-straight’ tunnel must be built instead that will be deeper in the centre to account for the curvature. This centre will be 100m deep underground. The beam in CLIC will also need to be much more focused than in the LHC as, being linear rather than circular, the particles only get one chance to collide. In the LHC the particles orbit 11,000 times per second and collide with a beam size of radius 16.7μm, whereas in CLIC the beam gets one chance only and so must be focussed onto a point almost one million times smaller, just 40nm wide and 1nm high, the size of a few water molecules. This essential level of focussing means that even seismic vibrations must be stabilised in order to maintain the laser-straight line.
CERN Director-General, Robert Aymar, opened the workshop by talking about the need to be ready to go beyond the energies of the LHC and stressed the importance of working on both projects in parallel. After three days of discussion, Mario Calvetti’s closing remarks summarised the challenges CLIC is facing but the mood coming out of the workshop was very positive, and its success suggested that a further meeting of minds would take place next year. As CERN staff made their way back to Building 2010, that year would have been on their minds as the next major milestone for CLIC and an exciting prospect.
