First phase of CLIC R&D complete

Let’s turn back the clocks to 2002: the LHC is still under construction, the wrap-up of the LEP physics programme is still in recent memory and the future of electron-positron accelerators at CERN is ambiguous. It was then that CLIC set out to prove the feasibility of their novel accelerator design in the CTF3 test facility. Though once a tall order for the collaboration, the recently released CLIC Conceptual Design Report has proven many of the major design elements… bringing to an end the first phase of CLIC R&D and pointing toward detailed performance optimisation studies in the next phase.


Streak camera images of the final beam, illustrating the combination of beams in the Combiner Ring.

Over a decade ago, the CTF3 team set up shop in the vacated LIL injector site, once home to the weathered machine that delivered electrons and positrons to LEP. Rebuilding and upgrading the machine piece by piece, the CTF3 team converted this mA linac into a high-current drive beam generator. Then, using recycled accelerators elements from across the world and a few pieces of new technology, they were able to add a delay loop, combination ring and two full test beam lines. “Our test facility is a complete hybrid,” says Roberto Corsini, CLIC Collaboration spokesperson. “But it has given us the ability to carry out far more than a single experiment. We’ve surpassed our original R&D goals, proving CLIC concepts once considered out of reach.”

At the heart of these new concepts is the generation of the drive beam; this beam would operate alongside the main CLIC accelerator, providing it with RF power without adding any extra length. But to make it, the CTF3 team had to create a high-current beam in a fraction of the required length. They set out using a delay loop to take beams slightly out of phase with each other, then overlapping them in a combiner ring (see first image). Today, CTF3 routinely converts 3.5 A beams into 30 A beams using this technique – establishing the combination principle required for CLIC.

CTF3 layout.

Their disparate lines of research have also allowed the CTF3 team to prove many other key concepts of the CLIC design, including the critical drive beam deceleration for RF power extraction. “In some respects, this proved to be more difficult for CTF3’s set-up than it will be for CLIC’s,” explains Roberto. “Our incoming drive beam energy is lower than the lowest CLIC drive beam energy. Of course, as energy decreases, the size of the beam grows – making the beam increasingly difficult to extract power from. We knew that accomplishing extraction in CTF3 meant guaranteed success with CLIC.” So far, CTF3 has succeeded in decelerating their beam to 30% in just 20 metres; they hope to push this by an additional 10% this year.

CTF3 have also studied the transfer of drive beam energy to a second accelerating structure – a key ingredient of the CLIC acceleration scheme. This has been demonstrated in their two-beam test station, where accelerating gradients of 145 MV/m are regularly achieved. Note that CLIC will require 100 MV/m – a challenging requirement, but one CTF3 has surpassed.

Proving these technologies is just the beginning for CTF3. With their first R&D phase winding up this year, they’ve started transitioning into a second, even more intense phase of research. For more about what’s in store for the facility, check out next week’s Bulletin.

Did you know?

- Visiting CTF3 is not only an exceptional experience for R&D fanatics; it’s also rather like celebrity-spotting, accelerator style. You can catch a glimpse of accelerator pieces from LEP’s accelerator chain, CERN's ISR, Super-ACO from France, and the CELSIUS storage ring from Sweden.

- CTF3 was the first facility to create isochronous beams – that is, beams orbiting with the same period independent of their energy in a circular accelerator.

- The CTF3 team have designed and tested novel Power Extraction & Transfer Structures (PETS) to provide energy to the main beam line. They operate with an on/off principle, allowing individual PETS to be turned off whenever there is an electrical breakdown.

- In its final stage at maximum energy, the CLIC facility will need a power of more than 500 MW. For this reason, the efficiency of RF to beam power transfers is especially important. CTF3 have already succeeded in providing 95.3% efficiency in drive beam acceleration, though they continue to push other systems towards similar values as a part of studies to optimize power and energy consumption of the machine.

Visit CTF3!

CLEX (the CLIC Experimental area).

CERN Internal Communication is organising a visit to the CLIC Test Facility CTF3 – an opportunity for you to explore the R&D facility.

If you wish to participate, you can sign up for a visit by sending us an e-mail. The visit will take place between 7 and 18 January 2013 (final date and time to be decided once visitor numbers are established). Note that visits are only open to CERN access-card holders.

The visit will last approximately 45 minutes and will include:
- an introduction to CLIC and the facility by experts,
- a tour of the CLEX (CLIC Experimental area) of CTF3 and the CLIC showroom,
- a few minutes for questions.


by Katarina Anthony