Onward and upward into the terascale

The 1-TeV upgrade challenges scientists develop better beams at lower rates of power

| 20 October 2011

The technical design of the International Linear Collider is nearing completion, and it comes with a stipulation: If the Large Hadron Collider should direct scientists to an energy range beyond what the ILC accommodates, they should refer to its 1-teraelectronvolt contingency plan.

Over the next several months, researchers will complete a preliminary study of the 1-TeV ILC as an energy-doubling upgrade of the current 500-gigaelectronvolt (GeV) technical design. Having developed the outline for a possible upgrade from day one, researchers are now conceptually defining the elements of a higher-energy machine.

Straw man beam parameters for a 1-TeV upgrade

“Our focus has been very strongly on designing the best 500-GeV machine we can,” said Global Design Effort (GDE) Project Manager Nicholas Walker. “At the same time, we do have to do a little more work on the TeV upgrade than we did for it in the Reference Design Report,” he said, referring to the 2007 document.

Upgrade parameters are largely based on a cavity gradient goal of 45 megavolts per metre (MV/m) and a power consumption limit of 300 megawatts (MW) for operating the collider. Using these straw man parameters as a foundation, scientists will continue to develop them for the forthcoming ILC Technical Design Report (TDR) using LHC results, cost studies and technical reviews as guides.

The toughest challenges lie in developing accelerator technology befitting one teraelectronvolt of linearly directed collision energy.

ILC scientists are focused on developing superconducting accelerator cavities with a 45-MV/m cavity gradient, up almost one-and-half times the 31.5 MV/m specification for the 500-GeV accelerator. These higher-gradient cavities would be developed during the ILC’s initial 500-GeV run. If there were an upgrade, they would be added to those already operating in the collider.

“The linac technology is the jewel in the crown of the machine,” Walker said. “It’s the high-tech element and the cost driver. We take the view that we should always use state-of-the-art technology whenever we do anything there.”

Scientists are also chasing higher cavity quality factors to help cut down on power losses as the beam is propelled towards collision.

Conserving power, even as the beam energy is hiked up, is a goal unto itself. By setting a power consumption ceiling of 300 MW to for collider operation, ILC scientists compel themselves to find a different way to reach higher luminosities without drawing it from the wall plug.

“We’re trying to be greener,” Walker said.

The path to a greener machine and luminous beam involves hitting on the right beam structure: reducing bunch repetition rate, taming Beamstrahlung, squeezing the beam size. Scientists are currently deciding on the beam’s working parameters based on the straw man limits. The parameters will also be used in simulations for the detector community’s detailed baseline design.

Linac construction scheme for 1-TeV upgrade

While enforcing technological rigour in certain aspects of the 1-TeV design, ILC researchers are necessarily flexible about the future machine’s collision energy.

“One has to bear in mind that the LHC results are rewriting the textbooks, so in the backs of our minds is the potential to react to whatever comes out of the LHC,” Walker said. Having borne this in mind from the time of the GDE mandate in 2003, ILC scientists pursue the current 1-TeV design in accordance with the original charge of designing an upgradeable machine that begins its life as a 500-GeV collider. Though resources aren’t available to do a full-fledged 1-TeV design, parameters are expected to be specific enough that the upgrade from the TDR 500-GeV collider will merit a summarising chapter in the report.

At the same time, the likelihood that the LHC will come out with meaningful results as early as 2012 requires researchers to be even more nimble, ready to build, from the get-go, a machine whose centre-of-mass energy may be lower or higher than 500 GeV. The 1-TeV study, in addition to prescribing a set of upgrade changes, also gives scientists a means to explore the design of a machine engineered for a different energy, one they may tackle in the post-TDR era should the LHC guide them elsewhere.

In contrast, other factors specific to the 1-TeV scheme have long been established, built into the original 500-GeV programme at minor cost. The beam delivery system includes room for the installation of longer bending magnets to take care of the increased beam energy. The current beam dumps can handle up to 18 megawatts of cast-off power – more than enough for a 1-TeV beam – precluding the need to weather a radioactive environment to change them later.

And, of course, doubling the energy would mean adding about 20 kilometres of linac to the present design.

Other technology architecture remains much the same. Damping rings, positron and electron sources and injection won’t fundamentally change for the 1-TeV scenario, though provisions are made for improvements.

Most are optimistic that they can design a 1-TeV machine within reasonable environmental, scientific and cost limits.

“It drives a lot of the R&D, keeps everyone excited – the whole technology and everything that goes with it,” Walker said. After the final report is delivered, the realisation of a 1-TeV design and the challenges of its attendant hurdles will encourage researchers to maintain their momentum. “The R&D in this technology will not stop.”

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