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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  • Richard Mitnick says:

    Forgive my total ignorance: The Tevatron did great things; but its 1 TeV and it was insufficient to get Higgs to come out and play. LHC is aiming at 7 TeV. How does a 1 TeV linear collider fit into this puzzle?

  • Warren Funk says:

    I’m not a particle physicist, Richard, but since no one has responded to you yet, let me take a crack at it.

    The difference lies in the nature of the particles being collided. The Tevatron collided protons and anti-protons, which are composites of three quarks and numerous gluons. Think of it as throwing two bags of marbles at each other, and then trying to sort out what happened during the collision! The ‘bag of marbles’ analogy understates the problem, since the quarks and gluons do not share the total energy equally, the way the marbles would, at least approximately. The ‘actual’ collisions are between a quark or a gluon from the proton with a quark and a gluon from the antiproton, neither of which has a pre-determined energy. This is a disadvantage, because the many collisions that do not occur at the energy of interest create a huge ‘background’ that challenges the ingenuity of particle physicists to build detectors that can sort it all out. The silver lining on this cloud, however, is that the broad range of initial conditions available means that hadron colliders like LHC (p-p) and the Tevatron can quickly survey broad regions of the available parameter space. They can reasonably expect to produce the reaction of interest, if it can happen, even if it is buried in background. Note that the Tevatron was barely able to reach the predicted energy range for the Higgs (roughly 100 – 150 GeV), even though its center of mass energy was about 2000 GeV. The LHC, however, with its current center of mass energy of 7000 GeV, is covering it easily, but because the collisions are so confusing, it will take them a while to collect enough data to make a statistically significant statement about whether the Higgs is there or not.

    The ILC on the other hand, will collide electrons with positrons, which are, so far as we know are fundamental, with no substructure. This means that we know precisely (well, within measurement error) what energy each of the colliding particles brings to the party. Think of it as a microscope, compared with a wide-angle lens. Because the collision energy can be carefully tuned, background issues are different and significantly less. This makes it possible to make very precise measurements and observe very subtle effects.

    It is in those subtle effects that the secret of the ILC’s large “energy reach” lies. In the incomprehensibly short duration of the collision, nature takes advantage of quantum mechanics to “borrow” energy from the vacuum, enough energy to create particles more massive than the total energy supplied to the collision. Those particles can effectively pop briefly into existence, influence the collision, and disappear to balance the books. If the detectors are sensitive and precise enough, and they are, they can disentangle the effect of these evanescent particles and learn a lot about them. The combination of precision, sensitivity and low background is what allows lepton colliders (electrons and positrons now, muons in the future?) to learn about physics well beyond their nominal energy range. But they need the hadron colliders to map out the territory for them to examine in detail.

    • Richard Mitnick says:

      Mr (Dr?) Funk-Thanks thanks for your reply. There is a glimmer of understanding coming through. I was not aware of what the differences were is colliding what particles.