Friday, September 29, 2017

Innovating the Colliders of the Future: the Electron Lens

Particle accelerators have opened a unique window into the subatomic world, revealing some of the most fundamental components of our universe. In the last ten years, CERN’s Large Hadron Collider (LHC) has taken us to new energy frontiers that resulted in the detection of the Higgs boson among other accomplishments, including the recent discovery of a doubly charmed particle. But there is still much to learn.

Published Wednesday in the American Physical Society’s journal Physical Review Letters, new research by a team of scientists from Fermi National Accelerator Laboratory, better known as Fermilab, could help pave the way for next generation particle accelerators like the Future Circular Collider (FCC). The FCC will be a higher-performance accelerator than the LHC, with proton collision energies approaching 100 TeV—more than seven times the current collision energy of the LHC. Such an accelerator would be capable of probing the mysteries of dark matter, neutrinos, and the prevalence of matter over antimatter much deeper than anything in existence today.

A schematic map showing where the Future Circular Collider tunnel is proposed to be located.
Image Credit: CERN.
Planning a next-generation high-energy particle accelerator is an enormous task. Although designs are currently being developed for the FCC, construction is not expected to break ground until after 2035. This is partly because you can’t just incrementally upgrade existing technologies to make such a high-energy collider—it requires vision, cooperation, infrastructure, money, and innovation.

According to Fermilab’s Vladimir Shiltsev, “Super large colliders require different methods than traditional ones in order to keep their cost under control and their performance as great as possible.” Designing an accelerator like the FCC requires some completely new approaches to challenges like beam stability.

Imagine a line of ships leaving a port one right after the other, following similar paths. As the first ship heads out, a wake forms behind it. As the second ship leaves, it hits the wake and rocks a bit. The third ship hits the turbulent water caused by the first two ships and wobbles even more. You can see how this approach to releasing ships would quickly lead to problems.

In particle accelerators like the LHC, beams are released in very high intensity bunches of closely spaced protons. These particles radiate electromagnetic wake-fields as they travel through the accelerator. Like a ship leaving behind a wake, each bunch of particles leaves behind electromagnetic field remnants that can impede the next particle in the bunch or the next bunch, causing it to “wobble” a bit. The cumulative effect of all of these wobbles is beam instability, a factor that can seriously limit the performance of an accelerator.

Most modern particle accelerators counteract beam instability using a system based on special magnets. These magnets have an effect analogous to spreading out the paths of the ships just enough to keep them stable. However, this approach becomes less efficient as the intensity of the beam increases. For example, operating the LHC at 7 TeV requires 336 of these magnets. Operating the FFC at 50 TeV would require tens of thousands of these magnets. Each of these magnets costs time, money, and room along the accelerator path.

In this new work, Shiltsev and his colleagues Yuri Alexahin, Alexey Burov, and Alex Valishev propose replacing the magnet-based system with something called an electron lens. This proposal is so innovative and different, says Shiltsev, that the scientists reviewing this new paper were shocked by the scale of the improvement an electron lens offers. The team’s calculations show that an electron lens just 1m long could replace tens of thousands of magnets that would take up 10km of space along the FCC’s path.

Electron lenses are a relatively new concept, but the technology has been well established thanks to Fermilab. For many years Fermilab was home to one of the most powerful particle accelerators in the world, the now-retired Tevatron, which collided beams of protons and antiprotons at nearly the speed of light. In 1997, scientists realized that introducing a well-controlled cloud of low-energy electrons, an “electron lens,” into the Tevatron would help compensate for unwanted effects caused by interactions between the two beams. The electron lens was built, installed, and made a world of difference. Two more have since been installed in Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) for the same purpose.

Another application of the electron lens soon followed, also at Fermilab’s Tevatron. It turns out that sending a proton beam through a 2m long electron lens with a hole in the middle, essentially a hollow tube made of electrons, is a great way to keep the beam well-aligned and reduce background noise. The LHC collaboration is considering installing some electron lenses for this purpose.

In this new research, Shiltsev and his team explored how sending a beam of protons though electron lenses of different sizes affects beam instability. Using theory and simulations, they show how the lens spreads out the natural resonant frequencies of the protons enough to dramatically reduce beam instability even in high-energy colliders like the LHC and the proposed FCC.

People have been building particle accelerators and beams for more than 75 years. We’ve gained tremendous insight that has led to exciting discoveries and the revamping of fundamental ideas. As we continue moving forward on the path to understanding our universe at its most elementary level, it’s worth remembering that great advances often come at the places where curiosity and innovation collide.

Kendra Redmond

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