Tuesday, November 06, 2018

Surprising Discovery Gives "Plasma Lenses" a Clearer Future

It might surprise you that scientists at CERN—the home of the world’s largest particle accelerator—don’t always think bigger is better. At 17-miles in diameter, CERN’s Large Hadron Collider (LHC) is the biggest and most powerful particle accelerator in the world. It’s the hub of high energy physics research, drawing scientists from around the globe and managing to make the Higgs boson a household name. But as CERN researchers look to the future, some of them are thinking small. Well, small in size, not small in impact.

An active plasma lens during discharge in both helium (left) and argon (right).
Image Credit: Lindstrøm et al, Physical Review Letters
“There are a number of new and powerful ways of accelerating particle beams these days that promise to build significantly smaller physics machines (like linear particle colliders or free-electron lasers) as well as spin-offs like cancer radiotherapy centers,” explains Carl Lindstrøm, a graduate student working with Erik Adli at the University of Oslo.

Smaller, less expensive accelerators mean greater access to these tools. And not just greater access for physicists, but also for researchers in other fields, the medical community, and cancer patients. That’s why CERN is also home to CLEAR, the CERN Linear Electron Accelerator Research Center. At CLEAR, researchers prototype and study techniques for shrinking down accelerators alongside components for LHC upgrades.

In a surprising new discovery, an international team of scientists and CLEAR collaborators that includes Lindstrøm have made a critical advance in a key area of small accelerator design—beam focusing. Their work will soon be published in the American Physical Society’s journal Physical Review Letters.

To be useful, a particle accelerator needs to do more than accelerate particles—it must also focus those particles into a beam and focus that beam on a target. Whether your goal is smashing atoms or irradiating cancerous tissue, if you can’t focus the beam there's no point in accelerating anything!

In conventional accelerators, particles are focused by powerful magnets. Carefully arranged groups of magnets placed throughout an accelerator create a strong magnetic field that guides the charged particles to the target. In general, the higher the magnetic field, the shorter the distance over which a beam can be focused.

“As the accelerator devices get smaller, so must the focusing devices,” says Lindstrøm. Therefore, miniaturizing the focusing section of an accelerator requires a completely different approach than the magnets used by large accelerators. One of the most promising approaches is active plasma lensing, in which the magnetic field is generated by sending a pulse of electric current through a thin tube of ionized gas, a.k.a. a plasma.

Active plasma lensing is an attractive option for rapidly focusing a particle beam, but there are drawbacks. One of the largest is that zapping the plasma with electricity makes the gas in the tube heat up unevenly. Experiments show that this uneven heating causes an aberration, a problem with the system that interferes with its ability to focus the beam clearly.

Two years ago, Lindstrøm was part of a team led by Erik Adli (University of Oslo in Norway), Simon Hooker (University of Oxford in the UK) and Jens Osterhoff (DESY in Germany) whose goal was to measure the effect of this aberration. First, they designed and set up a plasma lens using helium. Here’s the rest of the story, as told by Lindstrøm.

We were using practically the same setup that other labs had used before us, but due to small changes in our design, it was not working. So eventually, we gave up and decided to go for Plan B—using argon [instead of helium], as it is easier to break down into plasma, which would solve our problems. This worked just fine!

As we proceeded to do the intended experiment, which was to measure the effect of the aberration, we immediately observed that it was not there. We were all a bit baffled, and confused. We measured it again and again—always the same answer: no aberration.

First, we thought that maybe the other labs had made a mistake—our measurement was a more direct and precise one, whereas the other labs were using different methods. We had no reason to believe that the change of gas would suddenly fix things—but we had to check. After some thinking and some modifications, we finally managed to also use a light gas—helium—and immediately, we saw evidence of the aberration.

All the evidence now made sense, and we knew we had stumbled upon something very interesting!
Team members (from left) Kyrre N. Sjøbæk, Carl A. Lindstrøm, Erik Adli (principal investigator), Martin Meisel, and Lucas Schaper in the CLEAR control room.
Image Credit: Wilfrid Farabolini / CLEAR User Facility.
What they stumbled upon—and then demonstrated theoretically and experimentally—is that the troublesome aberration caused by active plasma lensing completely disappears if you fill the tube with argon instead of helium, the traditional choice. After doing some experimental and theoretical analysis, here’s the simple reason they think this is the case: Heat transfer happens slower in heavier gases than in light gases. Argon is much heavier than helium. In argon, they think, the heat transfer happens slow enough that the aberration just doesn’t have enough time to form.

This discovery means that researchers can now design higher-quality plasma lenses and, as a result, better focusing systems for small accelerators. This is great news for researchers—and really, for all of us, since we’d all benefit from greater access to particle accelerators. It also highlights how important it is to get your hands dirty.

“I find it incredible—and very inspiring—how much you can learn and discover from simply doing the experiment,” says Lindstrøm. “We set out to do a relatively straightforward measurement of a ‘known’ effect, and ended up finding a solution to the problem by chance. You can’t always expect a discovery, but if you don’t do the experiment, you definitely won’t learn anything new,” he says.

Kendra Redmond

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