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Fermilab attains unprecedented quality factor for LCLS-II dressed cavity

A Technical Division team rallies around a dressed cavity from the LCLS-II project. Photo: Reidar Hahn

Members of Fermilab's Technical Division are working on superconducting radio-frequency cavities that are shaped like squatty beads on straight string. These prone, uniformly bulging tubes accelerate the particle beams that shoot through their hollow insides.

The team recently achieved a record-high quality factor with a fully dressed cavity for a SLAC-headed project, Linac Coherent Light Source II.

"This has taken a lot of hard work from a very dedicated crew," said Rich Stanek, Fermilab LCLS-II senior team leader. Stanek acknowledged the entire cavity dressing team and all of the SRF scientists that helped reach this record quality factor.

Quality factor, Q, is a measure of how efficient a particle acceleration cavity is. A higher Q means a cavity is losing less energy, which is more cost-effective.

The two LCLS-II free-electron lasers will produce X-rays to probe a wide variety of materials, exotic and otherwise, at the nanoscale. Fermilab is responsible for designing, developing, building and testing about 150 nine-cell cavities for the LCLS-II superconducting accelerator. The R&D process began one-and-a-half years ago. It includes ensuring that the cavities meet certain Q values during testing.

"This is the first integrated test we did," said Nikolay Solyak, project support group leader. In an integrated test, everything is checked under real conditions. "The conditions were very close to the cavity's final condition in a cryomodule."

In this integrated test, the fully dressed 1.3-gigahertz cavity's quality factor was 3.1 x 1010 at 2 Kelvin and at a 16-megavolt-per-meter peak surface electric field. This Q exceeds LCLS-II's goal of 2.7 x 1010 and far surpasses current state-of-the-art standards.

"This quality factor is an extremely important step," said Slava Yakovlev, SRF department head. "It's a victory."

SLAC physicist Marc Ross, LCLS-II cryogenics systems manager, says he's pleased with the results.

"It's definitely a victory," Ross said. "These are some of the highest-quality-factor practical resonators ever built."

A fully dressed cavity is outfitted with all the components it will wear in the LCLS-II accelerator. This includes a titanium jacket filled with liquid helium chilled to 2 Kelvin, a temperature at which niobium is superconducting. It's also furnished with power-providing couplers, cavity-squeezing tuners to control frequency, and magnetic shielding. These components add heat and can lower Q, so the team had to develop a way to carry this heat away and keep Q high.

"This record Q is really the sum, the final point, of many years of research," said Anna Grassellino, Fermilab Technical Division scientist who leads cavity testing and processing for LCLS-II. "It's really a miracle of science and technology and engineering coming together and producing an unprecedented quality factor. It opens up a way for machines to operate much more efficiently at a much lower cost."

Grassellino led the Fermilab effort to apply the breakthrough technology, dubbed nitrogen doping, that helped achieve this record Q. It involves infusing nitrogen into a cavity's inner niobium surface. Nitrogen doping and other Fermilab discoveries that led to this Q value, such as the removal of magnetic flux through rapid cooling, will become new standards for achieving highly efficient accelerators worldwide.

"This is a critical milestone not only in LCLS-II design, but in other modern accelerator projects including our own project, PIP-II," Yakovlev said.

But there's more to be done for LCLS-II.

"We still need to show that the full cryomodule with eight cavities meets specifications," Grassellino said. "There's always a next step."

Chris Patrick

An LCLS-II-type accelerator cavity prepares to be treated with nitrogen, a process that increases the cavity's quality factor. Fermilab recently reported a record quality factor for LCLS-II-type cavities. Photo: Reidar Hahn
In the News

Finally some answers on dark energy, the mysterious master of the universe

From ars technica, Nov. 5, 2015

Unless you're an astrophysicist, you probably don't sit around thinking about dark energy all that often. That's understandable, as dark energy doesn't really affect anyone's life. But when you stop to ponder dark energy, it's really rather remarkable. This mysterious force, which makes up the bulk of the Universe but was only discovered 17 years ago, somehow is blasting the vast cosmos apart at ever-increasing rates.

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Frontier Science Result: CMS

Bigger is better

Since its discovery in 1995, our understanding of the top quark has improved, mostly due to improvements in accelerator and detector technology. Today's article describes an early analysis using the LHC data taken in 2015. The fact that it is about top quark production is a testament to changes in technology and the superb effort by the scientists involved.

I'd like to start by stating the obvious. Analyzing frontier physics data is hard. When you record data using a cutting-edge particle accelerator like the LHC or the Tevatron in its heyday, you encounter lots of ways of getting things wrong. It can take years for scientists to understand the little imperfections of their detectors and figure out how to correct for them.

The LHC just resumed operations in June, this time at a collision energy of 13 trillion electronvolts, which is a little over 60 percent higher than when it last ran in 2012. Given that scientists have been looking at the data for under half a year, the first physics analyses are of the simplest topics — ones for which a subtle understanding of the detector's performance is not needed. Thus it is (in my not very humble opinion) a breathtaking achievement that one of the first papers submitted for publication using data recorded in 2015 was on the production rate of top quarks.

Some of you will remember the excitement in 1995 when the top quark was discovered here at Fermilab. After painstaking effort, DZero and CDF scientists teased out a tiny signal from an overwhelming background. It was a very hard thing to do. So what changed?

We are told that bigger is better, whether it be for football players, paychecks or slices of Mom's apple pie. But that aphorism really does apply when it comes to particle accelerators. The LHC collides beams at an energy that is 6.5 times higher than was possible at the Tevatron and at a much higher rate of collisions per second. Both of these contribute to a much improved ability to produce top quarks. In fact, the rate at which top quarks are produced at the LHC is about 100 times that of the Tevatron for the same amount of delivered beam. What was once hard is now easy.

This CMS analysis is a simply brilliant accomplishment that exploited the enhanced capabilities of both the accelerator and the detector. The result is that the increased production rate is in exact accordance with the predictions of the Standard Model.

This is both good and bad. It is good because it again affirms just how good the theory is, and it is bad because it says that there are not yet any surprises in the production of even the heaviest particle ever discovered. This suggests that the road to a paradigm-changing discovery might be steeper than we hoped. But the universe is not obliged to make things easy for us, and it is our job to accept its secrets, whatever they may be.

Don Lincoln

Mirena Paneva of the University of California, Riverside, made important contributions to this analysis.
Photos of the Day

By the dusk's early light

nature, sky, sunset, landscape, buildings, architecture, MC-1 Building
Clouds appear to radiate from the MC-1 Building. Photo: Josh O'Connell, AD
nature, sky, sunset, landscape, buildings, architecture, sculpture, Tractricious, IARC, Wilson Hall
Pinks and violets reflect off IARC and through "Tractricious." Photo: Barb Kristen, PPD
In the News

Physicists probe antimatter for clues to how it all began

From NPR, Nov. 4, 2015

Our world is made of matter. "Everything you see and feel — your laptop, your desk, your chair — they are all ordinary matter," says Aihong Tang, a researcher at Brookhaven National Laboratory.

But matter has a counterpart called antimatter. Each kind of fundamental particle of matter has an antimatter nemesis lurking in the shadows. And true to science-fiction stereotype, if matter and antimatter ever meet, they annihilate in a flash of light.

If you've never run into "antimatter" outside of a Star Trek episode, you're not alone. There's not a lot of antimatter in our universe. And that has physicists confused.

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