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Testing Superconducting Radiofrequency Cavities at Jefferson Lab’s Main Particle Accelerator

Since it first went online more than 30 years ago, the Vertical Test Area at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility has gotten used to superlatives. One of the biggest testbeds of its kind. The busiest. The most versatile. 

Now, the Vertical Test Area that was created to help build Jefferson Lab's main particle accelerator has hit another milestone: In 2022, it conducted a mind-boggling 470 different superconducting radiofrequency accelerator cavity tests. In the rarified world of accelerators, that's an Olympic-level achievement. 

By comparison, the facility tested 277 superconducting radiofrequency (SRF) cavities in 2017, 400 in 2018, 349 in 2019, and 351 in 2021.

Jacob Harris, a quality engineer for Jefferson Lab's SRF Operations group, attributes the record number of tests in 2022 to increased demand but also to its early career operations engineer, Justin Kent, who took on the position just over two years ago. 

Since starting at the lab, Kent has implemented improvements in planning, scheduling and prioritizing. His previous experience as a facility cryogenics engineer also informed his moves to make more efficient use of the liquid helium that's key not only to the operation of particle accelerators but to testing their components. 

"This is no 'average Joe' type of position," Kent said. "It definitely took a lot of prior experience."

Kent's predecessor was part of the original team of scientists, engineers and technicians that was assembled more than three decades ago to build the test facility. It was built to support the construction of CEBAF, the Continuous Electron Beam Accelerator Facility. CEBAF is a DOE user facility that now supports the research of more than 1,850 nuclear physicists worldwide. 

When Kent stepped into the operations engineer role, he was only 27 — younger than the facility he was now tasked to help lead.

"It was very interesting to see drawings and paperwork and purchase requests that are older than me in my office," Kent said. "It was a challenge all over again. Taking over a very complex vertical test area is kind of overwhelming at first. Your eyes are pretty googly at the beginning and, as old as the system was, you don't really know what needs to be tackled first.

"But one of the things I devoted myself to pretty much my entire life is accepting near-to-impossible challenges and fighting through adversity at any point. I'm a very science/math-driven nerd." 

Putting Superconducting Radiofrequency Cavities to the Test

SRF cavities accelerate particles as they shoot through; as such, they make up the backbone of an accelerator. The cavities are strung together inside larger units called cryomodules. There, they are supercooled in a bath of liquid helium to 2 Kelvin, or about minus 456 degrees Fahrenheit (-456° F), to quash any electrical resistance. Finally, cryomodules are hooked up in a row to make an accelerator. Throughout the process, quality control is paramount.

"Physicists always demand absolute state-of-the-art, best-ever, never-before-seen performance," Harris said. "So, oftentimes, our testing criteria are set very aggressively." 

Before a test, a cavity must be preternaturally pristine. It is acid-cleaned, high-pressure rinsed and assembled in a clean room. Then it's placed inside a dewar, or a metal test vessel, that's supplied with liquid helium and pumped to sub-atmospheric pressures, plunging the temperature to 2 Kelvin. Once radiofrequency testing is done, the liquid helium is removed and purified, and the cavity is warmed back up. A full warm-to-warm cycle for a dewar is about three days.

One of the most common reasons a cavity may fail such a test isn't due to the cavity itself. Typically, it's because some particulate matter escaped the rigorous cleaning process.

The Vertical Test Area, commonly called the VTA, has eight dewars ranging from 16 to 34 inches across and 6 to 11 feet deep. These dewars can hold between 150 and 1,200 liters of liquid helium. The largest dewars can handle more than one cavity at a time, which helped achieve another facility record in 2022: six tests in one day. 

Cavity tests have been performed almost continually since the VTA was built, and engineers continue to boost its capabilities. The facility has three different stand-alone testing stations, for instance. Upgrades for some components are underway to build a fourth, so the VTA can more consistently test multiple cavities at once.

Since Kent came onboard, improvements include overhauling the facility's monitoring system to help coordinate and track operations, continuous upgrades to systems to help increase throughput, and designing and developing new magnetic shielding and cooldown methods so that the largest dewar can test up to three cavities at once.

About half of VTA operations supports the lab's own SRF research and development to design new cavity geometries, new coatings and other innovations to boost accelerator performance.

The other half is testing cavities and other SRF components for other facilities, such as the Large Hadron Collider upgrade at CERN and the multi-billion-dollar Electron-Ion Collider under development at DOE's Brookhaven National Laboratory in New York. 

Other projects that have benefited from the VTA's unique facilities include the Spallation Neutron Source Proton Power Upgrade project at Oak Ridge National Lab in Tennessee and the Linac Coherent Light Source II upgrade at the SLAC National Accelerator Lab in Menlo Park, California. LCLS is the world's brightest X-ray laser, and the upgrade will make it even brighter.

The Jefferson Lab team that built the VTA also lent its expertise to help build other testing facilities in this country and others, including at DESY in Germany in 1996, at DOE's Fermi National Accelerator Laboratory in 2006 and at Daresbury Lab in England in 2016.

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