US scientists build toolkit for improving particle accelerator’s performance

Scientists have developed a new toolkit to investigate Superconducting radiofrequency (SRF) cavity topography and its impact on performance.

SRF cavities form the backbone of advanced particle accelerators cavities and are part of the systems that power the electromagnetic fields that accelerate subatomic particles. The shape, roughness, and cleanliness of cavities’ inner surfaces contribute to their efficiency.

Developed by researchers at Thomas Jefferson National Accelerator Facility, the toolkit helps accelerator builders better monitor and control the characteristics of inner cavity surfaces.

In tests of the toolkit, scientists found that smoother SRF cavities function more efficiently. This means that the smoothness of the cavity surface indicates its performance, according to the study.

Toolkit built on decades of empirical research

Built on decades of empirical research in surface processing of niobium SRF cavities, the toolkit can also predict cavity performance by quantifying a cavity’s surface smoothness.

In the study , the team used the toolkit to investigate samples treated with the same recipe applied for cavities adopted by upgrade projects at the Linac Coherent Light Source, a Department of Energy (DOE) user facility. These upgrades are the DOE’s latest additions to its SRF accelerator fleet.

The toolkit includes scanning electron microscopy, secondary ion mass spectrometry, atomic force microscopy, and electron backscatter diffraction.

Researchers revealed that the grain boundaries, formed as the niobium metal is made, play a role in performance. Developed along grain boundaries after chemical processing of nitrogen-doped niobium, grooves can degrade the performance of the accelerator .

Grooves are found to degrade SRF cavity performance

Researchers claimed that the grooves are found to degrade SRF cavity performance because of the early breakdown of doped surfaces. Thus, a smoother surface would give better performance for higher fields.

“They also made new measurements of niobium samples prepared with a simplified oxygen-doping process. These cavity samples showed better topography. This indicates that controlling the surface smoothness and impurity profile may help boost performance both in high efficiency and high fields to help DOE’s future SRF accelerators, such as the Electron-Ion Collider (EIC),” said a press release .

Addition of contaminants to niobium cavities degrades efficiency

Niobium-made SRF cavities are the standard for efficient, high-power acceleration of particle beams. Their efficiency can be degraded when contaminants are added to niobium cavities.

But these enhanced cavities can’t withstand high-power operations as well as pure niobium cavities can.

Researchers studied the surface roughness of cavities with added nitrogen or oxygen.

Surface topography plays a crucial role in performance

“The result highlights the crucial role that surface topography plays in performance. It also hinted that oxygen would provide the cheapest gains in efficiency. The goal for the toolkit this research developed is to help accelerator scientists make better SRF cavities for future accelerators by controlling surface smoothness and impurities,” added the release.

Published in Physical Review Accelerators and Beams, the study investigated several representative surface treatments to test their methodology. They’ve found that it not only successfully predicts performance, but also points toward even better surface treatments not yet tested on a large scale.