Smoother Surfaces Make for Better Particle Accelerators

A closeup view of a the silver colored LCLS-II SRF cavity

Researchers have long thought pure niobium superconducting radiofrequency cavities were best for particle accelerators. Researchers are now using a toolkit to learn how to add impurities to the cavities to make them smoother—and more efficient.

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The Science

Superconducting radiofrequency (SRF) cavities form the backbone of advanced particle accelerators. SRF cavities are part of the systems that power the electromagnetic fields that accelerate subatomic particles. The cleanliness, shape, and roughness of the inner surfaces of these cavities contribute to their efficiency. Scientists have developed a new toolkit to help 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. The toolkit can also predict cavity performance by quantifying a cavity’s surface smoothness.

The Impact

SRF cavities made of niobium are the standard for efficient, high-power acceleration of particle beams. Adding contaminants to niobium cavities can further enhance their efficiency. But these enhanced cavities can’t withstand high-power operations as well as pure niobium cavities can. This research studied the surface roughness of cavities with added nitrogen or oxygen. 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.

Summary

Particle accelerator scientists have developed a novel toolkit for investigating SRF cavity topography and its impact on performance. The toolkit was built on decades of empirical research in surface processing of niobium SRF cavities. In this work, 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.

Their study revealed that the grain boundaries, formed as the niobium metal is made, play a role in performance. Grooves develop along grain boundaries after chemical processing of nitrogen-doped niobium. Atomic force microscope measurements combined with an algorithm based on differential surface geometry predict a suppression factor of the superheating field due to these grooves. The grooves are found to degrade SRF cavity performance because of early breakdown of doped surfaces. Thus, a smoother surface would give better performance for higher fields. The researchers 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).

Contact

Eric Lechner
Thomas Jefferson National Accelerator Facility
lechner@jlab.org

Funding

This material is based on work supported by the Department of Energy Office of Science, Office of Nuclear Physics, by an Office of Nuclear Physics Early Career Award, and by the DOE Office of Science Office of High Energy Physics.

Publications

Lechner, E.M., et al., Topographic evolution of heat-treated Nb upon electropolishing for superconducting rf applications. Physical Review Accelerators and Beams 26, 103101. [DOI: 10.1103/PhysRevAccelBeams.26.103101]