Higher operating temperatures will allow companies to cool their cavities with a small cryocooler device instead of a large, complex cryogenic plant - paving the way for greener technology.

A team of scientists at the Center for Bright Beams, a National Science Foundation Science and Technology Center, are working on the next generation of superconducting materials that will greatly reduce the costs associated with operating large particle accelerators.

Accelerators use superconducting radiofrequency (SRF) cavities, made from the element Niobium (Nb), to propel particles like electrons to nearly the speed of light. At superconducting temperatures, SRF cavities are nearly unencumbered by electrical resistance and contribute to areas such as medicine, industry, and basic research.

By making these cavities more efficient, researchers aim to lessen their environmental impact while also increasing the ability for smaller institutions and industry to use these critical research tools.

Particle accelerators are critical tools in the fight against climate change as they help strengthen new technologies such as lithium-ion storage capabilities and next-generation solar panels. However, the amount of energy required to operate some of these large machines is enormous, not only adding to the carbon footprint they seek to reduce, but also limiting their accessibility.

Their large consumption of power is a major hurdle for researchers, and is mostly due to one straightforward fact: the typical operating temperature of Niobium cavities is 2K, or - 456° F. In order to cool an accelerator cavity to these temperatures, it requires an intense amount of liquid helium based cooling (cryogenics), which is not readily available at smaller institutions.

At this rate, it is not feasible for industry, let alone smaller universities, to invest in SRF accelerator infrastructure needed for research and development. But imagine if they could.

Making accelerator technology more efficient can unleash a torrent of research. Higher operating temperatures could make an impact in not only energy development and storage but also medical treatments, large-scale sanitation, the semiconductor industry, as well as the critical endeavor of basic research.
The current energy requirements of the world’s largest particle accelerators are immense, explains Matthias Liepe, the head of the SRF cavity team at Cornell University, the lead institution of the Center for Bright Beams (CBB).

"Some of these really large machines that we’re dreaming about—they’re barely feasible with current technology," says Liepe. "You’d have to build a nuclear plant to run them, and that’s certainly not within what’s possible or [what] can be funded."

Liepe explains that the complexity, size, and power consumption of the cryogenics needed to operate these machines severely limits them to primarily large-scale, non-industrial accelerators. "Everything is reliant upon the performance and cost of the Niobium SRF cavities," says Liepe.

An interdisciplinary team of scientists at the Center for Bright Beams - from universities and national labs across the U.S. - are making significant strides in tackling this problem by revolutionizing SRF technology and employing next-generation materials, primarily Nb₃Sn.

The team is pursuing a more materials science-based approach to push RF performance of materials well-beyond the current level. CBB has already laid the foundation for further exploration of compound superconductors for SRF use, especially with Nb₃Sn, and has demonstrated that significant progress is achievable using its interdisciplinary skillset and tools.

"Niobium has been the state-of-the-art SRF material for decades due to its very good performance. And since it is a pure metal, it is fairly easy to make these cavities," says Sam Posen, former Cornell SRF graduate student and now Scientist and Deputy Division Head at Fermilab, and faculty at CBB. "While Nb₃Sn is a specific mixture of Niobium and Tin, it is somewhat harder to make cavities, but it can be very much worth the effort in a number of applications."

Posen explains that Nb₃Sn turns superconducting at 18 Kelvin, allowing researchers to operate SRF cavities at much higher temperatures. This significant jump in operating temperatures can drastically alter the industrial landscape.

"The higher operating temperature will allow a company to cool their cavities with a small device called a cryocooler instead of a large, complex, and difficult-to-maintain cryogenic plant"

"The higher operating temperature will allow a company to cool their cavities with a small device called a cryocooler instead of a large, complex, and difficult-to-maintain cryogenic plant," says Posen. "This could enable new industrial particle accelerator applications, in domains like isotope production, wastewater treatment, and tools for the semiconductor industry."

High-efficiency SRF cavities employing CBB’s next-generation surfaces will help overcome the cost barrier of future large-scale science accelerators and will make a critical contribution towards sustainable science, thereby advancing the energy and intensity frontier of science.

Within the first four years of the Center, CBB researchers have developed powerful theoretical and experimental tools for material screening and RF performance testing of samples and have used these tools to demonstrate promising material and growth options for further development including not only Nb₃Sn, but also NbN, and NbTiN.

CBB has made key contributions to reducing excess surface resistance of state-of-the-art, vapor-diffusion Nb₃Sn films and has demonstrated that vortex entry at grain boundaries, enhanced by overall surface roughness, is the likely source of premature failure of superconductivity (quench) of these films. Posen points to this cross-collaboration of CBB as a main reason for the Center’s success, as CBB brings together experts in a range of fields, not just experimentalists, but also theorists who help better understand the results and guide improvements to the cavities. This collaboration between experiment and theory is not only applied to the acceleration of particles but also the production and transport of electron beams researched within CBB.

CBB has used this knowledge to develop improved Nb₃Sn nucleation (growth), coating, and post-processing procedures, which have already halved surface resistance. Nb₃Sn has now become the first-ever usable alternative to Nb for SRF cavities, enabling ~2x higher temperature operation and ~70% (~3x) lower cooling power needs, compared to clean bulk Nb.
Significant challenges remain in order to bring Nb₃Sn to full fruition. Energy losses must be reduced even further and higher accelerating fields would increase the range of applications. Other promising compound superconductors and structured layers should also be explored.

While these feats are being pursued, Posen says that CBB scientists and others will continue to build on the work that has come before them. Much of this research has been conducted over the past decade at Cornell University with funding provided by the Department of Energy (DOE). This funding allowed researchers Liepe and Posen to achieve a first ever demonstration of a high-performance Nb₃Sn coated SRF accelerator cavity.

In the longer term, the CBB team hopes that Nb₃Sn cavities could be used for high-energy linear colliders, as Nb₃Sn is predicted to withstand higher electromagnetic fields than niobium. "So far this hasn’t been demonstrated yet," says Posen, "but we’ve been making steady progress towards this exciting goal."