Jefferson Lab Dedicates Niobium-tin Particle Accelerator Prototype

NEWPORT NEWS, VA – More than 30 of the world’s most advanced particle accelerators for research are built on one technology: superconducting radiofrequency (SRF) technology. It’s typically powered by base components made of pure niobium metal, but recent advances have shown that parts coated with a niobium-tin alloy may allow for greater efficiency.

Now, the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility and its partners are preparing to surge forward into the next era of SRF with tests of a new accelerator based on niobium-tin alloy technology. The team has successfully tested the first niobium-tin alloy cryomodule, a prototype section of particle accelerator, that is capable of accelerating electrons to energies exceeding 10 million electron-volts (10 MeV). The cryomodule was recently dedicated as Gray Enid I and is now being prepared to accelerate its first electron beams.

According to Grigory Eremeev, a former Jefferson Lab staff scientist who is now a senior scientist and deputy division director of SRF at DOE’s Fermi National Accelerator Laboratory, Gray Enid I is the culmination of decades of research that was propelled forward with a grant from DOE’s Early Career Research Program.

“This cryomodule was built with my early career award and partnerships among researchers and national labs,” Eremeev said. “I hope it works well in the upgraded injector test facility and that it does what we hope it will do. This would be a significant milestone.”

At a recent internal event to dedicate the new cryomodule, Michelle Shinn, DOE’s program manager for Industrial Concepts in the Office of Science, Office of Nuclear Physics, remarked that achieving this milestone was a real challenge.

“To actually not just coat a cavity, but coat two cavities, put them into a cryomodule, fully checked, fully ready, put them in a beamline and accelerate electrons – that’s gutsy,” said Shinn.

Superconducting Radiofrequency Particle Accelerators

The metal niobium is widely used for many different applications. Typically alloyed with other metals, it can strengthen and corrosion-proof building materials and pipelines, help improve the efficiency of solar cells, and help give jewelry a rainbow shine. This silvery metal is also a boon to applications that require superconductivity. Niobium loses its resistance to the flow of electricity at low temperatures, making it particularly useful as a base superconductor in high-powered magnets and quantum computers.

In particle accelerators, specially formed niobium components are typically chilled to just a few degrees above absolute zero, around 2 Kelvin or minus 456 F. These components, called cavities, are shaped somewhat like a stack of donuts with a hollow tube connecting them through the middle. The exact shape of each donut in the tube is determined by the type of particles the accelerator is designed to accelerate.

Particles travel through the center of the hollow tube as the niobium cavities are pumped with radio frequency waves of energy. The cavities harness and impart that energy onto the particles, thereby “accelerating” them. When superconducting, the cavities can store energy with almost no losses, allowing the structures to accelerate a continuous beam of particles.

Jefferson Lab’s Continuous Electron Beam Accelerator Facility was the first large-scale linear accelerator to use this technology. Because of its efficiency while delivering beams of high-energy-precision, CEBAF has been used to conduct many experiments in the nucleus of the atom that weren’t thought possible before, and a recent upgrade of the machine has taken advantage of new technology advances, yielding even more efficient accelerator cavities. Today, CEBAF is a DOE Office of Science user facility that supports the research of more than 1,650 nuclear physicists worldwide.

The successful demonstration not only set the stage for a revolution in nuclear physics research capabilities, it also set accelerator science onto the path of SRF, leading to future research machines based on the now-tested technology. Since those early years, Jefferson Lab has continued to provide leadership in SRF particle accelerator research and development, fabrication of new accelerators, and improving operations.

The result of this work can be seen in the successful use of SRF particle accelerator technology in several unique and world-leading research machines, such as the recently upgraded Spallation Neutron Source at DOE’s Oak Ridge National Laboratory for which Jefferson Lab provided the design and construction of new cryomodules. It can also be seen in the Linac Coherent Light Source, an X-ray free-electron laser for research at DOE’s SLAC National Accelerator Laboratory for which Jefferson Lab supported the development of new cavity processing recipe and provided construction of a significant portion of the cryomodules. With national lab and industry partners, Jefferson Lab played a key role in building both of these state-of-the-art research machines.

Several decades of operational experience have now shown that today’s machines are reaching the limit of efficiency that can be gained with niobium alone. Researchers have now focused on alternate materials, such as niobium-tin, updated fabrication processes and improving technologies to continue upgrading this successful technology for use in research and beyond.

Advancing Particle Accelerators with Prototypes and Partnerships

Now, SRF particle accelerators are poised to move beyond the laboratory and into the mainstream. With recent improvements to the technology’s efficiency and design, these machines are being developed with advanced materials and off-the-shelf technology for use in disciplines such as manufacturing and environmental remediation.

One of these improvements is by using alloys of niobium with other elements to increase its superconducting prowess. The expectation is that these niobium alloys will allow cavities to better dissipate heat generated in operations or enable cavities to maintain superconductivity at higher temperatures, requiring less electricity for intense refrigeration.

Tin is another shiny metal that becomes superconducting when cooled to low temperature. In Gray Enid I, the researchers used a furnace system to vaporize tin inside a standard niobium accelerator cavity. At temperatures greater than 1,100 C, tin atoms bind with niobium, resulting in the formation of a thin niobium-tin (Nb₃Sn) layer on the surface of the cavity. Niobium-tin is an intermetallic alloy that transitions into a superconducting state at 18.3 Kelvin, approximately double the superconducting transition temperature of niobium.

The researchers started the cryomodule by employing this vapor diffusion technique to create a thin niobium-tin layer on the internal surfaces of two SRF accelerator cavities.

The next step was testing to ensure the cavities were capable of particle acceleration. In a vertical testing phase, the cavities were cooled to 4.4 Kelvin and ramped up with SRF fields to see if they could hold the fields needed to accelerate electrons. The test was repeated at 2 Kelvin. The cavities did not perform well in tests.

“We started with some original CEBAF cavities that we still had available, but we realized that we couldn’t get a good coating on the older surfaces,” Eremeev said.

What’s more, these two initial prototype cavities demonstrated in testing that the order of assembly for this new type of coated cavity is important.

Typically, a niobium cavity would undergo full assembly and final “fine tuning” of its frequency before testing. All cavities require a tuning phase, where they are “tuned” to a particular frequency. To “tune” the cavity, it is mechanically deformed until it focuses the SRF energy at the frequency it is designed for. However, the researchers found that deforming the cavity would also warp the carefully crafted niobium-tin surface.

The researchers delayed the project as they worked out a solution to the issue. They surmised that tuning the cavity before the niobium-tin coating was added, as well as some other process improvements, would alleviate this issue.

“We went and bought some new cavities from the vendor. These new cavities had a good niobium substrate to work with,” Eremeev said.

According to Uttar Pudasaini, a Jefferson Lab SRF scientist who worked with Eremeev on the project, the new set of cavities worked much better.

“We found that older coated cavities aren’t able to dissipate the heat quite as well as pure niobium cavities of the same design, but the new cavities deliver the performance that we were hoping for,” said Pudasaini. “They are able to remain superconducting at higher temperatures.”

A Quarter Enables 10 MEV

The successful tests with coated cavities encouraged the researchers. To get a better idea of how they would function in an actual accelerator, the next step was to install the cavities into a cryomodule for testing.

A cryomodule is a section of SRF accelerator. It includes all of the components needed to support, insulate and monitor the accelerator cavities, once the cryomodule is installed into an accelerator. In CEBAF, a full cryomodule holds eight cavities, whereas a one-quarter cryomodule holds two. Throughout its operational lifetime, CEBAF has maintained a one-quarter cryomodule for initial acceleration of its electron beams.

Once assembled, the one-quarter cryomodule was then put through its paces in Jefferson Lab’s Cryomodule Test Facility. The cavities were again cooled to 4.4 Kelvin and 2 Kelvin for testing with SRF waves. As expected, the two cavities together demonstrated the capability to accelerate electrons to greater than 10 MeV at both temperatures.

“It basically got similar results to what we saw in the vertical tests,” Eremeev said.

Operating an accelerator at 4.5 Kelvin versus the current 2.1 Kelvin, with the same RF heat load, can have a significant impact on both the complexity of the cryogenic plant as well as the power used to operate it.

If the result holds in electron beam tests, it would signal a new era in which future SRF accelerators may be designed to function reliably at 4 Kelvin, rather than the current requirement of 2 Kelvin. This reduction in necessary cooling capacity could lead to more efficient cryogenic supply plants that are designed from the ground up to accommodate the less demanding needs of this type of technology.

What’s more, these tests proved that the cryomodule is the first capable of delivering greater than 10 MeV electrons. This is an important threshold for accelerators, enabling applications such as cancer treatment, sterilization of medical devices, and bulk treatment of wastewater to remove forever chemicals.

Moving Forward with Gray Enid I

Now, the researchers are looking forward to installing Gray Enid I into a small accelerator to test how well the cryomodule accelerates electron beams for research. The one-quarter cryomodule will be installed in Jefferson Lab’s Upgrade Injector Test Facility.

The UITF is a testbed for accelerator technologies. Its focus is on technologies that will be installed in the portion of CEBAF that produces the electron beams that are accelerated in the machine, such as CEBAF’s one-quarter cryomodule.

“Gray Enid I is named after earlier technology installed here in CEBAF in 1990,” said Rongli Geng, who heads the SRF Science & Technology department in Jefferson Lab’s Accelerator Division. “Gray Ghost II was the first cryomodule that was installed in CEBAF in the injector region. And so, this new unit is named and based on how they made the Gray Ghost.”

The success of this new cryomodule is the culmination of four decades of investment in the SRF innovation at Jefferson Lab by the DOE’s Office of Science, Office of Nuclear Physics and tireless efforts by three generations of the lab’s SRF practitioners. 

“If there is a will, there’s a way. After all the hard work and overcoming obstacles, it’s good to see this project finally coming to fruition,” Eremeev said.

Further Reading
Award Enables Research for More Efficient Accelerators

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Jefferson Science Associates, LLC, manages and operates the Thomas Jefferson National Accelerator Facility, or Jefferson Lab, for the U.S. Department of Energy’s Office of Science. JSA is a wholly owned subsidiary of the Southeastern Universities Research Association, Inc. (SURA).

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science