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You Visit Tour. Webb Lion Fountain. June 1 2017. Photo David B. Hollingsworth

DOE Invests $1.5 Million in CAS Particle Accelerator Innovation

Niobium has been a superconducting superstar in the world of particle accelerators for the past half century, but there is good evidence that advances in resonator cavity technology has put this metal near the fundamental limit of its performance, and that the march toward ever more powerful and efficient accelerators will require new superconducting materials.

This is a challenge that Old Dominion University physicist Alexander Gurevich has been working on for eight years, and now the U.S. Department of Energy has invested $1.5 million in the proof of principle of his basic theory that niobium may not need to be replaced, but rather "refaced" in the next generation of the most powerful accelerator cavities.

Gurevich has more than 20 years of research experience in theoretical investigations of superconductors under the extreme conditions of high direct currents and magnetic fields and high radio frequency (RF) fields. He joined ODU in 2011 as a professor of physics and an affiliate of the Center for Accelerator Sciences (CAS), which ODU formed in 2008 in conjunction with the Jefferson National Accelerator Facility in Newport News.

Jean Delayen, ODU professor of physics and director of CAS, is co-investigator on the three-year DOE grant. He has more than 40 years of experience in the physics and technology of superconducting particle accelerators.

"When we recruited Professor Gurevich, it was my opinion that his work would be highly synergistic to that of Professor Delayen and others at the Center for Accelerator Sciences," said Chris Platsoucas, dean of the ODU College of Sciences. "Now we have a most significant grant from the Department of Energy to show that this synergy indeed exists."

Added Charles Sukenik, chair of the ODU Department of Physics: "The project demonstrates the highly interdisciplinary nature of the research at CAS. Here accelerator science and condensed matter physics are blended at the forefront of science and technology. It is very exciting for everyone involved and provides a wonderful opportunity for our faculty, postdocs and students to collaborate at the cutting edge of basic and applied research."

Niobium is never found as a free element, but instead is derived from mineral compounds mined mostly in Brazil and Canada. It is an attractive, shimmering gray metal that is gaining popularity for its use in expensive jewelry, and a little of it goes a long way in the making of strong steel alloys. Also for more than 70 years it has been known than niobium goes to the superconducting state, in which it exhibits no resistance to the direct electric current, upon cooling below the critical temperature of 9.2 degrees of Kelvin. It is this last feature that makes superconducting materials invaluable in particle accelerators.

A typical particle accelerator, such as the linear accelerator called a "linac," consists of a series (sometimes many thousands) of RF niobium cavities that power electrons or other particles along a course at speeds that can approach the speed of light. Some of these accelerators are also powerful light sources -free electron lasers - that can be used by industry. Others are becoming more and more useful in medical diagnostic and therapeutic instruments. And the largest of these accelerators provide the atom smashing and particle collisions that allow nuclear physicists to explore the fundamental nature of matter and the origins of the universe.

"With niobium cavities in accelerators, technology advances over the last 50 years have been amazing," said Gurevich. These advances come from eliminating impurities in the metal and from defects management. "Now we are close to the theoretical limit, and the question is, 'Can we do better than niobium?'"

Advances in niobium superconductivity over the years have pushed the accelerating electric field in state-of-the-art niobium cavities to over 50 million volts per meter, but Gurevich says developers of next-generation accelerators would want to double the strength to 100 million volts per meter. The power leap is most important for research done by nuclear physicists at particle accelerators.

"The problem is, how much power can you dump in the cavity before you destroy superconductivity?" Gurevich added.

Indeed, as the peak magnetic field at the equatorial cavity surface exceeds the so-called lower critical field, quantized vortices, which can be viewed as tiny nanoscale tornados of electrons, start penetrating in the cavity, causing significant dissipation and eliminating the main advantage of superconductors. Eventually, the cavity quenches and becomes nonsuperconducting as the peak RF magnetic field reaches the breakdown field, about 200 millitesla for the best niobium cavities.

In addition, users of accelerators, most notably in industry and medicine, are calling for more efficient instruments. For example, cavities made from a material with a critical field potential higher than niobium could cut the number of cavities needed for an accelerator, streamlining its size.

Users are also eager to cut the cost of operating accelerators. At present the liquid helium refrigerant costs are very high to cool niobium cavities and magnets down to 2 Kelvin (about minus-456 degrees Fahrenheit), at which the RF power dissipated in the niobium cavities is about a million times lower than in good non-superconducting metals like a clean copper.

Many superconducting materials , most notably triniobium-tin or magnesium diboride, have critical temperatures more than twice that of niobium, which could significantly reduce the RF dissipation and increase the breakdown fields of accelerator cavities. At the same time, the RF dissipation power in these materials at 4 Kelvin could be as low as RF dissipation in niobium at 2 Kelvin, which could present a significant refrigerant cost savings. "Move from 2K to 4K, and suddenly this technology can become a lot more affordable, comparatively speaking," Gurevich said.

So why not build cavities out of magnesium diboride or some other new superconducting metal?

Here, according to Gurevich, is where it becomes abundantly clear why niobium became such a superconducting superstar in the first place. Despite being outperformed by new materials in several performance/cost categories, niobium has a huge advantage over other superconductors in another category, namely the lower critical field. As a result, all non-niobium superconductors suffer from dissipative penetration of vortices at magnetic fields much lower than 200 millitesla, which does not allow achieving high accelerating electric fields.

To address this problem, Gurevich first proposed in a 2006 research paper his theory of "refacing" cavities. He suggests that very thin multilayers - alternating between superconducting metals with higher critical temperatures and magnetic fields and layers of dielectric materials - be deposited onto the surface of niobium cavities. In essence, he projected a "best of both worlds" solution in which the basic niobium construction with the multilayered high critical field surface would block dissipative penetration of vortices while producing more powerful, more efficient accelerators.

Subsequent tests at the Argone National Laboratory in Illinois, Los Alamos National Laboratory in New Mexico, Saclay Nuclear Research Centre in France and the College of William and Mary have demonstrated the feasibility of his approach. In some of those tests, the multilayers were deposited onto the inner surface of niobium cavities by a process known as atomic layer deposition, which can add layers that are only the depth of one atom.

"Our theory shows that one could double the breakdown field of niobium cavities if they are coated with these multilayers," Gurevich said. "Since then, several labs have tried it and the results look very promising. But we must decide what will be the best multilayer materials and how we can optimize RF performance. It has been trial-and-error so far, and although this is a good first step, in the end we want a multilayer technology that could be better understood and tuned in the right direction."

The DOE project will involve three years of research and development in which Gurevich, Delayen and their colleagues at the CAS will evaluate single layers and multilayers incorporating high performance materials such as magnesium diboride, triniobium, niobium nitride and the diverse family of the recently discovered iron pnictides.

Creation of the thin film and multilayer samples for the research will be the task of two noted experts in materials sciences, professors Chang-Beom Eom of the University of Wisconsin and Rosa Alejandra Lukaszew of the College of William and Mary, who are subcontractors on the project.

The materials will be tested and characterized at the CAS on the ODU campus and at Jefferson Lab, where measurements of multilayer performance will be made in specially designed niobium cavities. Delayen, an inventor of new-generation cavities, has developed measurement techniques and equipment that will be used in these experiments. .

"This is long-term R&D," said Gurevich. It took 50 years to get the most from niobium. Hopefully it won't take that long for us. Our job is to understand the fundamental RF physics and materials science of new superconductors and demonstrate the potential of the multilayer technology .

"Can multilayers change the game, doubling the accelerator power gradient while also operating with savings at 4K? We hope our technology can address both."

The Gurevich grant is the second that DOE has awarded to CAS researchers in the past 15 months. Lepsova Vuskovic, Eminent Scholar and professor of physics, leads a CAS team that received nearly $600,000 last year to pursue a plan to improve the efficiency of particle accelerators.

Vuskovic's grant is titled "Plasma Processing of Superconducting Radio Frequency (SRF) Cavities for the Next Generation of Particle Accelerators." Her co-investigators are two other ODU physicists, Alexander Godunov, associate professor, and Svetozar Popovic, research professor, as well as two researchers from Jefferson Lab, Larry Phillips and Anne-Marie Valente-Feliciano.

The team's innovations with plasma processing include new and safer ways to keep tabs on and eliminate cavity surface "bumps in the road" that can impede an accelerator's efficiency. In addition to its potential to improve existing accelerator technology, the work is expected to make it easier for scientists and engineers to design the next generation of accelerators.