Oxygen tweaking could be the key to accelerated adaptation

Oxygen tweaking could be the key to accelerated adaptation


Newswise – Newport News, VA – Particle accelerators are expensive, but their cost comes with good reason: These one-of-a-kind, state-of-the-art machines are intricately designed and built to help us solve mysteries. Till our universe. Still, the scientists and engineers who build these machines should do their best to save where possible. Researchers at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility are supporting the mission by exploring how to optimize the cavities, one of the most important parts of the accelerator.

The cavities are tubes made of niobium, a metal that becomes superconducting at extremely cold temperatures, allowing the cavities to conduct large electromagnetic fields which they channel to efficiently move the particles. Higher fields in the cavity mean that the overall accelerator can be smaller.

“Your accelerator may only be 10 miles long instead of 20,” said Charles Ries, a senior accelerator physicist who retired from Jefferson Lab last year. “That’s real estate. That’s a huge cost savings.”

However, the area of ​​a cavity cannot be changed infinitely without consequences. If it is too high, the cavity will overheat and lose its capacity. superconductivity, To produce cavities that support the highest velocity fields, laboratories use different recipes to prepare niobium. For example, a process developed at Fermi National Accelerator Laboratory bakes the cavities at 300° Celsius.

“Using this process, they saw an increase in the performance of their cavities, but no one really understood what was happening,” said Eric Lechner, a staff scientist at Jefferson Lab. In work published three years ago, Lechner, Rees and other researchers examined samples prepared following this recipe using secondary ion mass spectrometry, which allowed them to study the surface structure of niobium.

They found that oxygen was being inadvertently introduced. When a pure niobium cavity is exposed to air, oxides form on its surface. During heating, the oxides disintegrate, and their constituent oxygen atoms dissolve into the cracks of the niobium crystals up to a few micrometers deep.

After that, Jefferson Lab accelerator scientists began developing a mathematical model that describes this oxygen diffusion. In work published in April Applied Physics JournalThey further expand and validate this model, which has since matured to predict how more sophisticated recipes affect oxygen diffusion and cavity performance.

“This model explains how the original oxide on the surface of niobium disintegrates and diffuses into the surface as a function of temperature and time,” Rees said. “We can use this to precisely tailor surface preparation to achieve the best possible and most reliable performance.”

Wide access to recipes

Previously, the model described a 300°C vacuum heat treatment. During that process, only niobium pentoxide breaks down onto the niobium surface. However, higher temperatures, including longer baking times at 300°C, are also commonly used to prepare niobium cavities where the excess oxide components decompose.

Lechner prepared samples of niobium according to these other recipes. Secondary ion mass spectrometry, which was conducted by former Virginia Tech graduate student Jonathan Engle, showed that the model could capture the essential features of oxygen migration in these more complex vacuum heat treatments.

with funding from DOE Early Career Award from the Office of Nuclear Physics, the researchers extended the model to describe variations in superconducting properties due to oxygen content introduced during surface preparation. They applied the model to previous experiments to link oxygen content with resulting cavity performance.

Low temperature baking typically involves heating a cavity to 120°C for 24 to 48 hours. This type of tip has been used for the past 20 years because it produces cavities that can handle high fields – but why it works remains a mystery.

“we asked ‘“Can we use our model to investigate whether oxygen diffusion is related to this phenomenon?” Lechner said. The team compared the model to previous low-temperature experiments, and they agreed well, suggesting that oxygen diffusion is indeed behind the performance boost.

Further analysis allowed the researchers to link variations in surface oxygen content with the extreme area of ​​the cavity. The results led researchers to believe that oxygen was changing the behavior of niobium to prevent the formation of magnetic vortices in the material that can form at high fields. These magnetic vortices generate heat, which limits the performance of the cavity. The oxygenated niobium allows the fields to rise higher without creating these vortices and generating too much heat.

“This work sheds light on the potential mechanism behind low-temperature baking, which has remained a mystery. Our modeling suggests where to look for additional experimental confirmation of this hypothesis,” said Lechner. “There are other materials that are being developed for particle accelerator cavities, and understanding this phenomenon should translate into those as well. Could.”

predictive power

In addition to explaining why previous recipes worked, the model shows how they can be improved.

“We have made significant progress in understanding the physical characteristics that allow us to gain some predictive power,” Rees said. “We understand enough now to make predictions. This could lead to huge savings in accelerator construction.”

Teams preparing cavities for various accelerator projects can use the model to develop a process that will achieve the desired properties. These processes may involve preparing the initial conditions, such as deliberately adding a specific type of oxide to the surface of niobium. The model also suggests that more oxygen dispersed deeper into the niobium would better prevent the formation of vortices.

“For baking at lower temperatures, our model suggests that if you can fill the surface with oxygen, you may be able to get better performance,” Lechner said.

Niobium processing is expensive and specialized; Only a few places in the world can do this. Accelerator scientists hope to one day replace the niobium cavities entirely with copper cavities coated in a thin film of niobium using deposition techniques.

“This work, which describes the dissolution of oxides in thin films of niobium, shows how you would do that,” Rees said. “We have a research program at Jefferson Lab that has been struggling for a long time to develop the technology to do this, and they are making progress.”

In the meantime, the researchers hope their model will help replace recipes for cavities in future experiments.

Further reading
Accelerator can get a boost from oxygen
Smoother surfaces make better accelerators
Creating a new theory for better accelerators

by chris patrick

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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 Southeastern Universities Research Association, Inc. (SURA) is a wholly owned subsidiary.

DOE’s Office of Science is the 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. Visit for more information https://energy.gov/science

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