Damping Down for More Efficient Accelerators

  • Frank Marhauser, in front of an SRF accelerator cavity
  • Design of single-cell accelerator cavity with dampers

New accelerator component under development could improve future particle accelerators and colliders

NEWPORT NEWS, VA – One of the ways that scientists reveal the fundamental building blocks of the universe is by smashing things together. Using particle accelerators, they accelerate bunches of charged particles – electrons, protons and ions – to a whisker below the speed of light. Then they run streams of these bunches into fixed targets or into each other in spectacular collisions. The resulting wreckage contains clues about what’s inside those charged particles and all matter.

Now, work underway at the Department of Energy’s Thomas Jefferson National Accelerator Facility aims to make the devices that make this research possible more efficient. Frank Marhauser, a Jefferson Lab senior scientist, is leading an effort to redesign aspects of particle accelerator components.

“You’d be improving the technology beyond what’s possible today,” Marhauser said of the new accelerators. He hopes to deliver a working prototype of a new particle accelerator component design in mid-2020.

Energy of a Higher Order

The particle beams in particle accelerators are powered by devices that concentrate and confine electric and magnetic fields. These devices are called accelerator cavities. Cavities have a fundamental, or resonant, frequency that is dictated by the shape and size of the cavities. The cavities figuratively ring - like an electromagnetic bell - with energy at their resonance frequency.

As charged particles move down the center of the tube and through the cavity, those strong fields inside the enclosure give the particles a kick. Repeated kicks make the particles go faster and faster and gain more and more energy, speeding their way to a collision.

However, those particles don’t just get pushed quietly along. The cavities can “ring” at a number of frequencies, and the particles may interact with the electric and magnetic fields inside the cavity and excite resonances at those other frequencies. As a result, the cavity may ring not only at the resonant frequency, but at these other frequencies, called “higher order modes.”

“A cavity should only be there for acceleration. But now you excite a lot of higher order modes. All of them are parasitic. All of them are bad. They create something you don’t want,” Marhauser said.

These modes, he said, could kick the beam off orbit, potentially killing it. Another possibility is that some power and heat could end up in the cavity walls. The most efficient accelerator cavities are made of superconducting niobium, which must be kept at about two Kelvin, or about 3.6o Fahrenheit, above absolute zero. Thus, too much heat raises the temperature and destroys superconductivity; this makes the superconductivity-dependent cavities stop working.

Scientists and engineers, therefore, put in components (called “dampers”) that extract the energy driving the higher order modes before it can do harm.

Marhauser’s innovation is to put these dampers directly on the cavity cell, attached through openings in the cavity walls. These dampers, which look a bit like rocket fins, are equally spaced around the cell to extract the energy generated by a variety of these higher order modes. 

According to Marhauser, numerical calculations and simulations done over several years show that this approach does a better job of getting rid of higher order modes than other solutions.

Toward Building a Prototype

Of course, there’s still the challenge of building the cavity. It could be machined from a block of special (and expensive) niobium. However, that, Marhauser pointed out, is wasteful of the niobium, because nearly all of it would end in scrap material.

Marhauser has been developing a new solution as part of a project funded by the Laboratory Directed Research and Development (LDRD) program, which provides resources for Jefferson Lab personnel to make rapid and significant contributions to critical science and technology problems of mission relevance to the lab and the DOE. The project has received funding for fiscal years 2019 and 2020.

Marhauser’s solution builds on Jefferson Lab’s expertise in forming thin niobium sheets through deep drawing niobium and electron-beam welding the formed structures together. Deep drawing involves putting the high-purity niobium through a press-driven die that forces it into the desired shape.  The approach minimizes niobium waste. A similar process, using aluminum or steel, is how your kitchen pots and pans are made.

The simplest approach to create the openings in the cavity walls for the dampers is to do it in one step. However, Marhauser found that forming the walls of the right depth in this way leads to premature material cracks where the material is being stretched.

Marhauser’s previous research and work on the LDRD-funded project has revealed that using multiple steps, i.e. using two or three smaller forming steps with heat treatment after each one, seems to solve the cracking problem.

Making the openings deep is critical to the whole design, because that means the electron-beam welds that attach the fins can be made away from where the most intense magnetic fields occur and prevents the welds themselves from also reducing the accelerating fields that can be achieved.

New Designs of Accelerator Cavities

While results have been promising so far, Marhauser is continuing with further testing of the special deep-drawing dies he has fabricated for forming the cavity wall openings. He also plans to continue tweaking the deep-draw-and-heat-treatment technique to fine-tune it and ensure it works well without introducing any killer problems.

In 2020, he plans to use the technique and his newly refined dies to fabricate a full-size test cavity to see if it performs as expected.

“The deliverable is a tested prototype accelerator cavity,” Marhauser said.  

If successful, accelerator cavities built or modified with this higher order mode damping technique could boost performance in particle accelerators.

Success, however, isn't guaranteed. It’s a situation that is tailor-made for investigation and prototyping through the LDRD program, a critical tool for directing the lab's forefront scientific research capabilities towards vital, excellent and emerging scientific challenges.

As Marhauser said, “That’s a good opportunity to do something that is risky but could transform the technology beyond the state-of-the art.”

By Hank Hogan and Kandice Carter

Contact: Kandice Carter, Jefferson Lab Communications Office, 757-269-7263, kcarter@jlab.org

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Jefferson Science Associates, LLC, a joint venture of the Southeastern Universities Research Association, Inc. and PAE, manages and operates the Thomas Jefferson National Accelerator Facility, or Jefferson Lab, for the U.S. Department of Energy's Office of Science.

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.