• Number 378  |
• December 17, 2012

Mirrors may reflect good performance

Just as the shiny baubles and lights have begun to appear for the winter holidays, so have the first mirror-like accelerator components at DOE's Jefferson Lab. Researchers have buffed the interiors of some accelerator component prototypes to a high-mirror shine in hopes of finding a more environmentally friendly method for manufacturing ever-more-efficient accelerator components.

The research is being led by Ari Palczewski, a Jefferson Lab staff scientist. He's experimenting with the process of cleaning and preparing the surface of accelerator components made of niobium, a rare metal. The components, called cavities, are designed to harness and focus the energy used to accelerate a beam of particles. Research has suggested that the most efficient accelerator cavities have interior surfaces that are both smooth and defect-free.

"There's this thing called the quality factor, which tells you how efficient the cavity is. That may be able to go up with a smoother surface," Palczewski explains.

Currently, a niobium accelerator cavity's smooth surface is achieved with a long process that includes a heavy acid wash called electropolishing. The process involves a high-pressure rinse, a high-temperature bake and a second, though lesser, session of electropolishing, rinsing and baking. In the electropolishing phases, a mix of caustic acids and electricity are used to strip away a thin layer of the interior surface of the cavity, exposing a new, smoother surface. This process is the current baseline procedure for producing the so-called high-gradient cavity surfaces required for the International Linear Collider, a project that’s in the planning stages. Through a globally coordinated R&D effort, this process has been made repeatable and reliable across all three regions in the world over the past several years. 

The alternative process Palcezwski is working on aims to replace the heavy electropolish with a far more environmentally friendly mechanical polish method. This method consists of several steps of filling a cavity with polishing media of ever-finer grit, such as ceramic, wood and a sand-like powder, and then tumbling the cavities in a barrel-shaped centrifugal spinner.

"Usually, a cavity goes through a heavy electropolish, high-temperature bake, and then a light electropolish. Barrel polishing can eliminate one of the electropolish steps, so it's more environmentally friendly," he says. "It's also safer, because it's just rocks - it's not anything harmful."

This method was first successfully demonstrated by a group led by Kenji Saito at KEK, a Japanese national laboratory. The extension of this method to yield a mirror finish was pioneered by Charlie Cooper, a staff scientist in the Fermilab SRF Materials Group. Cooper applied the new method to prototype cavities for the ILC.

Palczewski is also working with prototype ILC cavities. He first attempted the mechanical polishing process on a single-cell cavity, which features the prototype shape of an ILC cavity, but contains only one accelerating cell. A full prototype ILC cavity contains nine cells. 

"We started with Cooper's idea and improved upon it. We modified the process to make it cheaper, while requiring less cycles, and we think we get a better polish with less man-hours. We mechanically processed two single-cells, and we showed that our recipe for removal rates and smoothness worked," Palczewski explains. "We wanted to move up to multi-cells, because we use multi-cells in accelerators."

An opportunity soon arose to try out the improved cavity processing on a multi-cell ILC cavity. It involved one of the first nine-cell cavities produced by Niowave, an industrial vendor in the U.S. producing ILC prototype cavities, according to Rongli Geng, who leads the ILC prototype cavity processing and testing effort at Jefferson Lab.

The cavity failed to reach its performance goal during testing, and Geng and Palczewski worked to find a reason for the cavity's failure. They eventually found a pair of defects on the cavity's interior surface that had not been removed in its prior processing.

"It went through the standard processing for an SRF cavity," Palczewski says. "It was limited by defects in cell 5. Cell 5 is in the middle."

After identifying and documenting the location of the defects, Palczewski and Geng determined that they might be removed by a mechanical buff.

Palczewski filled the cavity with polishing media and placed it inside the large barrel polishing machine. The piece was then spun inside the polisher through four cycles, using four different sizes of polishing media for 110 hours to buff out the defect and smooth the interior surface.

"All of this is standard, off-the-shelf, buffing media that we're now using for niobium cavities," he says.

The result was an accelerator cavity sporting a surface with a high-mirror shine. 

"This is the very first full-scale cavity processed by this alternative process at Jefferson Lab," Geng says. He and other researchers have high hopes for the now-shiny cavity.

"There are theoretical expectations that smoothness could be beneficial for better performance," Geng explains. "There are debates as to whether an optically smooth surface means good performance; having this new procedure now available in-house allows us to research the topic."

Performance testing of the Niowave cavity is expected to resume as soon as it can be scheduled in the lab’s Vertical Testing Area, a facility used to test the performance of accelerator components.

In the meantime, the researchers are looking forward to applying the new cavity processing to the surfaces of other cavities. Prior to Palczewski's work, other colleagues at Jefferson Lab already applied the mechanical polishing method for various R&D cavities. Now, it's expected that use of the method will boost Jefferson Lab cavity processing expertise to a new level.  

"So part of this research is to prove and to optimize the process. We're trying to make it available for any needs of the community. So, the next project that comes up, we want to be able to use this to replace heavy chemistry - the heavy electropolish," Palczewski says.

Geng agrees. 

"This started from the ILC R&D. We're hoping to continue R&D, but we're also hoping in the meantime, that this will benefit other projects. The technique is now available in-house, for future R&D in new cavity shapes," he says.

Submitted by DOE’s Thomas Jefferson National Accelerator Facility

For More Information:

ILC Treatment of JLab Cavity Garners Excitement
Reaching New Heights in Accelerator Technology