A joint Fermilab/SLAC publication
Dirk Nölle, DESY

Accelerator, stonewashed


Taking a page out of the jeans manufacturers' playbook, researchers use stonewashing machines to perfect equipment for a future International Linear Collider.

Quite often it’s basic research that pioneers new technology. But in one case, researchers might very well have benefited from a little inspirational snoop at Levi’s or another jeans manufacturer.

Scientists at the German laboratory DESY are currently using stonewashing equipment to surface-treat specialized superconducting cavities used to accelerate particles: accelerators stonewashed, so to say. This technique originated at KEK in Japan. It has been further developed at laboratories around the world, including Fermilab, Jefferson Lab, and Cornell University in the United States, DESY in Germany, and the Raja Ramanna Centre for Advanced Technology in India.

While jeans are only stonewashed for show, there is a fundamental scientific reason to do this with the accelerator cavities. The cavities, which create electromagnetic fields used to accelerate particles, work best when their interiors are as smooth as a mirror. One way to achieve this is to repeatedly polish the cavities' insides.

In this way, a group of scientists from DESY and the University of Hamburg is working to produce test cavities for the proposed International Linear Collider.

“We want to reach gradients of 35 megavolts per meter and more for the ILC. This requires a surface that is very clean and smooth up to a few nanometers,” says Aliaksandr Navitski, who conducts the testing. 

The stonewashing treatment has four stages. The cavities are filled with different polishing granules and rotate on two axes—a bit like the Earth rotating around the Sun and also around itself—for hours in a type of cement mixer. The first filling, a mixture of stones and water, eliminates impurities that develop mainly in areas where individual half cells were welded together. It removes ten micrometers per hour from the surface. After eight hours, the mixture is exchanged: Fine stone granules melted into synthetic material provide an increased degree of fineness. A fine sanding is achieved in the final two stages of stonewashing: The cavity is filled with small hardwood blocks and fine-grained aluminum oxide and water, and in the last stage with colloidal silicon oxide, and mixed for 30 to 40 hours before the highly polished cavity is finished.

Using polishing materials ranging from centimetres to millimetres in size, the cavities are refined in a tumbler (pictured at top of page).

Dirk Nölle, DESY

After each “wash cycle,” the cavities are precisely measured in DESY's ILC HiGrade laboratory. “We want to analyze the systematics of stonewashing,” Navitski says. “The evenly distributed filling of the cavity with the appropriate mixture of abrasives and liquid is as important as the tumbling time at each polishing stage.”

For the construction of the ILC, this technology could replace the current method of electropolishing with fluoric acid. This would not only mean getting rid of the undesirable acid, but also has the advantage of removing traces of other metals such as aluminum—something not possible with electropolishing.

A version of this article originally appeared in DESY inForm.


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