Lutz Lilje inspects a cavity.

The smooth inside surface of this cavity was created by electropolishing.

A FLASH cavity during electropolishing

Electrons are thrill-seeking particles. They get their kicks out of the superconducting cavities of the ILC. The higher their quality, the better the kick – so a team of scientists, engineers and technicians at DESY in Hamburg, Germany, makes sure that the cavities are buffed to the highest degree. A technique called electropolishing is currently emerging as the method to produce superclean superconducting radiofrequency cavities.

The cavities transfer energy to the particles. The field strength is in the order of tens of Megavolts. To reach this high voltage, the resistance within the cavity has to be as low as possible. The cavities are made of pure niobium, which becomes superconducting at very low temperatures. But every tiny speck of dust or dirt poses a threat to its superconductivity: the charge flowing through the material knocks electrons out of the inclusions or bits of dust, causing field emission; the area eventually warms up and the surface resistance rises. Finally, the cavity's surface becomes normal conducting: a thermal breakdown – a quench – occurs. "The ILC's success depends on all 16 000 cavities working perfectly," explains Lutz Lilje, scientist at DESY and expert for superconducting radiofrequency technology. "A quench from field emission might cause the whole system to switch off for some time. We don't want that!"

Several processes make sure that the niobium itself is as pure as possible. Once it is welded into its characteristic nine-cell shape, however, it has passed through all sorts of machines, hands and apparatuses and urgently needs a proper wash. And a heat treatment: the cavity is heated in a furnace, all hydrogen is removed, it is cleaned in a cleanroom up to six times with high-pressure ultra-pure water. Before these water rinses, it used to be etched with an acid bath, which usually gave them a relative rough surface – a potentially dangerous thing if you want to avoid quenches.

This is where electropolishing comes in. The process still needs strong acids, but it is not the acid itself that removes impurities but a current applied to the bath that causes smoothening of the surface. Electrolysis works with the exactly the same principles. "This gives a really smooth surface and a much better gradient," says Lilje. Gradient is the magic word in accelerator technology: the higher the gradient, the higher the energy supplied to the particles and thus the acceleration of the electrons. "The funny thing is: cavities that have gone through electropolishing show a higher gradient, but nobody exactly knows why until now. But many people want to understand exactly that," says Lilje.

Several experts on electropolishing are investigating another possible advantage. It might be possible to spare the cavity one trip to the oven if the results are good enough – which would save money. The process itself is an invention from the 70s; Siemens used it for X-band niobium cavities then, and experts at KEK tried it on the KEK-B accelerator and TRISTAN in the 1980s.

The technology is currently used at DESY for ‘FLASH', the free-electron laser prototype for the future XFEL. It is being worked on in several laboratories to arrive at a possible mass-production process for the many cavities of the ILC.

-- Barbara Warmbein