Thin Coats Galore
A refrigerator-sized machine inside SLAC's Klystron Coating Lab hums and clangs as it deposits a few layers of titanium nitride molecules onto a cylindrical piece of ceramic for Argonne lab's Advanced Photon Source. Since 1985, the coating lab has been applying similar thin films to klystron microwave windows and other components used in accelerator labs from around the world.
Klystron windows provide a seal for the vacuum inside the klystron—a device that powers a linear accelerator. These white ceramic disks are called windows because they are designed to transmit the microwave energy generated inside the klystron without much absorption or reflection.
"Klystrons always have difficulties with windows breaking. The higher in power you go, the harder it is to make a window that doesn't destroy itself," said Dale Miller, the principal technician at the lab. One process that destroys these windows is called multipacting. Electric fields near the windows crash electrons into the surface of the window. These impacts eject electrons that can then be accelerated into the window or other klystron surfaces and jar more particles loose. The energy created by this cascading effect eventually breaks the window.
To protect the windows from multipacting, a film of titanium nitride 10 angstroms—only a few layers of molecules—thick is applied to the window surface. A titanium nitride surface is less susceptible to multipacting than the ceramic material of the window.
The lab uses two different techniques to apply thin films. Sputtering involves bombarding material that will form the thin layer with ionized gas molecules. These gas molecules hurtle at the material with enough energy to eject atoms from its surface. The displaced atoms then fly toward the object being coated and deposit on the surface.
Reactive evaporation uses a less-violent method to produce thin layers. "Basically, it's like a toaster: we heat up a titanium wire," Miller said. The wire reaches high enough temperatures that the metal begins to sublimate, or evaporate, into the coating chamber. These evaporated molecules condense onto the cooler surface of the object being coated. To produce the coating material, a gas such as nitrogen is pumped into the coating chamber to react with the metal atoms.
Besides producing films of titanium, the lab technicians also coat objects with many other materials, such as gold and, in the future, niobium. Their machines can also cover components with separate layers of different material types.
"We prefer to sputter rather than do evaporation. But with evaporation, we can coat any surface," Miller said. While the sputtering machine easily coats the flat surfaces of klystron windows, the evaporation machinery can adapt to the more complicated surfaces the lab encounters when they work on custom projects.
A thick binder on a bookshelf in the lab contains Polaroid snapshots of the different custom coating projects the team has completed through the years. "One of the more interesting projects we've had was the BaBar beam pipe," Miller said. The lab coated the vertex chamber inside the BaBar detector with a 7-micron layer of gold atoms. Other SLAC projects have involved the B Factory and PEP klystrons. Outside SLAC, the coated surfaces for groups at CERN in Geneva, DESY in Germany, and KEK in Japan.
To find more information on the coating lab's capabilities and past projects, or to request a custom coating project, explore their SLAC intranet page.