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CEBAF's beam polarization gets a boost

Phil Adderley (below) and Jim Clark install a "getter" in an electron gun. The "getter" improves vacuum quality inside the electron gun by removing contaminants. A view down the barrel of the electron gun. The dime-sized superlattice photocathode sits in the center of the metal disk. Click here for a high resolution photograph.

In the last year, the polarization of CEBAF's electron beam has increased by more than 10 percent to over 86 percent polarization. This vast improvement in polarization, or the percentage of electrons spinning in one direction, has reduced the amount of beam time needed to complete precision experiments like G-Zero and HAPPEx. It's the result of work by the Electron Gun Group, which has spent the last two years pushing the boundaries of photocathode physics.

Matt Poelker, Electron Gun Group leader, says the group has been working to increase the performance of the electron gun -- the part of CEBAF that provides the initial electron beam. In this section of the injector, lasers liberate electrons from a dime-sized, thin slice of material called the photocathode, or cathode for short. The electrons are then pulled into the accelerator by an electric field. Over the years, the quality of the electron beam provided by the injector had been improved by upgrading the lasers used to knock out the electrons. But the group thought that a better cathode could also help.

Two years ago, a new cathode being used at Stanford Linear Accelerator Center (SLAC), called a superlattice photocathode, caught the group's attention. At the time, CEBAF's cathodes were made of a single layer of strained gallium arsenide that's 100 nanometers, or about one hundred thousandth of a millimeter, thick. The new cathode is more like a Dagwood sandwich, comprised of 14 layers of gallium arsenide alternated with 14 layers of gallium arsenide phosphide. Each layer is roughly three to four millionths of a millimeter thick. The group procured a few samples of the material and installed it for testing.

"We had a procedure to put a photocathode into our gun. When we used that standard procedure, the superlattice photocathodes were a failure: they didn't provide any electrons," Poelker says.

Marcy Stutzman, an Electron Gun Group staff scientist, cast around for the cause. She reevaluated the procedure used to prepare cathodes for installation and found that the problem might be related to the anodization process. Anodization is used to destroy the active surface around the edges of the cathode. This process prevents beam steering problems in the injector by allowing only electrons from the center of the cathode to leave the gun.

"No one had ever anodized these before, and we have to anodize to run," Stutzman explains.

Only one side of the cathode is anodized. The other side is glued to a protective glass slide to prevent anodization. Once the top side of the cathode is anodized, the glass slide is removed, solvents are used to remove residual glue, and additional cleaning steps are performed. The Gun group thought that this extensive cleaning process was killing the new cathodes. A surface analysis class she had taken gave Stutzman the idea to try a new tactic. She eliminated the messy glue from the process, attaching the protective glass to the cathode for the anodization process instead with a soft, silvery metal called indium. Indium becomes sticky when heated, allowing it to bond materials together. Removing the indium didn't require solvents, and the entire cleaning process could be eliminated.

"You just heat it back up and slide it off," Stutzman laughs.

The new process worked. Polarization went up more than 10 percent, and the yield of the cathode that provides the electrons for CEBAF improved.

"We've doubled the amount of quantum efficiency -- more electrons out per photons in, and we've improved the polarization by 10-12 percent to 86 percent polarization," Stutzman says.

Poelker says he's glad his group was able to improve the quality of the electron beam by switching to the new superlattice photocathodes. "Marcy really saved the day by developing a new technique that allows us to use this commercially available material in our guns," Poelker says.

Accelerator Division Associate Director Swapan Chattopadhyay agrees, "I am very proud of our Polarized Source Group -- they are highly sought after by premier institutions such as MIT and SLAC for collaborations and they have given us a solid foundation in performing precision measurements with parity-quality beams to better understand what's at work in the heart of matter. I expect even greater things to emerge from this group in the years ahead!"



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