When problem-solving is concerned, two heads are said to be better than one. Likewise, JLab's recent installation of a second polarized electron gun is designed to resolve one of physics researchers' most important issues: uninterrupted beam delivery.

"The additional gun helps us maximize the Lab's operational capabilities. We need a stable polarized electron source to meet our user's experimental requirements," explains Charles Sinclair, Injector Group leader. "About 50 percent of the Lab's experiments require polarized beam; and an even larger fraction of the highly rated experiments need polarized beam. We schedule the accelerator to deliver beam to two or three halls simultaneously, so in practice, we need to run the polarized source 100 percent of the time. The Lab has been running polarized beam since April 1998 and plans to continue polarized beam operation into the spring of 2001 - as far as the long-term schedule is developed."

A close-up look at the Continuous Electron Beam Accelerator Facility's new polarized electron gun system.

The second gun was designed, built, tested offline, and installed in nine months. Both guns sit horizontally, a change from the first polarized gun's vertical orientation. Commissioning of both is nearly complete.

"We've delivered beam to users from both guns," says Injector Group staff scientist Paul Rutt. "We haven't fixed all the bugs, but many of the issues have been resolved. Now, when one gun needs work done on its photocathode, we can deliver beam to the physics program from the other gun. I wouldn't call it a perfectly seamless operation, but we've definitely made progress."

Several other improvements were made to the polarized source at the same time the new guns were installed. For example, in the past, temperature fluctuations near the lasers which illuminate the polarized gun cathodes affected beam stability, so an air-conditioned housing has been built around the three lasers. Improved laser controls and electronics hardware were also installed. Work is in progress to remotely reconfigure the laser systems so that switching beam delivery between the polarized guns can be done with the push of a single button.

"We're staying on the cutting edge of laser technology, even inventing new technology," says John Hansknecht, Injector Group engineering coordinator. "The experimenters will always want more current and better polarization. We are constantly looking for ways of getting higher laser power and more efficient photocathodes. Hopefully we'll find ways to keep everyone satisfied."

Happy Users, Productive Experiments

A key component of both electron guns is a small disc of material that sits within each injector's ultra-high-vacuum chamber. Made of gallium arsenide, this dime-size photocathode (often called a crystal or wafer) emits electrons when struck by laser light. Yet the wafer loses its emitting properties over time, as more electrons are delivered. Although the laser can be refocused on different sections of the photocathode, eventually the material's effectiveness decreases and the entire crystal must be replaced.

The Injector Group includes (front row, l to r) Jim Clark, Kim Ryan, Reza Kazimi, Matt Poelker, Group Leader Charlie Sinclair and Paul Rutt, and in the back row (l to r) Peter Hartmann, Michael Steigerwald, Tony Day and John Hansknecht. Injector Group member Philip Adderley was unavailable for the photo.

During replacement, Injector Group personnel must open the ultra-high vacuum chamber within the injector. Although this process lasts just minutes, reestablishing the ultra-high vacuum - better than that found on the surface of the moon - can take up to 50 hours and involves enclosing the gun in an oven-like structure. The entire array is subsequently heated to a temperature of 250 degrees Celsius (482 degrees Fahrenheit). The net effect of the heat is to desorb or drive out gas molecules from the gun's internal surfaces that otherwise would physically impair photocathode operation.

"Electrons are found everywhere," says Philip Adderley, a high-vacuum technologist with the Injector Group. "If you have residual gas molecules remaining as you accelerate electrons from your cathode, you produce oppositely charged ions. These ions are heavier than electrons and can damage the cathode as they are accelerated into the cathode material. The better the vacuum the lower the number of these possible interactions."

The presence of two polarized guns enables one to be taken out of operation as necessary, while the other continues to supply beam. With planned upgrades to the laser system and continued investigations of more efficient and durable photocathode materials, researchers should be able to take full advantage of their allotted beam time in the coming months and years.

"We now have a more versatile injector system that should provide beam to the physics users without interruption," says Matt Poelker, a staff scientist with the Injector Group. "It's been a good project. A lot of challenges, sure, but people seem to be pretty happy."

"Because of the work of our Injector Group, the Lab is now a worldwide leader in polarized beam technology," says Accelerator Division Associate Director Christoph Leemann. "They are an exceptionally talented team who have in a sustained effort over many years, worked long hours and met the challenges they've encountered in their effort to develop and enhance the accelerator's polarized beam capabilities. The group has the Lab poised to meet all of our user's near and mid-term needs.

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