Jefferson Lab's accelerator is undergoing a period of intense conditioning much like a world-class athlete preparing to set a new record.

Coaching the accelerator to a higher performance level is a task shared by many in the Accelerator Division. About 18 months ago a few individuals were designated "Performance Integrators" for different aspects of CEBAF (Continuous Electron Beam Accelerator Facility). The challenge for each was to identify improvement goals, then to coach and train various accelerator subsystems so overall peak performance could be achieved.

The most visible performance parameter of CEBAF is the output energy of its electron beams. For Charlie Reece, deputy head of the Accelerator Development Department, and one of the accelerator system's Performance Integrators, this is part of his game.

Work has been underway since September 1996 to increase the Lab's electron beam to a stable, predictable 6 GeV (billion electron volts) physics operation. Its top end energy output is presently near 5.5 GeV at modest currents; in September 1996 it was near 5.2 GeV.

Reece is using a two-pronged approach to economically get the electron beam up to 6 GeV. "First we're evaluating the accelerator's operating parameters using the experience we've gained while running it. We're examining the soft and hard limits of the machine. We have found some room to squeeze the accelerator and make it run a little harder. We can run it more aggressively and the equipment will meet the demand," Reece explained. The upgrade team has reviewed, evaluated and fine tuned the accelerator's operating parameters, control mechanisms, and hardware tolerances.

But, that will only give the Development team part of the energy increase needed to get beam to the higher operating level. The other approach used by Reece and his team, which includes contributors from the Accelerator Division Operations, Development and Beam Departments, is improving individual cavity function making the equipment run better by improving its ability to "kick" the electrons harder as they go by.

For decades, a particular processing technique you could think of it as a "conditioning drill" in keeping with our sports metaphor has been used to improve the performance of such superconducting cavities. But it had been done in laboratory settings never used on installed cavities, like those in CEBAF.

The process consists of putting a small amount of helium gas inside each superconducting cavity pair and pushing them unusually hard. The interior of the cavities normally is kept at ultra-high vacuum so the accelerated beam does not get scattered by particles present in normal atmospheric pressures.

One of the chief limitations on cavity operation is undesired emission (or release) of electrons from microscopic defects on the cavities' surfaces. Running the cavities with the helium gas inside induces many of these defects to "burn themselves up" giving a cleaner, more stable surface on the inside of each cavity.

"But the big question was: Could we successfully do this in situ? It had never been done in situ because it is a complex task and the specter of possibly damaging an $800k cryomodule was not pretty," Reece admitted.

A particularly troublesome cryomodule presented Reece with an opportunity to test helium processing in situ. Would other components be damaged; would the treatment affect the vacuum? With a draft helium processing plan in place, the team tried their idea out on the malfunctioning cryomodule. They figured they had nothing to lose. They had already decided to pull the poorly-performing unit out of the accelerator for over-haul a time consuming and costly procedure. If helium processing worked they might not have to rebuild the cryomodule and they would have developed an innovative and resourceful process for improving the accelerator's beam energy.

It worked! Helium processing significantly improved cavity performance, without any unpleasant side effects. Each cavity pair is conditioned for 45-90 minutes. The treatment reduces the electron field emission inside the cavities and the secondary effect of arcing at the cold ceramic RF (radiofrequency) windows, allowing them to operate in a more stable fashion at higher voltages. The technique was refined through use on two cryomodules at a time until the accelerator upgrade team was ready for a big push which came last summer.

Sixteen cryomodules were put through helium processing during the 1998 Fourth of July shutdown. Nearly every component treated showed a significant improvement. Another eight cryomodules were treated during the January 1999 shutdown, with the last group possibly undergoing processing this summer.

"We are closing in on 6 GeV," Reece said. "Several experiments using 5.5 GeV beam are scheduled for this spring and we should bring 6 GeV within reach in the next year."

The race is on and the Development team is well on its way to bringing the accelerator to a new personal best. However, attaining 6 GeV won't mark the finish line, instead it marks a new starting point for a much bigger goal increasing the beam energy to 12 GeV. Many people are working that goal, too. "Doubling beam energy involves a lot of up front work creating a conceptual design and detailed drawings, developing test components capable of energizing and delivering beam at 12 billion electron volts, and creating demonstration hardware proving that we can do what we say we can," Reece explains. "All of that will be necessary before we get funding for the upgrade from DOE."

Editor's note: This is the first in an on-going series of stories dealing with the accelerator beam upgrade. Next month ON TARGET will highlight some of the many people involved in the cavity pair helium processing.

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