News Release

Jefferson Lab’s Hall C G-zero experiment completes first engineering run

Proposed more than a decade ago to precisely quantify the contribution of the strange quark — one of the "lightest" of the six varieties of quarks known to exist — the G-Zero, or G0, experiment is set for a second engineering run in Hall C later this year

Peer-Reviewed Publication

DOE/Thomas Jefferson National Accelerator Facility



Standing in the CEBAF Injector aremembers of the Accelerator Division's Polarized Source Group: (foreground left to right) John Hansknecht, Joe Grames, Group Leader Matt Poelker, and Reza Kazimi are flanked by (background, l. to r.) Tony Day, Phil Adderley, Marcy Stutzman, Clyde Mounts, John Musson and Jim Clark.

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Proposed more than a decade ago to precisely quantify the contribution of the strange quark -- one of the "lightest" of the six varieties of quarks known to exist -- the G-Zero, or G0, experiment is set for a second engineering run in Hall C later this year. Accelerator Division specialists -- have spent 25 person-years in preparation for the experiment's run this fall, along with several years of work contributed by roughly 100 researchers from around the world who are part of the G0 collaboration. Participants have addressed an array of technical requirements, including a made-to-order detector, a 50-ton superconducting magnet, a precision electron injector and a uniquely calibrated electron beam.

"G0 is a huge undertaking and Jefferson Lab, with its one-of-a-kind accelerator capabilities, is on the verge of bringing this experiment to reality," says Andrew Hutton, deputy director of the Accelerator Division. "In the process of providing G0 with the unique beam it requires, the Accelerator Division has confronted a variety of challenges. Skills and knowledge from across the Division's areas of expertise were called upon to meet the beam-run parameters."

"It has been a huge team effort," he adds. "There are a few bugs we are addressing, but we anticipate a fruitful run when the experiment starts [up again] in October. We're all looking forward to giving our G0 collaborators the beam they need."

Because G0 researchers require an exact electron-pulse size and rate, extensive modifications were made both to the devices that create and then direct the accelerator beam to Hall C. Those changes were evaluated in the first G0 engineering run that concluded in late January. Technicians repositioned steering magnets, adjusted beam apertures, changed out certain pieces of hardware and made refinements to existing software systems. To adjust for specialized beam-injector requirements, new diagnostic equipment in Hall C was also installed.

"This [engineering] run was intended to shake things down; we wanted to get everything in place," says Jay Benesch, with Jefferson Lab's Center for Advanced Studies of Accelerators (CASA). "The run did expose problems in the machine and in configuration management that need to be addressed, but we did meet most of the requirements. My expectation is that we'll get everything addressed before October. The Accelerator Division believes the experiment will be a successful one."

G0 design necessitates a modification of the normal beam delivery system usually employed to meet Hall C's demands. Key to the experiment's success will be the accelerator's capability to precisely change the orientation of the spin of the electrons in the beam -- a parameter known as helicity -- relative to their direction of motion, without changing any other beam properties, such as energy, position and direction.

In addition, the electron-pulse repetition rate must be reduced to 31 megahertz, 16 times smaller than what researchers normally expect. But despite the lowered rate, average Hall C beam current needs to remain relatively high. Like water pulsing in bursts from an ultra-high-speed, off-and-on faucet, the accelerator's injector "drip" of electrons must come less frequently but in the form of larger droplets.



Accelerator Division staff in the Machine Control Center troubleshoot beam application software early in 2002, during G0 beam development. Mike Spata, Operations Group leader (sitting, front to back); confers with Ken Surles-Law, shift supervisor; and Roger Housman, on-duty operator; as Jay Benesch, accelerator physicist (standing front to back); and Joan Sage, computer scientist, assist.

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"You've got to squeeze many more electrons into the individual bunches," says Matt Poelker, head of the Polarized Source Group, part of Jefferson Lab's Accelerator Operations Department. "G0 users need many more electrons -- a factor of 16 more -- per bunch. Because electrons naturally repel each other, it was a big challenge."

Although Lab scientists would have preferred to use a JLab-designed and built laser in the Injector, attempts to make such a device locally didn't pan out. Three lasers were tried in successive attempts to obtain the required bunch structure. The third, a device made by Time Bandwidth Inc., and delivered to the Lab in August 2002, functioned adequately but needs further tweaking to provide optimum beam quality.

The laser boasts a dedicated titanium-sapphire drive able to create the kind of electron density required for the G0 experiment by precisely setting the frequency of optical pulses. The laser pulses in turn excite electrons present in a connected gallium-arsenide photocathode. The electrons bunch together and can be emitted in denser packets, traveling from the photocathode, into the accelerator "racetrack" and on to Hall C.

Because other experiments will be conducted in Hall C during the early- and mid-part of 2003, the G0 laser was moved out of the injector area during scheduled down time in February to a temporary stand in Building 58. There it will be used to conduct further tests to enhance beam quality and electron-bunch delivery prior to reinstallation in the fall.

"It's a technical challenge to create a laser like this," Poelker points out. "You need feedback electronics and precision engineering. The cavity length has to be constant, within a few microns of the 4.6-meter length."

"We all hoped it would go more easily than it did," he continues. "The beam-quality requirements were hard to meet. Beam current, position and energy in the two different states of polarization that users needed had to be identical. That places strict demands on the laser. We came pretty close to meeting the requirements; when the users return, we'll do even better."

Historically, the measurements that G0 scientists seek to make have been extremely difficult to conduct because the sought-after effects are quite small. Even using the G0 approach -- looking for changes in the scattered proton when the orientation of the spin of the colliding electron changes -- particle involvements remain rare. To collect a statistically significant sample, scientists must be assured that large numbers of polarized electrons will be available. Their generation and collection mandates sustained and inventive engineering.

The G0 experiment will be reinstalled in Hall C in September; G0 equipment was placed on rails and slid out of the way for other scheduled Hall C experiments underway this spring and summer. A second G0 engineering run should begin in October, with the first experiment measurements slated for early 2004.

"We've accomplished a lot in a relatively short period of time," Poelker says. "We're learning how to deliver the beam G0 users want. We're improving our ability, thanks to a group effort that involves a lot of people who have worked and are working really hard on the beam. Given everything we had to do, and the tight schedules, I think it's been a big success."

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