FACET Prepares to Shine

A new SLAC test facility has passed two important milestones and is gearing up for experiments aimed at dramatically shrinking the size and cost of future particle accelerators.

FACET, the Facility for Advanced Accelerator Experimental Tests, uses two-thirds of the famous two-mile-long linear accelerator at SLAC National Accelerator Laboratory. Major construction was completed last month, and last Thursday the first electron beam traveled 1¼ miles through the linac to the location where the FACET experiments will be installed.

”It is really exciting to see first beam in this new facility after almost three years of preparations,” said Nan Phinney, the FACET project director.

FACET is a user facility, which means scientists from all over the world submit proposals and compete for time on the beam. They’ll use FACET’s extremely short, intense pulses of electrons to investigate two innovative methods of accelerating electrons and positrons to exceedingly high energies in extraordinarily short distances. These “wakefield acceleration” techniques could ultimately lead to powerful tabletop accelerators for science, medicine and industry, as well as the ability to reach much higher energies at accelerators like the SLAC linac.

The FACET wakefield acceleration experiments will build on work begun at SLAC’s Final Focus Test Beam, where in 2006 scientists more than doubled the energy of electrons over a distance of about one yard by accelerating them about 1,000 times faster than conventional accelerators could. A major goal for the FACET experiments is to learn how to control the beam and plasma so an entire pulse – not just a small fraction of it – is uniformly accelerated.

Current plans call for installing and commissioning the experimental equipment in July and bringing in the first four teams of experimenters in August for what is called user-assisted commissioning.

“This early beam time will give everyone a quick, hands-on look at how FACET operates and will identify for both SLAC and the experimenters any areas that need improvement or modification,” said Mark Hogan, head of the plasma acceleration group in the Advanced Accelerator Research Department.

“Of course we’ll all also be trying to get as many scientific results during that short time as we can,” he added, smiling. “I’m ecstatic that we’re finally getting going again after years of talking about it. I’m so glad to be back in the tunnel again doing physics. I live for doing these experiments.”

As part of the recent $14.5 million construction, workers installed a new beamline, which includes the experimental area, a final focus and a “bunch compressor chicane.” The chicane squeezes the linac’s electron pulses to 15 microns, about one-fifth the diameter of a human hair. Yet each pulse reaches an extremely high peak current of 20,000 amps, some 1,000 times higher than the current that would trip a typical home circuit breaker. In Thursday’s milestone, the beam travelled successfully through the accelerator and the chicane. The next goal is to reach FACET’s full design intensity.

“Our job is to make these extremely small balls of charge at an energy of 23 billion electron volts,” said Uli Wienands, the commissioning team leader.

Jerry Yocky, who has been organizing the commissioning of the new beamline, said the accelerator is “extremely complex, and requires tenacity and experience to bring up and operate.”  When consulting some of the linac’s original 1966 blueprints while trying to figure out a problem, he once exclaimed, “These drawings were made before I was born!”

Wienands had high praise for his tight-knit commissioning team, and in particular for the many contributors from SLAC departments such as Power Electronics Maintenance, Mechanical Fabrication, Controls and Facilities.

“They are the heroes of this operation,” he said, noting that two weekends ago, several controls technicians and engineers came in and spent a good number of hours troubleshooting to get the accelerator working.

When it’s fully operational, FACET will shoot ultra-short pulses through a sealed chamber full of hot gas – lithium to start; cesium later – where it will briefly create the ideal conditions for extreme particle acceleration. The electrons in each pulse will strip a single electron away from the gas atoms along a narrow channel to form a plasma. Then the pulse will push those liberated electrons out of the way, leaving in its wake a tiny region that has, for just an instant, an extremely strong electric field caused by the briefly separated charges of the negative electrons and positive atomic nuclei. This plasma wakefield accelerates a second bunch of already-high energy electrons to exceedingly high energies in a very short distance, like surfers riding a wave.

A second FACET experiment will test the possibility of a related “dielectric wakefield” acceleration technique, in which the action takes place within a hollow fiber made of a dielectric material, or an insulator whose material can be polarized by an applied electric field, such as glass or silicon. Two additional experiments will use FACET’s short, high-current pulses to develop a technique for precisely measuring the length of such short pulses without destroying them and to explore details of how magnetic fields switch orientations in disk drives.

The roots of wakefield acceleration go back to a 1979 theoretical proposal by UCLA researchers Toshiki Tajima and John Dawson. SLAC’s involvement began in 1996 at the Snowmass, Colorado, Workshop on “New Directions for High-Energy Physics,” at which a University of Southern California professor suggested a wakefield experiment to the late Bob Siemann, SLAC’s iconic leader of advanced accelerator research. Siemann was immediately excited by the idea and led the effort to convert the Final Focus Test Beam, which received electron pulses from the SLAC linac, into a place for conducting high-energy plasma wakefield experiments.

After the first round of experiments is complete, the team will install more components and experimental equipment in the beamline to get ready for full-fledged runs in early 2012. SLAC will host a users' meeting for FACET’s scientific users in late August. A new round of experimental proposals will be reviewed in early winter in preparation for FACET’s second user run, expected later in 2012.

Related Links

• "Crashing the Size Barrier," symmetry magazine, October 2009