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People: Mark Hogan and the FACET Team Move from Theory to Experiments

(Photo - Mark Hogan)
Mark Hogan.
(Photo courtesy Mark Hogan.)

Mark Hogan wears a T-shirt and running shorts most days of the week, donning slacks and a tie for appropriate events. But his wardrobe is less a fashion statement than a way of life. Hogan has made it a point to participate in at least one major triathlon every year, making time for smaller races when he's not coaching Little League or getting approvals for a state-of-the-art science experiment.

Hogan is the head of the plasma acceleration group in the Advanced Accelerator Research Department. He is a member of the team building FACET, the Facility for Advanced Accelerator Experimental Tests, which will make use of the first two-thirds of the SLAC linac. FACET's plasma acceleration experiments will accelerate electrons on waves of plasma, an approach that promises to accelerate particles up to 1000 times faster than existing technologies. In July, the project received both its Critical Decision 2 and 3 approvals, which recognized the cost, design and schedule of the experiment and authorized the start of construction, respectively.

"I'm looking forward to being a part of all the science," said Hogan, who arrived at SLAC in 1998 as a postdoc, and has been working with the FACET team specifying experimental requirements for the past several years. In a sense, FACET will continue the work accomplished by the Final Focus Test Beam, which operated for user-proposed experiments at SLAC from 1997 until 2006. Due to the unique capabilities of the SLAC linac, these world-leading experiments quickly advanced the field of plasma wakefield acceleration. The FFTB shut down in spring 2006 to make way for the Linac Coherent Light Source.

With FACET construction scheduled to be complete in April 2011, the team plans to spend roughly the first year of operation commissioning the machine and conducting experiments to diagnose its electron beam and instrumentation. One of their primary goals is to create sets of two electron bunches spaced about 300 femtoseconds apart in the beam—a key ingredient for next-generation plasma wakefield acceleration experiments and something that the SLAC linac can't currently provide.

(Photo - Mark Hogan)
Mark Hogan installs mirrors at the FACET facility. (Photo courtesy Mark Hogan.)

The plasma team plans to insert an obstruction in the beam's path, which will split each bunch into two shorter ones, a 30-micron bunch and a 10-micron bunch, separated by about 120 microns. When the first bunch hits the plasma, the electrons in the plasma are repelled, but then quickly rush back to their original spots after the second bunch passes. The wake of the rushing plasma electrons generates fields 1,000 times larger than the SLAC linac and quickly boosts the energy of the second electron bunch in the process. This high-energy push is the backbone of plasma wakefield acceleration.

"The work is appealing and on the 'R' side of R and D," said Hogan, who has been working for years with his colleagues on the experimental foundations behind plasma acceleration and its capabilities. "We were doing very early research, asking things like 'What's the physics?' and 'How can we make this work?' There was a lot of opportunity for new ideas." Now that FACET is under construction, Hogan is eager to try all the latest ideas at this state-of-the-art SLAC facility.

SLAC has already received many FACET user proposals, which will compete for beam time during the four months that the facility will run each year, beginning in 2012. (Construction and upgrades to the linac for the proposed LCLS-II project are tentatively scheduled during FACET down time.) Once these user runs begin, the team will have a five-year window to run the machine.

"When that happens, I'll be the guy walking around with a smile on my face," Hogan said. Until then, you can spot him as the tall guy in running shorts.

—Lauren Rugani
SLAC Today, September 22, 2010