July 11, 2013
The Facility for Advanced Accelerator Experimental Tests (FACET) ended its 2013 user run on a high note by creating the best environment for plasma wakefield acceleration (PWFA) yet achieved.
This advance was made possible by a new 10-terawatt laser that was installed and commissioned at FACET in only six months. The laser creates a tunnel of highly consistent, homogeneous plasma – hot, ionized gas – for electron bunches from SLAC's linear accelerator to shoot through. This enables more energy from the electron bunch to go into making the accelerating wakefield.
Sebastien Corde, a postdoctoral researcher at SLAC, says the data are so new the researchers are not entirely sure how big a boost the accelerated electrons received. Boosted, they were, though – the results are clear on that count. Plus, he added, "The laser had a huge impact on the strength of the interaction" between the electrons and the plasma.
Another key component: the electron beam itself. "The electron beam was amazing," despite high temperatures that can often degrade beam quality, said UCLA graduate student Navid Vafaei-Najafabadi, who was present that final weekend. "I think we have several scientific data sets of very high quality. I’m very excited."
PWFA is one of the key technologies being investigated at FACET to bring down the cost of future accelerators by reducing their size. Conventional accelerators like the linac use electromagnetic waves to accelerate electrons. PWFA uses plasma to give electron bunches from the accelerator an extra kick. This has been shown in previous experiments to double the energy of some of the electrons in a bunch in the space of a meter.
Previously the plasma was both formed and surfed by the same electron bunch – difficult to control, impossible to optimize.
But in the new method, a piece of equipment called a notch collimator slices each bunch neatly in two. The first part of the bunch generates the wave in the plasma, while the second one surfs it. Collaborators at the University of California-Los Angeles Plasma Accelerator Group figured out how to make long tunnels of uniform plasma and came up with specifications for the laser and the lenses necessary to focus it correctly, said Mark Hogan, who is head of SLAC's Advanced Accelerator Research Department and SLAC principal investigator on the PWFA team.
As for the actual process of installing a powerful laser in an above-ground building, plus the system for transporting terawatt laser pulses 30 feet below ground to the accelerator tunnel, and then compressing and focusing the pulses correctly, and – once all that was done – adjusting the timing and placement of the pulses so they created a 40 centimeter-long, hair-thin tunnel of uniform plasma in just the right spot for each pair of electron bunches to travel through – and getting it all done in six months – that was all SLAC, led by Selina Li and Mike Litos, two of FACET's physicists and members of the PWFA collaboration. Hogan counted off groups at the lab that had contributed. "The laser group, the beam operators, everyone in Controls, Mechanical Engineering, Fabrication, Radiation Protection, Test Facilities…" Indeed, it seemed every group at SLAC was involved in some capacity.
An often-unsung contribution to experiments like this is the diagnostics and imaging equipment. After all, if the researchers can't tell what is happening during an experiment, even conducting the experiment seems pointless. And here, the FACET team shines. "It's really incredible what they've put together in the FACET test area in the way of beam diagnostics," said Ken Marsh, the leader of the UCLA team.
Next steps for the PWFA researchers: analyze the data and prepare for the next run. A common theme brought up by several scientists is that when everything is working together, it doesn't take long to do good science. With the laser in place, a recipe in hand for tuning the beam, a robust set of diagnostics and a group of excited researchers, good PWFA science is practically a given for the next run.