First beam at SLAC plasma facility

First beam

FACET-II, a new facility for accelerator research at SLAC National Accelerator Laboratory in California, has produced its first electrons. FACET-II is an upgrade to the Facility for Advanced Accelerator Experimental Tests (FACET), which operated from 2011 to 2016, and will produce high-quality electron beams to develop plasma-wakefield acceleration techniques. The $26 million project, recently approved by the US Department of Energy (DOE), will also operate as a federally sponsored research facility for advanced accelerator research that is open to scientists on a competitive, peer-reviewed basis.

“As a strategically important national user facility, FACET-II will allow us to explore the feasibility and applications of plasma-driven accelerator technology,” said James Siegrist of the DOE Office of Science. “We’re looking forward to seeing the groundbreaking science in this area that FACET-II promises, with the potential for a significant reduction in the size and cost of future accelerators, including free-electron lasers and medical accelerators.”

Whereas conventional accelerators impart energy to charged particles via radiofrequency fields inside metal structures, plasma-wakefield accelerators send a bunch of very energetic particles through a hot ionised gas to create a plasma wake on which a trailing bunch can “surf” and gain energy. This leads to acceleration gradients that are much higher and therefore potentially to smaller machines, but several crucial steps are required before plasma accelerators can become a reality. This is where FACET-II comes in, offering higher-quality beams than FACET, explains project scientist Mark Hogan. “We need to show that we’re able to preserve the quality of the beam as it passes through plasma. High-quality beams are an absolute requirement for future applications in particle and X-ray laser physics.”

Aerial view

SLAC has a rich history in developing such techniques, and the previous FACET facility enabled researchers to demonstrate electron-driven plasma acceleration for both electrons and positrons. FACET-II will use the middle third (corresponding to a length of 1 km) of SLAC’s linear accelerator to generate a 10 GeV electron beam, kitted out with diagnostics and computational tools that will accurately measure and simulate the physics of the new facility’s beams. The FACET-II design also allows for adding the capability to produce and accelerate positrons at a later stage, paving the way for plasma-based electron–positron colliders.

FACET-II has issued its first call for proposals for experiments that will run when the facility goes online in 2020. In mid-October, prospective users of FACET-II presented their ideas for a first round of experiments for evaluation, and the number of proposals is already larger than the number of experiments that can possibly be scheduled for the facility’s first run.

Last year, the AWAKE experiment at CERN demonstrated the first ever acceleration of a beam in a proton-driven plasma (CERN Courier October 2018 p7). Laser-driven plasma-wakefield acceleration is also receiving much attention thanks to advances in high-power lasers (CERN Courier November 2018 p7). “The FACET-II programme is very interesting, with many plasma-wakefield experiments,” says technical coordinator and CERN project leader for AWAKE, Edda Gschwendtner, who is also chair of the FACET-II programme advisory committee.