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SRF-Based Energy-Recovering Linear Accelerators (ERLs)

IRUVFEL schematic

A 160 MeV, 10 milliampere energy-recovering linear accelerator (ERL) drives JLab's upgraded free-electron laser (FEL). The FEL produces up to 10 kilowatts of light when the electron beam slaloms through the array of magnets in the infrared wiggler, or up to 1 kilowatt when directed through the recently installed ultraviolet wiggler. The electron beam recirculates through the linear accelerator and is decelerated for energy recovery, recycling the beam's energy for accelerating new electrons. The beam itself is dumped. Besides driving the wigglers, the ERL yields terahertz light directly when the beam passes through a bending magnet.

Energy recovery was demonstrated as early as the mid-1970s, but from 1999 to 2001 at JLab, the first ERL with high average current drove the first kilowatt-scale FEL. That FEL, which has since been substantially upgraded, gave users infrared light at 3–6 microns for 1800 hours—the most achievable with available funding—and led to publications by some 30 groups. Research topics included nanotube production, hydrogen-defect dynamics in silicon, and protein energy transport. (See the JLab FEL's research highlights.) The experimentation influenced thinking about linear and nonlinear dynamical processes.

Moreover, the ERL itself directly produced broadband light in the terahertz region between electronics and photonics, at over four orders of magnitude higher average power than anywhere before. In Nature (420, 131), Mark Sherwin of the University of California, Santa Barbara, predicted "new investigations and applications in a wide range of disciplines."

At 5 milliamperes and 42 MeV, JLab's original SRF ERL was a much-higher-current cousin of CEBAF, the five-pass, 6 GeV recirculating linac that enables the laboratory's main mission of research in nuclear physics. The JLab ERL/FEL has now been upgraded to produce light at 10 kW in the infrared, with a 1 kW capability recently added in the ultraviolet. For infrared operation, the average beam current has been doubled to 10 milliamperes. In the further evolution of ERLs, high average current will be crucial. Optimal performance, in fact, is a trade-off between that and beam degradation. Envisaged ERL projects involve average currents about an order of magnitude higher than those demonstrated so far.

In 2005, Cornell University's serious pursuit of a design for an ERL light source yielded funding from the National Science Foundation (NSF) to begin developing a major ERL-based upgrade of the Cornell High Energy Synchrotron Source at the Cornell Electron Storage Ring. With the NSF funding, Cornell is developing an injector to deliver low-emittance beams at 100 milliamperes. At JLab — Cornell's partner in preparing the NSF proposal — collaborative experiments are being conducted concerning other issues in ERL development: beam breakup in the ERL/FEL and RF control in both the ERL/FEL and CEBAF.

To complement the JLab FEL's demonstration of high average current, CEBAF was specially configured briefly during 2003 for a single-pass proof-of-principle study of energy recovery at the giga-electron-volt energy scale.