BIG FEL Grows in Power

Scientists at the US Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Laboratory; Newport News, VA) have pushed the facility's free-electron laser (FEL) to a world-record output of 1.72 kW at 3.1 μm, setting the stage for a host of practical uses for the device. Emitting a close-to-diffraction-limited beam, the laser produces pulses of about 1 ps in duration at an 18-, 37-, or 75-MHz repetition rate. It saw "first light" in June 1998, at which time it produced 155 W-topping the previous high for such FELs by a factor of ten.

The FEL is powered by a superconducting radio-frequency (RF) linear accelerator fed in turn by a laser-driven electron source; total length is approximately 50 m. Consisting of a frequency-doubled Nd:YLF (yttrium lithium fluoride) laser that causes electrons to boil off a photocathode, the electron source emits at a repetition rate that defines the repetition rate of the FEL. The superconducting accelerator itself has very low heat loss, according to Fred Dylla, FEL program manager, who notes that all but 0.01% of the RF energy produced by the accelerator ends up as electron kinetic energy. This efficiency is in contrast to the competing copper-cavity technology, which has a tendency to melt at high average powers.

The laser cavity of the FEL is an ordinary concentric cavity, with a high-reflection dielectric mirror and a 90% reflecting output mirror. The electron beam is passed into and out of the laser cavity with the aid of magnets that deflect the beam around the cavity mirrors. Within the cavity is a "wiggler," a linear series of closely spaced magnets that excite the electrons.

Crucial to the high output of the FEL is its electron-recycling technology, says Dylla. He explains that electrons leaving the FEL are circulated back into the superconducting accelerator, increasing both energy efficiency and optical output by a factor of five or more. Because the recycled beam is degraded both in spatial quality and energy spread, it is not directly recirculated. Instead, it is sent into the accelerator 180° out of phase with the virgin beam, thus decelerating the recycled beam and causing it to dump its energy back into the system in the form of RF power.

FEL upgrade

Aided by funding from the Depart ment of Energy, the US Navy, the Commonwealth of Virginia, and industry, Jefferson Laboratory is undertaking an upgrade of its FEL that includes boosting electron energy and adding additional wigglers. If upgrade goals are met, the FEL will emit >10 kW in the 1-10-µm range and >1 kW in the 0.2-1-μm range and will be wavelength-selectable from 0.2 to 60 μm. "At double the cost and ten times the light output, the planned upgrade will lower the cost per watt by five times," explains Dylla.

The researchers have already coaxed the FEL to lase at its fifth harmonic-another first, according to George Neil, FEL deputy program manager. They achieved this by inserting a cavity mirror that suppressed both the fundamental and third-harmonic light. Benefits will include the rapid introduction of shorter wavelengths.

The FEL shows advantages for material processing (see figure). Jefferson Laboratory has a lineup of industrial users who are already beginning to exploit these advantages. Researchers from DuPont (Wilmington, DE) are using the laser to treat nylon cloth to modify its surface texture, giving it the look and feel of natural cloth. Northrop Grumman Corp. (Los Angeles, CA) and others have begun to experiment with the laser glazing of metals, in which the high power of the FEL briefly melts a thin surface layer of the metal, causing it to become amorphous and thus very hard and corrosion-resistant.

Free-electron laser (FEL) drills a clean hole through a 4-cm-high piece of ceramic (top to bottom). Tuned to the 4.8-μm absorption of the ceramic, the FEL was adjusted to emit 102 W at a 37-MHz repetition rate. Conventional laser drilling usually cracks this material.

Solarex (Frederick, MD) is collaborating with North Carolina Central University (Raleigh, NC) in developing a large-area apparatus supplied with FEL light for the annealing of amorphous silicon used in photovoltaic solar cells; the two entities will also use the laser as a light source for high-sensitivity infrared spectroscopy intended for defect characterization of silicon. Dylla notes that the FEL's picosecond pulses-which can be adjusted in width by a factor of two by tuning the accelerator-are ideal for developing ablation and machining techniques based on ultrashort phenomena.