Jefferson First Light from New Free-Electron Laser (CERN Courier)
Jefferson First Light from New Free-Electron Laser
Researchers at the Jefferson Laboratory, Newport News, Virginia, have delivered first light from their Free-Electron Laser (FEL). Only 2 years after ground was broken for the new facility, on June 17, Jefferson's FEL produced 155 W of continuous-wave (cw) power at 4.9 micron wavelength. No previous FEL had exceeded 11 W.
FEL development is a spinoff from the lab's main mission of basic studies of the quark structure of hadrons and nuclei, and the superconducting radio-frequency (SRF) electron accelerator that drives the FEL derives from the technology of Jefferson Lab's 4 GeV, 200 microampere, cw main machine.
For the FEL, electrons are accelerated in a cryomodule before transiting a wiggler, where they supply energy for the production of light in an optical cavity. While a conventional laser typically produces a fixed wavelength, FEL light can be tuned by varying the electron energy or the magnetic field of the wiggler.
Jefferson's superconducting electron-accelerating technology offers two commanding cost advantages for FELs: the laser can stay on 100% of the time instead of only 1% or 2%, and 99% of the energy that is not converted to useful light in a single pass can be recycled back into radiofrequency power.
The device is the first in a series of high-average-power, wavelength-tunable FELs being developed at Jefferson Lab for basic science, for industrial applications, and for applied defense research. The laser is planned to reach 1kW. Envisioned upgrades would lead ultimately to a FEL reaching 20 kW in the infrared and 1 kW in the deep ultraviolet down to 0.2 microns.
The FEL accelerator, designed for 5 milliampere average current, is laid out in a racetrack configuration to recover energy from the spent electron beam. The electrons are produced in a 350 kV DC photocathode gun and accelerated to 10 MeV in a superconducting radiofrequency accelerating unit of 1 m active length - a pair of five-cell cavities like those in the main machine. The electrons are then accelerated to 57 MeV in a four-cavity-pair cryomodule.
After the FEL, the beam can be recirculated for energy recovery and dumped at the injection energy of 10 MeV. The recirculation loop is based on the isochronous achromat used in the MIT Bates accelerator but designed with an energy acceptance of 6%. With energy recirculation, estimated power output at 3 microns is 980 W with a small signal gain of 46%. The wiggler covers the wavelength range 3.0 to 6.6 microns using 40 periods of 2.7 cm.
Design values for the electron beam parameters are:
- Kinetic energy - 42 MeV (nominal)
- Average current - 5 mA
- Repetition rate - 37.425 MHz
- Charge per bunch - 135 pC
- Normalized transverse emittance - 13 mm-mrad
- Longitudinal emittance - 50 keV-degrees
- Beta function at wiggler centre - 50 cm
- Energy spread - 0.20%
- Peak current - 50 A
- Bunch length (rms) 1 psec
To minimize emittance-growth effects and speed the commissioning process, the wiggler and optical cavity were initially placed at the exit of the accelerator. Commission of the recirculation loop is the next objective, followed by the start of user experiments.
Envisioned applications include:
- Basic science: materials science and molecular and optical physics.
- Polymer surface processing: amorphization to enhance adhesion, fabric surface texturing, enhanced food packaging, and induced surface conductivity.
- Micromachining: ultrahigh-density CD-ROM technology, surface texturing, micro-optical components, and Micro-Electrical Mechanical Systems (MEMS).
- Metal surface processing: laser glazing for corrosion resistance and adhesion pre-treatments.
- Electronic materials processing: large-area processing (flat-panel displays) and a laser-based "cluster tool" for combined deposition, etching, and in situ diagnostics.
The industrial applications exploit the potential of SRF-driven FELs to overcome conventional lasers' cost, capacity, wavelength, and pulse-length constraints. Of additional interest for basic science is the potential for short pulses of x-rays from the IR FEL, which arise from Thomson scattering of the recirculating optical pulse against the drive electron beam.
As well as providing a unique tool for basic research in materials science and atomic and molecular physics, because of its efficiency it also offers the potential to produce light at a cost useful for industrial processing. Initial users include DuPont (polymer processing) and Armcd/Northrop-Grumman/Virginia Power (metals processing). In addition, Old Dominion University, the College of William and Mary, Christopher Newport University, and Norfolk State University are partnering with industries in the recently dedicated Applied Research Center built by the City of Newport News adjacent to the Jefferson Lab to take advantage of the new FEL.
User labs at the FEL facility have been equipped via substantial donations by universities and industries for use with the first experiments scheduled for this summer. The Free-Electron Laser project was funded by the Department of Energy, the Commonwealth of Virginia, and the Department of the Navy, and supported by industries, universities and the City of Newport News.