Free-Electron Lasers for U.S. Industry (Science & Technology)
Free-Electron Lasers for U.S. Industry
By Mark Wilsey, Science & Technology
April 21, 1997
The Department of Energy, in partnership with several U.S. corporations, is building a unique laser facility for industrial research. The new device is a free-electron laser (FEL); it will be the world's most powerful laser of this type. And, while FEL technology is in large part a spinoff from the Strategic Defense Initiative program, the primary mission of this new facility is to explore FEL commercial applications.
The laser is being built at the Thomas Jefferson National Accelerator Facility, TJNAF, in Newport News, Va. TJNAF - dedicated last year, and thus the newest of the DOE labs - is the home of the Continuous Electron Beam Accelerator. The latter is the premier tool for studying the fundamental structure of matter in nuclear physics research. Early in the planning for the laboratory, it was recognized that the accelerator's electron beam could be used as a source for a high-powered free-electron laser. And so, an FEL facility was incorporated into the plans.
The Laser Processing Consortium formed to support the FEL project, included the lab, a group of universities, and a number of U.S. firms, such as DuPont, 3M, Xerox, IBM, and Northro Grumman. The FEL is expected to be completed next year at a cost of $34 million, nearly 40% of which will come from U.S. industries. The rest of the funding will come from the DOE, the Commonwealth of Virginia, and the U.S. Navy. (The Navy's longstanding interest in free-electron lasers revolves around their potential use in shipboard missile defense systems.)
The principal advantage of a free-electron laser over conventional lasers is that, while nearly all other lasers operate at the fixed wavelength for which they were designed, the wavelength of the FEL light is tunable over a broad range, from infrared to ultraviolet. This gives the FEL, in turn, a broad range of industrial applications.
The FEL's Versatility
Industry's interest in the new FEL facility is twofold: Not only will the facility allow for research to tailor light sources so as to optimize specific processes, but the power of the FEL will approach a level useful for actual industrial applications. Thus, the FEL will be a test bed to explore new types of laser-processing for industry.
For example, scientists at DuPont know how to treat a nylon film with a specific wavelength of ultraviolet, to make it resistant to bacteria. However, conventional lasers are too feeble to do this well, and can process only a few square yards of film a day. But the new FEL facility will be 20 times more powerful than these lasers, and could boost production by a factor of 10, allowing for product-testing of the film.
In another case, Northrop Grumman would like to use the FEL to bore tiny holes in aircraft wings, to reduce drag and improve the performance of the aircraft. Other applications could include treating surfaces of materials to increase wear-resistance or to protect from corrosion.
The FEL will provide laser light at 1-3 kilowatts of power over a range of wavelengths, from 190 to 20,000 nanometers.
The first ingredient for a free-electron laser is a high-energy stream of electrons, and here the Continuous Electron Beam Accelerator facility at TJNAF is just the ticket. The accelerator is made up of a string of hollow metal cavities which are about the size of a slightly flattened soccer ball. The cavities are made of niobium, which becomes superconducting when cooled to -456 degrees F. Power to the accelerator is applied in the form of radio-frequency (rf) electromagnetic energy. The rf energy is converted into an electric field inside the cavities; in turn, the field accelerates the stream of electrons that have been injected into this string of cavities.
It is the superconducting technology involved here, which allows for the production of continuous electron beams, which is needed for the FEL, but there are also other advantages which apply to the nuclear physics research done at TJNAF. A superconductor conducts electricity without any energy loss to resistance. A non-superconducting rf accelerator can operate only for a fraction of a second, due to heating.
To create the laser light, the high-energy electron beam is passed through a magnetic field which varies along the path of the electrons. These alternating magnetic tugs cause the electrons to "wiggle," and this induces the electrons to radiate. The emission from the the electrons is captured in an optical cavity, and released as laser light. The wavelength of the light can be tuned by adjusting the velocity of the electrons through the magnetic fields of the "wiggler."
Northrop Grumman, which designed the magnetic wiggler for the FEL, is interested in studying the possibilities for marketing such devices, as FEL technology becomes more widespread. At present, the U.S., which developed both the FEL and the rf superconducting technology, has a substantial lead in the field. Other nations, such as Japan and Germany, are also considering the use of FELs for industrial processing.