Argonne engineers set new record

Engineers at Argonne National Laboratory’s Advanced Photon Source have set a world’s record for the most energetic beam of light using a fully operational mirrorless free-electron laser.

The beam of light produced in the experiment had a wavelength of 385 nanometers, placing it in the ultraviolet region of the spectrum and making it 1,000 times more energetic than the previous record beam from an operational free-electron laser of its kind.

Unlike a conventional laser, which requires mirrors, the Argonne free-electron laser uses a powerful electron accelerator in combination with arrays of very long and precise magnets and needs no mirrors for operation.

With further development, free-electron lasers promise to provide laser-like X-ray beams in ultrashort pulses that will enable scientists to study the properties and structures of materials in far greater detail and in far less time than is possible today.

X-rays are currently the most widely used scientific probe for studying the structures and interactions of crystalline materials at atomic and molecular levels. But many materials do not form crystals and many reactions take place too quickly to study adequately.

An ideal step forward for X-ray researchers would be to use lasers that produce intense, perfectly focused X-ray beams. Conventional lasers cannot produce beams of light more energetic than ultraviolet light, which is far less energetic than X-rays. This is because conventional lasers rely on mirrors, which become less efficient as they reflect higher-energy light.

Scientists have been trying to solve this problem by developing a version of the free-electron laser. This relies on ‘self-amplified spontaneous emission’ and does not require mirrors. Instead, a self-amplified spontaneous emission free-electron laser requires a high-quality electron beam and a long array of high-quality magnets, called an ‘undulator. ‘

When the electron beam passes through the undulator, the magnets vibrate the electrons from side to side, causing them to emit light as synchrotron radiation.

At high enough electron energies and with a long enough undulator system, a free-electron laser could theoretically produce an X-ray beam with a peak brightness more than one billion times greater than the brightest beam available today.

The success of the process is gauged by whether the free-electron laser effect has ‘saturated,’ meaning the point at which the maximum power has been given up by the electron beam and converted to coherent synchrotron radiation.

As electron bunches propagate down the undulator, they are bathed in the same light they generate. As they move back and forth through the magnets and interact with the electric field of this light, some gain energy, and some lose energy, depending upon their phase relationship with the light and the magnetic fields.

Consequently, two mutually reinforcing processes take place: In one process, the electrons begin to form microbunches separated by a distance equal to the wavelength of the light they generate.

In the second process, the light waves from the electrons begin to line up more in phase reinforcing and amplifying the light’s brilliance and intensity.

Eventually, a favourable runaway instability is said to develop. The light intensity grows exponentially along the undulator until the process ‘saturates,’ bringing the beam to its highest possible intensity. By the time the light beam emerges, its initial intensity is amplified more than a billion times.