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January, 2007
Setting Free the Electrons

Set a swarm of electrons free from any atoms, gather them into a tight bunch, and accelerate that bunch to relativistic (near-light) speeds. Then give them a vigorous series of shakes: they will radiate light. This is the principle behind a device known as the free electron laser (FEL), and it has been used to generate tunable, coherent, high power beams of light at wavelengths in the ultraviolet regime.

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The free electron lasers (FELs) of FERMI@Elettra will be located at Sincrotrone Trieste, a major synchrotron light source in northern Italy near the Adriatic Sea.

Researchers at Berkeley Lab's Center for Beam Physics are now in the process of designing critical components for a new ground-breaking, next-generation FEL facility at Sincrotrone Trieste, the Italian national synchrotron radiation laboratory in Trieste, which will be able to generate carefully controlled pulses of light into soft x-ray wavelengths. They are also in the preliminary planning stages for an even more advanced FEL facility to be located here at Berkeley Lab

The FEL facility to be built in Trieste is called FERMI@Elettra. Serving as a complement to ELETTRA, a synchrotron light source similar to Berkeley Lab's Advanced Light Source (ALS), it will provide intense, tunable, coherent light in the vacuum ultraviolet (VUV) and soft x-ray region of the spectrum on a femtosecond time-scale. A femtosecond is a millionth of a billionth of a second. How brief is that? A femtosecond is to one second what one second is to about 30 million years.

"The FERMI@Elettra facility will be able to generate short photon pulses, under 100 femtoseconds, or longer pulses of up to many hundreds of femtoseconds," says John Corlett, a physicist with Berkeley Lab's Accelerator and Fusion Research Division (AFRD), who heads the Center for Beam Physics and is leading Berkeley Lab's FEL design efforts. "It will also be the world's first FEL facility to employ seeded harmonic cascades."

To understand the technology behind seeded harmonic cascades, it helps to know the basic FEL set-up. A short-wavelength FEL consists of a linear accelerator (linac) which is fed electrons that have been bunched and focused into a narrow beam. The linac accelerates this beam of electron bunches to relativistic energies, then sends it through an array of dipole magnets with alternating north and south poles, called an "undulator." The magnetic field of the undulator forces each bunch of electrons to oscillate back and forth, like skiers zigzagging through an extremely tight slalom course. This fierce oscillating motion causes the electrons to radiate photons, and these photons then push back on the electrons, moving them together into tight microbunches. The microbunched electron beam then generates a coherent pulsed beam of light, which travels in the same direction as the electron beam.

In a free electron laser, a relativistic beam of electrons is sent through an undulator, an array of dipole magnets with alternating north and south poles. The magnetic field of the undulator forces each bunch of electrons to oscillate back and forth, causing them to emit a laser-like beam of light. (Illustration by Flavio Robles, Creative Services Office)

The type of light emitted by an FEL — whether infrared, visible, UV or x-ray — depends upon the energy of the bunched electrons and the strength and period of the undulator's magnetic field. While conventional FELs can generate pulses of light millions of times brighter (more intense) than those from conventional lasers, or even third-generation synchrotron radiation sources like the ALS or ELETTRA, they suffer from pulse-repetition rates, the number of light pulses emitted each second, that are usually far too low for many critical applications. Short-wavelength FEL pulse-repetition rates are typically less than 100 hertz.

"With new designs of photocathode electron guns, we can obtain megahertz pulse repetition rates," said Corlett. "This, together with superconducting linac technology and seeded harmonic cascades, gives us much greater control over beam pulse rate and duration and other characteristics of FEL light."
An FEL based on seeded harmonic cascades employs a separate laser and one or more two-stage undulators in addition to the linac. The laser is used to shoot a beam of light at a chosen wavelength through the linac's electron beam as it enters the first undulator stage. This action "seeds" the energized electron beam, causing it to microbunch at the same wavelength as the seed laser. When the seeded electron beam passes through the second undulator stage, it emits light at multiples of the seed laser frequency, called harmonics. If several two-stage undulators are arranged in a series so that each emits outgoing radiation at a harmonic of the incoming radiation, a cascade effect results, yielding light that is at a short wavelength not attainable from other sources with the required intensity, tunability, coherence, and extremely short pulse lengths and high pulse-repetition rates.

"The harmonic cascade effect allows us to get the controlled generation of shorter wavelength radiation, which is needed for experimentation in many areas of science," said Corlett. "Plus, we can produce these FEL beams using a conventional laser to seed the electron beam."

FERMI@Elettra will employ two FELs, one which will have a single harmonic stage and be able to produce light in the 40 to 100 nanometer frequency range, and the other of which will have two harmonic stages and be able to produce light as short as 10 nanometers in wavelength. Corlett and his colleagues at the Center for Beam Physics are currently collaborating with Sincrotrone Trieste personnel to design a high-brightness radio-frequency photocathode gun (which will emit electrons when illuminated by laser), a main linac and electron bunch compressors, and a beam spreader that distributes the electron bunches to the two FELS.

FERMI@Elettra will build upon existing facilities to create the world's first seeded harmonic cascade free electron laser.

"In this process, we're learning how to obtain high-quality electron bunches in the presence of strongly perturbative electromagnetic fields, how to optimize bunch compression for specific FEL requirements, and how to optimize the performance of seeded harmonic cascades," said Corlett. "The FERMI@Elettra facility will require ultrastable timing and synchronization, instrumentation, and radiofrequency power systems, all areas in which Berkeley Lab excels."

With 100 femtosecond pulse lengths, the FERMI@Elettra FELs can be used to study the motion of atoms in molecules during physical, chemical, and biological reactions, and to observe how molecular structures change. In the future, however, scientists would also like to be able to track the motion of electrons during such reactions. For this they will need to able to do spectroscopy on the attosecond timescale, the timescale of electron motions — it takes an electron about 24 attoseconds to orbit a hydrogen nucleus. How brief is this? There are more attoseconds in one minute than there have been minutes in the entire history of the universe.

To produce soft x-ray and UV light at pulse lengths on the attosecond timescale, Corlett and his colleagues at the Center for Beam Physics, along with collaborators in Berkeley Lab's Materials Sciences, Chemical Sciences, and Engineering Divisions, are proposing a new cascaded harmonic FEL facility. This new FEL facility will feature a single-pass linac equipped with powerful superconducting radio-frequency (RF) acceleration systems rather than the conventional pulsed RF systems that are to be used in the FERMI@Elettra linac. The superconducting RF will enable the Berkeley Lab linac to accelerate a high-repetition-rate electron beam to energies possibly as high as 2 billion electron volts (2GeV). This energized beam of electrons will then feed an array of ten FELs, each operating at a repetition rate of 100 kilohertz or higher.

One plan under consideration for a future facility calls for the superconducting linac to be located in a 350-meter-long tunnel that would stretch from south of the ALS to the site of the decommissioned Bevatron accelerator, which is now being dismantled. Housed at the south end of the tunnel would be a high-repetition-rate, low-emittance injector. At the north end of the tunnel, aboveground, would be the experimental hall housing the FEL array and the x-ray beamlines and experimental areas.

The combination of high peak and average brightness with control of pulse duration will provide a unique facility that would push the performance envelope for light source technologies and complement the scientific capabilities of both the ALS and the Linac Coherent Light Source (LCLS) now under construction at the Stanford Linear Accelerator Center.

"Our proposed FEL facility will be able to fully address a number of major scientific challenges with ultrafast attosecond pulses, and also with high-energy-resolution capabilities," said Corlett. "It will open up new areas of research and enhance the technology base for future accelerator-based light sources."

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John Corlett is a physicist with Berkeley Lab's Accelerator and Fusion Research Division who heads the Center for Beam Physics and is leading the design of next generation free electron lasers. (Photo by Roy Kaltschmidt, Creative Services Office)

In a white paper prepared this past summer for the U.S. Department of Energy by a team of Berkeley Lab scientists, which included Corlett, physicists William McCurdy and Robert Schoenlein made the scientific case for this new harmonic cascade FEL facility. One of the driving challenges in the high-tech world today, they said, is to be able to control the properties of matter. To achieve this goal, scientists will need a much better understanding of two fundamental issues: how do correlated electron interactions give rise to emergent properties in atoms, molecules, and solids? And how are material properties generated by the coupling of correlated electrons?

 "It's clear that in the future, direct quantitative measurements of the electronic and atomic structural dynamics on the ultrafast time scale of the underlying correlations will be indispensable for achieving new insight into the physics of complex systems and novel properties emerging from correlated phenomena in atoms, molecules, and complex solids," McCurdy wrote. "Time resolution, high average flux, high repetition rate, high resolving power, and soft x-ray tunability emerge as critical needs that are not fully met by existing, planned, or under construction light sources."

Schoenlein, a leading authority on ultrafast spectroscopy, wrote, "Strong interactions between electron charge, spin, and lattice give rise to exotic properties and new functionality. The unprecedented temporal precision and resolution of this machine in the soft x-ray spectrum will allow us to unravel these correlated effects for the first time by making measurements of the electronic structure on time-scales short compared to the correlation times. In addition the spectral brightness of this machine will allow us to use x-ray scattering techniques to resolve the electronic structure with energy and momentum resolution, and bulk sensitivity that is far beyond what can be done at present soft x-ray sources."

In addition to Corlett, McCurdy, and Schoenlein, co-authors of the white paper titled Scientific Challenges for Future Light Sources, and Accelerator Research and Development Required to Meet Them were Ali Belkacem, Roger Falcone, Graham Fleming, Zahid Hussain, Janos Kirz, Wim Leemans, Steven Leone, Daniel Neumark, Howard Padmore, David Robin, Christopher Steier, and Alexander Zholents.

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