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SPIE Professional January 2012

XFELs Emerge

Three active or planned hard X-ray free-electron lasers can produce extremely intense X-ray laser pulses, but each has distinct characteristics

By Jeff Hecht

IMAGE for article about XFELs in SPIE Professional

The free-electron laser (FEL) has come a long way since John M. J. Madey invented it at Stanford University in the 1970s. A major attraction has been its versatility. Madey's first FEL produced infrared light, but the wavelength is not limited to specific atomic or molecular transitions.

FEL emission comes from free electrons passing through a periodically varying magnetic field, and its wavelength depends on the electron energy and the magnetic field period. That allows FELs to operate across an exceptionally wide spectral range.

Short-wavelength FELs reached the soft X-ray band in 2005, when FLASH, the Free-Electron Laser at Hamburg, began operation. Powered by a one billion electron-volt (GeV) accelerator at Deutsches Elektronen-Synchrotron (DESY) in Hamburg, FLASH first operated at 13.5 nanometers, and now emits at wavelengths as short as 4.1 nm.

FELs reached the hard X-ray band, below 1 nm, when the Linac Coherent Light Source (LCLS) began operation in 2009 at the SLAC National Accelerator Laboratory in Stanford, CA.

A second hard X-ray FEL, the SPring-8 Angstrom Compact Free Electron Laser (SACLA) (xfel.riken.jp) in Japan, fired its first X-ray laser pulses in June 2011.

Aerial view of SACLA installation in Japan, showing the 700 meter structure that includes linac undulator rooms and experimental stations. (Courtesy of Riken)

A third hard-X-ray system, the European XFEL is under construction in Germany by a European consortium, with operation to begin in 2015.

The emerging generation of hard X-ray FELs share key features. All pass beams of electrons, accelerated to relativistic velocity, through long arrays of magnets with alternating polarity. All produce short, coherent, and extremely intense X-ray laser pulses — orders of magnitude more brilliant than the synchrotron radiation long used for X-ray research. But each has distinct characteristics.

U.S. Linac Coherent Light Source

LCLS got a head start because SLAC had a linac available, the two-mile Stanford Linear Accelerator, a Silicon Valley landmark that extends under Interstate 280 near its junction with Sand Hill Road. Built in the 1960s for particle physics, it is the world's longest linear accelerator but was no longer used for cutting-edge research.

SLAC overhauled its last kilometer to produce a high-quality beam of relativistic electrons with energies as high as 14.3 billion electron volts (GeV) to power LCLS.

Close-up of undulators at LCLS shortly after the XFEL's first operation in 2009. (Courtesy of SLAC National Accelerator Laboratory)

Electrons emerging from the accelerator pass through a series of 33 undulator modules, each 3.4 meters long, separated by 60 centimeters. Together, they stretch 132 meters along the beam tunnel. The magnetic field alternates directions with a 3-centimeter period along each module, bending the paths of the speeding electrons so they radiate X-rays.

As in a conventional laser, the initial spontaneous emission along the axis of the undulator stimulates emission from other free electrons in the undulator. However, X-ray lasers are limited to self-amplified spontaneous emission (SASE) because the lack of good resonator mirrors limits amplification to a single pass through the undulator.

Successful FEL operation requires "a very close coupling between the X-rays produced and the electrons producing them," says John Arthur, who leads the LCLS X-ray Facilities Operations Division. The electrons travel in groups that flatten in the direction of their travel, and they must be kept collimated and in phase with the emitted X-rays through the undulator. LCLS produces 120 pulses per second, each typically lasting 50 to 500 femtoseconds.

Self-amplified spontaneous emission is inherently chaotic, so the output has spikes lasting 1 to 2 fs for soft X-rays and as short as 300 attoseconds for hard X-rays.

The undulator period is fixed, but varying the number of acceleration stages used in the linac can change electron energy, tuning LCLS output between 0.12 and 3 nm. Increasing the wavelength from 0.15 to 1.5 nm increases the number of photons in a pulse by a factor of 10, keeping pulse energy roughly constant. A second-harmonic accessory can produce wavelengths as short as 0.07 nm, but at power sharply lower than the fundamental.

Grazing-incidence mirrors with about 95% reflectivity in the X-ray band deflect LCLS output to one of six stations housing specialized measurement instruments.

Most experiments require long set-up times, so typically only two instruments are used each week, one during the day and one at night. Meanwhile the previous week's two experiments are being disassembled and the next week's are being assembled.

Experiments are already yielding results unobtainable with less intense X-ray sources. The high photon flux can remove multiple electrons from atoms, revealing important details of intra-atomic quantum-mechanical forces.

Similarly, the intense X-ray emission allows scattering studies of crystals down to the nanometer scale, one to two orders of magnitude smaller than possible with synchrotrons. Arthur says that's important for analyzing the molecular structures of proteins, many of which are difficult to crystallize.

  Hard X-Ray FELs
SACLA in Japan

The Japanese SACLA X-ray FEL was built by RIKEN (a government-funded private research institute formally known as Rikagaku Kenky-jo) at the RIKEN Harima Institute in Hyogo Prefecture, a site it shares with the Spring-8 synchrotron source.

Commissioning began in late February 2011. Its first laser emission was at 0.12 nm on June 7. Just three days later, SACLA generated pulses at 0.10 nm, and on July 13 SACLA lased at 0.08 nm, a record for the shortest wavelength directly generated by a FEL. Testing is continuing, and current plans call for SACLA to open to outside users in March 2012.

Built over a five-year period from 2006 to 2010, the X-ray FEL has been designated one of Japan's "key technologies of national importance." The physics behind SACLA is the same as that of LCLS and the European XFEL, but the designs differ in important ways.

A crucial difference is the 400-meter linac that was custom-designed for X-ray FEL use rather than modified from an existing accelerator. A thermionic electron gun with a single-crystal cathode generates the SACLA input beam, and the accelerator can generate electric fields of 35 megavolts per meter, twice that of LCLS, so it can pump up electron energy faster.

The SACLA accelerator reached 8 GeV during preliminary testing, but the first laser tests were conducted with a 7 GeV beam. Cranking up the accelerator to 7.4 GeV was the key to reducing the laser wavelength from 0.1 nm in June 2011 experiments to 0.08 nm in July. Developers hope to reach 0.06 nm with 8-GeV electrons. The linac can fire up to 60 pulses per second, producing laser pulses as short as 30 fs.

SACLA's undulator operates in vacuum, allowing it to have a shorter magnetic-field period of 1.8 cm, which produces shorter wavelengths for a given electron energy than is possible with LCLS. The 90-meter undulator contains 18 segments, each 5 meters long, for a total of 5000 periods, more than the longer LCLS undulator.

The entire structure in Japan, including beam lines and instrument stations, stretches 700 meters, a little more than a third the length of the equivalent system at SLAC. Initially the system will include only a single instrument station, for hard-X-ray experiments. Four additional stations are planned in the same chamber, but only one will be usable at a time.

European XFEL in Germany

The most ambitious plans are for the European XFEL, to be completed in 2015. It will be the first FEL to use a superconducting linac, a dedicated 1.7-km accelerator now in the early stages of construction that will accelerate electrons to energies as high as 17.5 GeV.

Computer montage of the European XFEL main building at the site in Schenefeld: The five tunnels from which the laserlike X-ray flashes are led to the experiment stations will end in the underground hall beneath the main building. This will house labs and offices, seminar rooms, an auditorium, and the library. (Courtesy: European XFEL / Kontor B3)

It will fire 10 bursts of 2700 pulses each — a total of 27,000 pulses per second — which will be shared among several beam lines, making it the only XFEL to allow multiple users to perform experiments in parallel. The electron bunches "can be tailored to certain instruments, providing the corresponding experiments with the conditions they really want to have," says Thomas Tschentscher, scientific director of The European XFEL.

The electron beam from the linac will be routed through a "switchyard" that includes three undulators in different locations. An initial switch will select pulses to be routed down one of two beams, each feeding a 175-m undulator with 4-cm period designed to generate X-rays between 0.05 and 0.4 nm. (See figure above.)

Electrons passing through one undulator will be dumped, but electrons that pass through the other — having lost some of their energy — will be directed to a third undulator, 105 m long with a 6.8-mm period, to produce X-rays between 0.4 and 4.7 mm.

"It's a big effort to build the distribution system," says Tschentscher, who is co-chair of a conference on XFELs at SPIE Optics + Optoelectronics. However, it offers a big advantage to users because it allows multiple experiments to be run simultaneously. Plans call for installing 10 to 15 instruments for experimenters.

The whole beam line including linac, undulators, and experimental stations, will stretch 3.4 km, from DESY in Hamburg to the town of Schenefeld.

Peak brilliance of the European XFEL's beam is expected to equal SACLA's, a few times that of LCLS. But with a pulse rate much higher than either, the European XFEL's average brilliance will be a factor of 100 times higher. The high repetition rate should be particularly valuable for demanding experiments such as single-particle imaging of the three-dimensional structure of biological objects, which require very large data sets.

Initial switchyard for routing the European XFEL electrons through undulators to generate X-rays: A switch at left will split pulses between two undulators. Electrons that pass through the top undulator will be dumped. Those passing through the lower undulator will be passed through a third undulator to generate lower-energy X-rays. The X-rays then would be split among three stations, each serving two experiments.

With very short wavelengths, Tschentscher says, "We can penetrate more dense materials, and we plan to apply this to study dynamic processes in material science applications."

Future plans for XFELs

With users clamoring for more time on X-ray FELs, developers are already looking farther into the future. SLAC has proposed using another third (1 km) of the old linac to drive a parallel X-ray FEL with a new undulator directed into a second experimental hall.

"If all goes as planned, we expect that LCLS II will start producing X-rays in 2017 or 2018," Arthur says.

SLAC also is exploring designs for new undulators that would push to shorter wavelengths and higher brightness. One possibility is a two-stage XFEL, in which an initial SASE pulse would be taken from one undulator, and filtered to improve its quality, then passed through a second undulator that would amplify it to produce brighter and spectrally purer pulses for more demanding experiments.

Jaff Hecht




-Jeff Hecht is a science and technology writer and the author of Understanding Lasers, Laser Pioneers, and Beam.

Have a question or comment about this article? Write to us at spieprofessional@spie.org.

DOI: 10.1117/2.4201201.14

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