Diamond sharpens x-ray beam razor thin

A thin sliver of diamond can transform the Linac Coherent Light Source (LCLS) into a precise tool for studying and manipulating matter at the atomic level – and may enable experiments never before possible.

In the “self-seeding” technique, conducted by scientists at the US Department of Energy’s SLAC National Accelerator Laboratory, the diamond filters the laser beam to a single x-ray color, which is then amplified. The method yields laser pulses focused to higher intensity in a much narrower band of x-ray wavelengths and will deliver sharper images of materials, molecules and chemical reactions.

To create a precise x-ray wavelength band and make the Linac Coherent Light Source even more “laserlike,” researchers installed this chamber with a slice of diamond crystal. The new hardware sits halfway down the 130-m bank of magnets where the x-rays are generated.

“The more control you have, the finer the details you can see,” said Jerry Hastings, a SLAC scientist and co-author of the research, which appeared in Nature Photonics (doi: 10.1038/nphoton.2012.180).

Self-seeding has the potential to produce x-ray pulses with significantly higher density than the LCLS currently produces. The increased intensity in each pulse could be used to probe deeply into complex materials, helping answer questions about exotic substances such as high-temperature superconductors or the intricate electronic states found in topological insulators.

“People have been talking about self-seeding for nearly 15 years,” Hastings said. “The method we incorporated at SLAC was proposed in 2010 by Gianluca Geloni [of the European XFEL research center and by] Vitali Kocharyan and Evgeni Saldin [both of the DESY research center] ... in Germany. When our team from SLAC and Argonne National Laboratory built it, we were surprised by how simple, robust and cost-effective the engineering turned out to be.”

The Linac Coherent Light Source’s new self-seeding improvements yield laser pulses focused to higher intensity in a much narrower band of x-ray wavelengths, as seen in these spectrographs comparing a normal SASE (self-amplified spontaneous emission) pulse (left) and a seeded one (right). The results promise to speed research discoveries and may enable experiments that have never before been possible.

The LCLS generates its laser beams by accelerating bunches of electrons to nearly the speed of light and setting them on a zigzag path with a series of magnets. This forces the electrons to emit x-rays, which are gathered into laser pulses a billion times brighter than any available before and fast enough to scan samples in quadrillionths of a second.

Without self-seeding, the x-ray pulses contain a range of wavelengths in unpredictable patterns that cannot all be used by experimenters. Until now, a narrower-wavelength band could be created at LCLS only by subtracting the unwanted wavelengths, resulting in a substantial loss of intensity.

To create a precise x-ray wavelength band and make the LCLS more “laser-like,” researchers installed a diamond crystal slice halfway down the 130-m bank of magnets where the x-rays are generated.

“The resulting pulses could pack up to 10 times more intensity when we finish optimizing the system and add more undulators,” said Zhirong Huang, a SLAC accelerator physicist and study co-author.

The self-seeding system has excited the international scientific community. Representatives from other x-ray laser facilities, including FEL in Switzerland, SACLA in Japan and the European XFEL, helped with it and are studying how to implement their own systems.