New tool to measure X-ray pulses borrows from SLAC history (w/ Video)

June 27, 2013 by Glenn Roberts Jr., SLAC National Accelerator Laboratory

The XTCAV device installed at SLAC's Linac Coherent Light Source X-ray laser, pictured here with a fish-angle lens, provides precise measurements of X-ray pulse characteristics. Credit: Matt Beardsley
( —A tool developed half a century ago for sorting subatomic particles has been redesigned to measure X-ray laser pulses at SLAC's Linac Coherent Light Source (LCLS).

The result is a new device, called an X-band radio-frequency deflector, or XTCAV, that pinpoints the duration of X-ray pulses to within a couple of , or quadrillionths of a second, giving scientists a much more detailed view of the individual pulses that interact with their samples. The XTCAV was successfully tested this month, concluding a two-year fabrication and installation project.

"The ultimate goal is to screen every pulse, so researchers know the pulse length and pulse shape of every single they're getting" for every experiment at LCLS, said Patrick Krejcik, a SLAC accelerator physicist and XTCAV project manager.

Until now, scientists got information about the lengths of LCLS X-ray pulses indirectly, by measuring the that give rise to the X-ray pulses and making certain calculations.

The new method, proposed in 2011, allows them to measure the X-ray pulses in a more direct way.

In the LCLS, bunches of electrons wiggle through a series of powerful magnets and emit ultrabright X-ray light in the form of . These pulses travel to the experimental halls, while the remaining electrons continue to a beam dump for disposal.

Because the lasing process along the electron bunch is typically not uniform, and some electrons emit more intense X-ray light than others, the spent electron bunches carry the indelible fingerprint of the X-ray pulses they had spawned upstream. The XTCAV examines this electron waste stream and extracts detailed information about the X-ray pulses.

This schematic (top) shows an electron bunch entering the XTCAV, located just downstream of the LCLS undulator. The XTCAV gives the bunch a "kick" of radio-frequency energy, and this kick affects the head and tail of the bunch differently. As a result, the bunch appears elongated or "streaked" on the screen image (bottom). The bunch is streaked horizontally in time and is spread out vertically in energy. The elongation of the streak is proportional to the bunch length. Credit: Yuantao Ding
"It tells us not only the length and energy profile of the electron bunches, but also the pulse length and arrival time of the X-rays," Krejcik said.

While a few other techniques for measuring the length of X-ray pulses have been tested, the XTCAV demonstrates a permanent solution that doesn't disturb the X-ray pulses and can produce higher-quality data in experiments.

"You learn more about the formation of the X-ray laser pulses – for instance, which portions of the electron bunches are driving them – which allows us to improve LCLS performance through tuning," said Yuantao Ding, a staff scientist and lead researcher on the XTCAV.

Already, the device has been tested in a special "self-seeding" mode at LCLS that produces X-ray pulses in a very narrow band of X-ray "colors," and has provided new insights about the pulse characteristics in this mode.

Zhirong Huang, who helped oversee research and development for the XTCAV, said it gives LCLS machine physicists and X-ray users a new set of eyes to see and study these ultrafast X-ray pulses with a precision of less than 10 femtoseconds.

"All atomic processes happen on the scale of a few femtoseconds," Huang said. "I think this will be a key diagnostic for those experiments."

Krejcik noted that the XTCAV project has involved both accelerator staff and LCLS staff, including many SLAC scientists, engineers and machinists, as well as Christopher Behrens, a visiting scientist from the German Electron Synchrotron (DESY).

This historic photograph shows an S-band transverse deflecting structure, or "LOLA," named for creators Greg Loew, Rudy Larsen and Otto Altenmueller. LOLA is a predecessor to an XTCAV device that will be used to gauge the properties of X-ray laser pulses at SLAC's Linac Coherent Light Source. Credit: Greg Loew
"It's quite specialized. Really, this project could only be done at SLAC," he said, because of the lab's unique expertise.

The XTCAV device resembles a mini accelerator. But rather than accelerating the electron bunches, it is designed to deflect and stretch or "streak" them to better reveal their shape and other key characteristics.

The device is coupled with a high-speed camera that captures information on the , revealing the length and energy of about one in 12 X-ray pulses headed for the LCLS. Planned upgrades include a faster camera that will measure every pulse.

SLAC's Patrick Krejcik talks about the XTCAV device, a powerful new tool to gauge X-ray laser pulse properties for SLAC's Linac Coherent Light Source. Credit: SLAC Multimedia Communications

The XTCAV modernizes and miniaturizes a 50-year-old SLAC creation known as S-band transverse deflecting structures or "LOLA," named for creators Greg Loew, Rudy Larsen and Otto Altenmueller. It was originally used to select specific types of particles produced in fixed-target experiments for analysis. Today the devices serve as diagnostic tools for electron bunches produced in SLAC's linear accelerator.

Loew, deputy director emeritus at SLAC, said it's gratifying to know that LOLA is still inspiring new uses: "I wish everything else would have this kind of long life."

Explore further: New X-ray tool proves timing is everything

Related Stories

New X-ray tool proves timing is everything

February 20, 2013

(—With SLAC's Linac Coherent Light Source X-ray laser, timing is everything. Its pulses are designed to explore atomic-scale processes that are measured in femtoseconds, or quadrillionths of a second. Determining ...

X-rays in the fast lane

May 10, 2013

X-ray free-electron lasers (XFELs) produce higher-power laser pulses over a broader range of energies compared with most other x-ray sources. Although the pulse durations currently available are enormously useful for the ...

X-ray laser pulses in two colors

March 27, 2013

( —SLAC researchers have demonstrated for the first time how to produce pairs of X-ray laser pulses in slightly different wavelengths, or colors, with finely adjustable intervals between them – a feat that will ...

X-rays capture electron 'dance'

January 31, 2013

(—The way electrons move within and between molecules, transferring energy as they go, plays an important role in many chemical and biological processes, such as the conversion of sunlight to energy in photosynthesis ...

Cool electron acceleration

June 4, 2013

Physicists from the Max-Planck-Institute of Quantum Optics produced electron pulses from a laser accelerator whose individual particles all have nearly the same, tuneable energy.

Recommended for you

New thermoelectric material delivers record performance

January 17, 2019

Taking advantage of recent advances in using theoretical calculations to predict the properties of new materials, researchers reported Thursday the discovery of a new class of half-Heusler thermoelectric compounds, including ...

Zirconium isotope a master at neutron capture

January 17, 2019

The probability that a nucleus will absorb a neutron is important to many areas of nuclear science, including the production of elements in the cosmos, reactor performance, nuclear medicine and defense applications.

Mechanism helps explain the ear's exquisite sensitivity

January 16, 2019

The human ear, like those of other mammals, is so extraordinarily sensitive that it can detect sound-wave-induced vibrations of the eardrum that move by less than the width of an atom. Now, researchers at MIT have discovered ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.