New pulse monitor advances data analysis at X-ray free-electron lasers
Researchers from DESY and the U.S. SLAC National Accelerator Laboratory have developed a new tool to measure individual pulses of X-ray free-electron lasers (XFELs) with unprecedented time resolution. At SLAC's Linac Coherent Light Source (LCLS), the scientists achieved a resolution of one femtosecond, or one millionth of one billionth of a second. The previous record for single-shot measurements was ten femtoseconds.
Owed to their extraordinary power packed into ultrashort light pulses, XFELs promise scientific breakthroughs in many areas, ranging from imaging of single molecules to filming the motion of electrons in atoms and molecules. However, the interpretation of such experiments is challenging because individual XFEL pulses vary in shape and length.
The new pulse monitor, unveiled in a study published by the journal Nature Communications, provides researchers with precise measurements of every single X-ray pulse - crucial information for the analysis of data collected at LCLS and potentially other free-electron lasers including DESY's future FLASH II facility and the European XFEL currently under construction in Northern Germany.
Irregular X-ray Pulses
XFELs are linear particle racetracks that accelerate bunches of electrons to nearly the speed of light before sending them through magnetic structures known as undulators. These cause the electrons to wiggle along their flight path and emit radiation that amplifies into very bright and ultrashort flashes of X-ray laser light.
Subsequent light pulses are not identical - they slightly differ in shape and duration. "The statistical nature of light production in XFELs as well as the acceleration process itself lead to fluctuations between separate shots," explains Christopher Behrens, DESY accelerator scientist and first author of the present study.
Such irregularities pose a significant challenge for experimenters. "Many processes studied with XFELs depend on how they are initiated or sampled by the X-ray pulse," Behrens says. "We therefore want to know the exact time-dependent pulse profile, which, for instance, could either be a single pulse or a sequence of two pulses."
Moreover, applications such as the imaging of single molecules rely on limiting the exposure of samples to damaging X-rays and, hence, would profit from a precise knowledge of X-ray pulse durations.
Yet, high-resolution pulse monitors for XFELs are not widely available. Until now, researchers estimated LCLS's X-ray pulse lengths indirectly from the lengths of the electron bunches that produce the X-ray pulses - an approximation often not accurate enough for the interpretation of experiments.
New Diagnostic Tool
Now, with the new method, scientists can measure X-ray pulses more directly. "We diagnose the electron beam after it has left the undulators," says SLAC's Yuantao Ding, one of the study's corresponding authors and a lead researcher on the project that is managed by SLAC accelerator physicist Patrick Krejcik. "Since the production of X-rays leads to an energy loss of the electrons, the electron bunch carries the 'footprint' of the X-ray pulse."
The centerpiece of the pulse monitor is a device called an X-band radio-frequency transverse deflector, which is located behind the XFEL undulators and kicks electrons out of their original flight path. Since the deflection differs between the head and tail of the electron bunch, its temporal profile along the accelerator is stretched or "streaked" in a direction perpendicular to it. Different positions in that direction correspond to different times in flight direction.
In a subsequent step, the streaked electron bunch traverses a dipole magnet that stretches the electron beam in yet another direction depending on electron energy - a process similar to a prism turning white light into a colorful spectrum.
The end result is a two-dimensional electron-bunch pulse profile, with time in one dimension and energy in the other.
The researchers then measure this profile when the XFEL produces laser light, and again when they suppress lasing. "The comparison of both profiles enables us to determine the time-dependent energy loss of the electron bunch due to the X-ray pulse," explains Behrens. "From the energy loss, in turn, we can reconstruct the power profile of the X-ray pulse."
Radio-frequency deflectors are routinely used for electron beam diagnostics in front of the XFEL undulators. "One new approach in our study was to insert a deflector behind the undulator, which enabled us to measure the X-ray pulses' footprints," says Behrens who laid the foundations for this project together with his SLAC colleagues while visiting the U.S. lab three years ago. Another feature of the new device is a tenfold increase in time resolution compared to conventional deflectors.
The pulse monitor is now available to LCLS users, providing them with accurate pulse information that is expected to advance their data analysis. The new tool can capture every single one of LCLS's up to 120 X-ray flashes per second and does not interfere with ongoing X-ray experiments.
Besides improving data interpretation, the method can also help enhance the performance of XFELs. "We know that we can taper undulators, for example, to increase their X-ray output," Ding says. "However, we do not understand all the details of this process yet and it needs to be optimized. With our tool, we now have a new set of eyes to look into these details."
Applications of the pulse monitor are not limited to LCLS. "It may also be of interest as a diagnostic tool for FLASH II and the European XFEL," says Behrens. "At FLASH II, for instance, it could be used to optimize the initiation of the laser process by an external laser known as seeding."
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