Making the Old New Again: Measuring Ultrashort X-ray Laser Pulses

Researchers have developed a powerful new diagnostic tool for the Linac Coherent Light Source (LCLS) with femtosecond resolution.

Image courtesy of SLAC National Accelerator Laboratory
The figure shows measured electron bunch images with lasing suppressed (top left) and full lasing (right). Horizontal dimension represents time and vertical represents energy. This lasing-induced time-dependent energy change is used to reconstruct X-ray power profiles. The inset shows the deflecting structure installed at LCLS for these measurements.

The Science

A tool developed a half a century ago for sorting subatomic particles has been redesigned to measure X-ray laser pulses at the 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 few femtoseconds, or quadrillionths of a second, giving scientists a much more detailed view of the individual pulses that interact with their samples.

The Impact

For the first time scientists can directly measure the X-ray power profile on a shot-by-shot basis with femtosecond resolution, providing a noninvasive diagnostic tool for photon experiments and new insight into lasing dynamics. LCLS users have already started to use this X-ray tool to assist in analyzing data from experiments.

Summary

Ultrashort X-ray pulses generated in free-electron lasers (FELs), which can be a few femtoseconds to several hundred femtoseconds in duration, enable ultrafast time-resolved X-ray studies. Temporal characterization of such X-ray pulses on a shot-by-shot basis is critical for photon experiments but is also very challenging. By utilizing an X-band radio-frequency transverse deflector (also called transverse cavity, or “XTCAV”) at the LCLS, researchers have demonstrated a new technique to measure X-ray pulses with femtosecond resolution. A section of the linear accelerator at SLAC National Accelerator Laboratory produces a powerful electron beam that is stimulated by an undulator – a series of powerful magnets – to emit pulses of ultra-bright X-ray light. The XTCAV produces an elongated "streak" of the electron beam traveling through the undulator and maps its duration, enabling measurements of the time-dependent FEL lasing effects on a transverse profile monitor. By comparing streaked images during lasing with images taken under suppressed lasing conditions, the X-ray temporal distribution can be determined. The data processing for X-ray reconstruction is simple and reliable, providing a constant stream of noninvasive, single-shot X-ray profiles that can add an important new layer of data analysis to user experiments.

Contact

Yuantao Ding
SLAC National Accelerator Laboratory
ding@slac.stanford.edu

Funding

U.S. Department of Energy, Office of Science, Basic Energy Sciences program and Early Career Research Program

Publications

C. Behrens, F.-J. Decker,  Y. Ding, V. A. Dolgashev, J. Frisch, Z. Huang, P. Krejcik,  H. Loos, A. Lutman, T. J. Maxwell, J. Turner, J. Wang, M.-H. Wang, J. Welch, and J. Wu, “Few-femtosecond time-resolved measurements of X-ray free-electron lasers.” Nature Communications. 5:3762 (2014). [DOI: 10.1038/ncomms4762]

Y. Ding, C. Behrens, P. Emma, J. Frisch, Z. Huang, H. Loos, P. Krejcik, and M-H. Wang, “Femtosecond x-ray pulse temporal characterization in free-electron lasers using a transverse deflector.” Physical Review ST Accelerators and Beams. 14:120701 (2011). [DOI: 10.1103/PhysRevSTAB.14.120701]

V. A. Dolgashev and J. Wang. “RF Design of X-band RF deflector for Femtosecond Diagnostics of LCLS Electron Beam”.  AIP Conf. Proc. Vol. 1507, 682 AIP, Texas, USA, 2012.  [DOI: 10.1063/1.4773780]

O. H. Altenmueller, R. R. Larsen and G. A. Loew, “Investigations of Traveling-Wave Separators for the Stanford Two-Mile Linear Accelerator.” Review of Scientific Instruments 35:438 (1964). [DOI: 10.1063/1.1718840]

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