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.
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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