Generation of high power, femtosecond to sub-femtosecond x-ray pulses is attracting much attention within the x-ray free-electron laser (FEL) user community. At the existing FEL facilities, such as the Linac Coherent Light Source at SLAC, several methods have been developed to produce such short x-rays. Low-charge operation mode and emittance-spoiling scheme have successfully delivered short pulses for user experiments with duration less than 10 fs. A nonlinear compression mode has been recently developed and the pulse duration could be about 200 as. We will review the recent experimental progress at the LCLS for achieving few-femtosecond x-rays, and also discuss other short pulse schemes for reaching sub-femtosecond regime.
Sanghoon Song, Diling Zhu, Andrej Singer, Juhao Wu, Marcin Sikorski, Matthieu Chollet, Henrik Lemke, Roberto Alonso-Mori, James Glownia, Jacek Krzywinski, Alberto Lutman, Yuantao Ding, Timothy Maxwell, James Turner, Oleg Gorobtsov, Ivan Vartanyants, Aymeric Robert, Yiping Feng
Intensity interferometry measurements were carried out to study the spatial coherence properties of a Free-Electron Laser (FEL) in the Self-Amplified Spontaneous Emission (SASE) mode in the hard X-ray regime. Statistical analyses based on ensemble averages of the spatial intensity correlation function were performed on a large number of pulses, overcoming challenges associated with the FEL beam being non-stationary in time and highly collimated. The second-order intensity correlation functions consistently show deviations from unity, reminiscent of the classical Hanbury-Brown and Twiss effect. They also exhibit a slow decaying spatial dependence at length-scales larger than the width of the beam, indicating a high degree of spatial coherence. These measurements are consistent with the behavior of a highly brilliant but chaotic source obeying Gaussian statistics as expected for a SASE FEL. Our study could be used to devise an in-line diagnostic capable of providing quasi real-time feedback for understanding and tuning the FEL process.
Analysis of single-shot, lasing-induced changes of the longitudinal electron bunch properties has proven invaluable for fs-scale reconstruction of otherwise difficult to measure x-ray FEL pulse profiles. In this talk, we report on measurements following the recent installation of an X-band transverse deflecting mode cavity at the LCLS. Limitations of the FEL pulse profiling technique employed are discussed. An unprecedented 1 to 3 fs RMS time resolution of x-ray and electron bunch profiles is demonstrated. Phenomena impacting x-ray FEL performance are also observed. The new tool is proven as a powerful diagnostic in support of user experiments and machine improvement studies.
Mina Bionta, Doug French, James Cryan, James Glownia, Nick Hartmann, David Nicholson, Kevin Baker, Christoph Bostedt, Marco Cammarrata, Matthieu Chollet, Yuantao Ding, David Fritz, Steve Durbin, Yiping Feng, Marion Harmand, Alan Fry, Daniel Kane, Jacek Krzywinski, Henrik Lemke, Marc Messerschmidt, Daniel Ratner, Sebastian Schorb, Sven Toleikis, Diling Zhu, William White, Ryan Coffee
A recently demonstrated single-shot measurement of the relative delay between x-ray FEL pulses and optical laser pulses has now been improved to ~10 fs rms error and has successfully been demonstrated for both soft and hard x-ray pulses. It is based on x-ray induced step-like reduction in optical transmissivity of a semiconductor membrane (Si3N4). The transmissivity is probed by an optical continuum spanning 450 - 650 nm where spectral chirp provides a mapping of the step in spectrum to the arrival time of the x-ray pulse relative to the optical laser system.
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