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.
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.
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.
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 (Si<sub>3</sub>N<sub>4</sub>). 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.