Subsequent projects

Prof. Dr. Reinhard Kienberger
Technische Universität München
Lehrstuhl für Physik E11

Dr. Ryan N. Coffee
The Linac Coherent Light Source (LCLS),
SLAC National Accelerator Laboratory

Characterization of the FEL pulse sub-structure in the time domain

Within the BaCaTeC project, measurements at the LINAC Coherent Light Source (LCLS) at Stanford, California, the world’s largest and brightest coherent X-ray facility at present, have been performed. The first results yielded a resolution of the ultrafast pulse substructure with sub-femtosecond precision utilizing photoelectron spectroscopy. Notwithstanding its importance for the characterization of LCLS itself, the precise measurement of such short FEL pulse durations is a prerequisite for the design of many future experiments with the goal to obtain detailed information on physical, chemical and biological dynamics, therefore paving the way to novel and attractive applications in medicine, especially in the fields of soft tissue imaging and cancer diagnostics.


Primary project: Measuring the shortest X-ray pulses in the world


Final Report

The goal of this project was the temporal characterization of ultrashort and ultrabright X-ray pulses at the Linac Coherent Light Source (LCLS) operated by the Stanford University in California. The ultrashort pulse duration available at this novel free-electron laser (FEL) is a fundamental and crucial parameter for a host of high-intensity dynamical experiments aiming to obtain detailed structural and functional information on physical, chemical and biological systems. The precise knowledge of the pulse duration of X-ray FEL pulses is a fundamental prerequisite for few-femtosecond X-ray pump/probe experiments at X-ray FEL facilities.

Up to now, only an upper border of the FEL pulse duration could be determined relying on the so-called linear streaking technique, which was established at LCLS during a former BaCaTeC-supported project. However, there was a lack of an unambiguous, direct and single-shot time–energy X-ray pulse characterization method. We demonstrate a new approach for the characterization of the FEL pulse structure with the two-color angular streaking technique, which uses a circularly polarized IR streaking laser and an array of 16 time-of-flight detectors for mapping the FEL pulse properties to the angular distribution of X-ray-generated photoelectrons.

We showed that the angular streaking technique is capable of measuring the temporal substructure and the chirp of arbitrary X-ray pulses with sub-femtosecond precision on a single-shot basis. Furthermore, the relative X-ray/optical arrival time jitter arising due to the statistical nature of the self-amplified spontaneous emission (SASE) process can be determined within the same measurement. Eventually, this method permits us to directly tag the substructure of FEL pulses, revealing single and double SASE spikes for X-ray pump/probe measurements with durations of only a few hundred attoseconds. The results have been accepted by Nature Photonics for publication in the immediate future.


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