FEL profiling - 'characterization of individual FEL pulses'

FEL profiling - 'characterization of individual FEL pulses'

Full temporal characterization of individual FEL pulses at the X-ray free-electron laser FLASH was achieved using independent, optical laser-driven, single-cycle terahertz radiation for photoelectron “streaking” spectroscopy.

A single-cycle terahertz field accelerates photoelectrons emitted from neon atoms irradiated by an X-ray free-electron laser. In this way, the X-ray pulse temporal profile and arrival time are uniquely retrieved on a pulse-to-pulse basis with femtosecond precision.

In the measurement, the FEL pulse ejects a burst of photoelectrons from a noble gas with a temporal profile that is a replica of the incident FEL pulse. The overlapping THz field accelerates or decelerates the photoelectrons within the burst depending on their instant of release. The resultant streaked kinetic energy spectrum of the accelerated photoelectrons, in conjunction with independent measurement of the THz streaking field, allows the FEL temporal profile to be uniquely mapped on a pulse-to-pulse basis, with femtosecond precision. Because an external source of THz radiation is used, the recovered pulse profile is naturally synchronized to the user experiment environment. As a result, the femtosecond X-ray pulse arrival time information is also obtained simultaneously.

Because the output characteristics of current generation FELs operating on the principle of self-amplified spontaneous emission (SASE) can fluctuate strongly from pulse to pulse, this information will be crucial to their future application.

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