Abstract
Free-electron lasers are unique sources of intense and ultra-short x-ray pulses that led to major scientific breakthroughs across disciplines from matter to materials and life sciences. The essential element of these devices are micrometer-sized electron bunches with high peak currents, low energy spread, and low emittance. Advanced FEL concepts such as seeded amplifiers rely on the capability of analyzing and controlling the electron beam properties with few-femtosecond time resolution. One major challenge is to extract tomographic slice parameters instead of projected electron beam properties. Here, we demonstrate that a radio-frequency deflector in combination with a dipole spectrometer not only allows for single-shot extraction of a seeded FEL pulse profile, but also provides information on the electron slice emittance and energy spread. The seeded FEL power profile can be directly related to the derived slice emittance as a function of intra-bunch coordinate with a resolution down to a few femtoseconds.
Highlights
The after peak values of the undulator K-parameters are Krmada,x1−3 = the undulator guides the electron beam around a mirror use
The presented study allows to map the regions of the electron bunch, that are capable of lasing with femtosecond resolution
The required assumptions on the initial energy spread restrict its applications to soft x-ray facilities, where collective effects do not spoil the bunch properties significantly
Summary
A radio-frequency deflector downstream of the radiator of an FEL is the ideal tool to non-destructively measure the longitudinal power profile of the emitted laser light generated from the signature imprinted to the electron bunches. Important electron beam parameters, i.e. slice energy spread and slice emittance are derived by evaluating the measured phase space distribution of unseeded electron bunches upon lasing. With these slice parameters at hand we were able to predict quantitatively the performance of the seeded FEL configuration with an established semi-analytical model. The recorded FEL peak power as a function of the relative timing between the optical femtosecond laser seed and the electron bunch is well described in our model. The effect of inhomogeneities along the electron bunch on the interaction with optical seed pulses become visible and can be studied in great detail on the micrometer length and femtosecond time scale, respectively. Novel phase-coherent multicolor lasing schemes - possibly with low bunch charges in order to reach the shortest pulses - will rely on the presented capability to analyse and control ultra relativistic electron and photon beams on their intrinsic timescale, which is essentially given by the speed of light
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