Abstract
Arbitrary manipulation of the temporal and spectral properties of x-ray pulses at free-electron lasers would revolutionize many experimental applications. At the Linac Coherent Light Source at Stanford National Accelerator Laboratory, the momentum phase-space of the free-electron laser driving electron bunch can be tuned to emit a pair of x-ray pulses with independently variable photon energy and femtosecond delay. However, while accelerator parameters can easily be adjusted to tune the electron bunch phase-space, the final impact of these actuators on the x-ray pulse cannot be predicted with sufficient precision. Furthermore, shot-to-shot instabilities that distort the pulse shape unpredictably cannot be fully suppressed. Therefore, the ability to directly characterize the x-rays is essential to ensure precise and consistent control. In this work, we have generated x-ray pulse pairs via electron bunch shaping and characterized them on a single-shot basis with femtosecond resolution through time-resolved photoelectron streaking spectroscopy. This achievement completes an important step toward future x-ray pulse shaping techniques.
Highlights
Indirect observation of the x-ray pulse structure has been achieved by analysis of the electron bunch after it has propagated through the Free-electron lasers (FELs) undulator [17]
In this work, we demonstrate all key elements required for advanced x-ray pulse shaping through electron beam manipulation by first generating x-ray pulse pairs and directly characterizing the x-rays to verify, control and tune the delivered pulse profile
When the duration of the ionizing x-ray pulse is shorter than the half-cycle of the THz streaking field, the FEL pulse structure can be reconstructed from the measured photoelectron spectrum provided that the instantaneous THz vector potential is known
Summary
Indirect observation of the x-ray pulse structure has been achieved by analysis of the electron bunch after it has propagated through the FEL undulator [17]. For very short x-ray pulses, especially sub-femtosecond x-ray pulses that may be generated in the future, the technique suffers from mismatch in the effective velocity of the electron beam in the undulator structure and the speed of light, which fundamentally limits the measurement resolution.
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