Ultrafast Source Science
The future performance requirements on accelerator-based photon sources demand methods for improving the control of photon pulse characteristics. To meet these challenges, PULSE is participating in a collaborative effort with SLAC Accelerator Physics and LBNL to move towards implementing Enhanced Self-Amplified Spontaneous Emission (ESASE) at the LCLS.
The ultrafast x-ray research enabled by LCLS in the areas of atomic, molecular, chemical, materials, and biological research are truly revolutionary, but an x-ray beam based on SASE presents severe challenges that must be overcome, particularly in the area of time-resolved pump-probe interactions. The incoherent x-rays produced by an undulator are relatively easy to time using techniques developed at the SLAC SPPS experiment and elsewhere to resolve the electron bunch; but the severe electron beam reshaping that occurs during the SASE lasing process makes upstream electron bunch measurements unreliable indicators of the x-ray laser pulse duration and arrival time.
We can remove the uncertainty by controlling the electrons, preferably with a laser that can be used to excite a system under study, or at least provide a timing mark. This is what ESASE provides. For this reason, a successful ESASE project could be integral to the success of pump-probe experiments at LCLS.
ESASE can do more than just set the location in time of the laser. Since the ESASE modulations are at the scale of the optical carrier, we have the opportunity to time events that last considerably less than the LCLS specified pulse duration, down to times less than one femtosecond.
For example, in the area of photoinduced isomerization of organic molecules, such as the trigger mechanism for Vitamin D production, or photosynthesis, optical spectroscopy experiments have already shown that the first stage of excitation only lasts for tens of femtoseconds. These structural changes could be tracked by observing changes in the structure factor for elastic scattering, thereby achieving both femtosecond and angstrom resolution.
Another area of high interest is electronic relaxation. The characteristic relaxation time in an atom following x-ray photoabsorption or photoemission is often considerably less than one femtosecond, and without very fine control over the XFEL beam, measurements on this scale are simply out of reach. Even in the case of bulk materials such as metals, the carrier dynamics have characteristic timescales set by the inverse of the plasma frequency, which is less than one femtosecond in most cases.
Any observation on these time scales could also be used immediately to verify the sub-femtosecond time structure of the ESASE beam, so basic research has a strong role in the development and commissioning of ESASE, and not only in its implementation.
The ultimate use of electron beam conditioning such as ESASE is to impose some coherent control on the output of the x-ray laser. Viewed in this way, the ESASE method provides a simple method for control: modification of the temporal coherence properties of the laser beam, its frequency, dispersion, and intensity envelope.
The ESASE technique uses an optical laser to control the part of the electron bunch responsible for lasing [A. Zholents. Phys. Rev ST Accel. Beams, 8, 040701 (2005).]. An optical laser interacting with the electron beam in an upstream wiggler induces energy modulation, which is then converted to a large density modulation prior to entering the SASE undulator. The uniformly-spaced, high-current spikes within the electron bunch amplify the SASE radiation much faster than the rest of the bunch, allowing for a shorter saturation length and hence the generation of ESASE x-ray spikes. The use of the optical laser provides natural synchronization for pump-probe experiments and x-ray pulse length control. When applied to the LCLS [A. Zholents, W. Fawley, P Emma, Z. Huang, G. Stupakov, S. Reiche, Proceedings of FEL2004 Conference, p. 582 (2004)], ESASE offers potential improvement of the lasing process, better synchronization capability, and flexible x-ray pulse structures. The number of attosecond x-ray spikes depends on the length of the optical pulse and can in principle be reduced to a single spike.