Juhao Wu (February 14, 2011)
Hard X-ray FEL self-seeding
Description of topic and motivation:
The success of the LINAC Coherent Light Source (LCLS) [1] motivates extensions of the capacity,
capabilities, and quality of this revolutionary new light source. One important research of all these
possibilities is to generate narrow bandwidth hard x-ray Free Electron Laser (FEL). Lacking the coherent
seed at hard x-ray, self-seeding scheme emerges as a promising solution [2]. In particular, the recently
proposed single-crystal scheme [3] excites research team at SLAC to experimentally realize it. Reaching
Terawatts (TW) level FEL is extremely useful for single molecule imaging experiments in various research
directions. With self-seeding preparing a single mode FEL, tapering can be more efficient in extracting
energy from the electron bunch. Tapering the LCLS-II undulator system to reach TW FEL within the selfseeding scheme is important from the FEL physics view point, and will be carefully studied analytically
and by numerical simulation.
Time tables of milestones:
- Simulation and calculation
For the original self-seeding proposal in Ref. [3], the calculation was performance for ideal electron
bunch, and ideal perfect crystal. In order to pave the path for a successful experiment, we have
extended the simulation/calculation to:



Include electron bunch centroid energy jitter (done)
Include x-ray divergence (done)
Use start-to-end electron bunch (partially done)
Possible extensions:


With the self-seeding preparing a single coherent mode, tapering the undulator can be even
more efficient to extract energy from the electron bunch. Study the possibility of generating TW
FEL at LCLS-II within the self-seeding scheme will push the state-of-art, and will be pursued.
(started and seems very promising)
Even though the originally proposed self-seeding scheme [3] is theoretically sound, improved
scheme or modified scheme should naturally come out once we have more thorough study of
that proposal. In particular, once we have more concrete test on the experimental side as well.
- Experimental tests
Since it is crucial to determine where we should insert the monochromator and the electron by-pass
chicane, study on the existing LCLS FEL beam is necessary to find out


The FEL power level, this is important for providing sufficient seed to the second part of the
undulator section.
The FEL divergence. This is closely related to the above tolerance study on the x-ray divergence.

The FEL pointing stability. This will be important for the overlapping of the seed with the
electron bunch when they meet again at the entrance of the second part of the undulator
Even though the LCLS undulator system cannot provide large tapering to reach extremely high power
level FEL, some experimental studies can be performed as follows.



The transition behavior of how the seeded FEL follows the tapering will be studied against
theoretical and simulation finding.
Stability of a tapered FEL
Transverse coherence of tapered FEL
- Writing Paper
Depending whether important discovers can be obtained, different level of paper will be composed.
Conference papers should be targeted for detailed simulation results of the LCLS or LCLS-II beam
parameters, based on the original scheme [3]. If improved scheme or modified scheme differs
significantly from the original one, journal paper should be considered. The tapering study for a selfseeded FEL would be sufficient to be a stand-alone paper.
- Commissioning of the single-crystal Hard X-ray Self-seeding (HXRSS) scheme at LCLS
Paul Emma is leading the effects to commission hard x-ray self-seeding scheme at LCLS. Participating in
the commission and analyzing the data are planned. Detailed simulation will be provided for the FEL
properties at various steps along the commission plan.
Deliverables:
1) Paper:
Simulation details on the self-seeding FEL and tapering will be reported in a few conference proceeding
papers for FEL 2011. If more improvement can be obtained, journal papers can be considered.
2) Commissioning:
According to Paul Emma, the commissioning of the HXRSS should be completed at the end of 2011.
References:
1. P. Emma et al., Nature Photonics 4, 641 (2010).
2. J. Feldhaus et al., Optics Communications 140, 341 (1997).
3. G. Geloni et al., DESY preprint, DESY 10-133, August 2010; arXiv:1008.3036v1.