1. LCLS efforts: Self-seeding -- status report
J. Wu
02/17/2011
FEL Physics Group
Beam Physics Department

2. Motivation and Progress
Given the unique facility: LCLS, the focus is to experimentally realize a hard x-ray self-seeding (HXRSS) FEL
Carefully checked the scheme: a single-crystal diamond C(400) with symmetric Bragg geometry as the monochromator
As an Accelerator Improvement Project (AIP), the HXRSS has been funded (J. Amann as projector manager)
P. Emma is leading the effort for commissioning LCLS in collaboration with ANL
Weekly (1:30 PM Friday) commissioning meeting

3. Hard X-ray Self-Seeding (HXRSS)
LCLS
e- chicane
1st undulator
2nd undulator e-
SASE FEL
Single crystal
Seeded FEL
e- dump
Und 1—14
Und 15
Und 16—28(33?)

4. Motivation and Progress
Theoretical merits
Seeded FEL to provide narrow bandwidth
Seeded FEL as a single mode perfect example to develop/verify tapering theory
Interplay of electron bunch energy chirp and undulator taper
Application
Narrow bandwidth, high spectrum brightness source excites users
Tapering with single mode can be more efficient to extract energy from the electron bunch  reaching Terawatts (TW) FEL  single molecule imaging

5. Status and Milestone Simulation Extension
Include electron bunch centroid energy tuning range (done)
Include x-ray divergence (done)
Use start-to-end electron bunch (partially done)
Provide step-by-step simulation for the FEL properties in the LCLS project (on-going)
Extension
Taper theory; reaching TW LCLS-II (on-going)
Monochromator (single crystal) physics (on-going)
Seed level: what is a sufficient seed to preserve good coherence (on-going)

6. An Example: tunability
Electron bunch centroid energy tuning range
Going through the chicane  electron timing variation (R56 ~ 12 mm, Dg/g ~ 3 %  Dz~ 0.4 mm or 1.3 fs)
FEL having different energy going through the crystal  photon timing variation  crystal orientation
Example: tuning from 1.4 Å to 1.6 Å.
Choose s –polarization

7. Rocking curve: p - polarization
0.16 nm
0.15 nm
0.14 nm

8. Rocking curve: s - polarization
0.16 nm
0.15 nm
0.14 nm

9. 8 keV energy-tuning case
Spectrum on the left, temporal profile on the right
l =1.4 Å; Bragg angle: 51.72o
p - polarization

10. 8 keV on-energy case
Spectrum on the left, temporal profile on the right
l =1.5 Å; Bragg angle: 57.25o
p - polarization

11. 8 keV energy-jitter case
Spectrum on the left, temporal profile on the right
l =1.6 Å; Bragg angle: 63.78o
p - polarization

12. 8 keV energy-jitter case
Spectrum on the left, temporal profile on the right
l =1.4 Å; Bragg angle: 51.72o
s - polarization

13. 8 keV on-energy case
Spectrum on the left, temporal profile on the right
l =1.5 Å; Bragg angle: 57.25o
s - polarization

14. 8 keV energy-jitter case
Spectrum on the left, temporal profile on the right
l =1.6 Å; Bragg angle: 63.78o
s - polarization

15. Example: undulator taper
Taper the 18 undualtors after the monochromator
Taper starts at 25 m, quadratic taper of 2 %
At the end of 18 undulator (~ 60 m magnetic length), FEL power reach 100 GW (< 1 mJ for low charge 20 pC)

16. Self-seeded FEL at exit of 10 undulators
There are 18 undualtors after the monochromator
FEL at the exit of 10 undulator
2.8E-5

17. Self-seeded FEL at exit of 18 undulators
There are 18 undualtors after the monochromator
FEL at the exit of 18 undulator
1.0E-4

18. Status and Milestone
Experimental LCLS Machine Development shifts (partially done)
FEL power level  important for providing sufficient seed to the second part of the undulator
FEL divergence  related to the tolerance on the x-ray divergence.
FEL pointing stability  important for overlapping of the seed with the electron bunch when they meet again at the entrance of the second part of the undulator
Tapering LCLS Machine Development shifts (planned)
The transition behavior of how the seeded FEL follows the tapering will be studied against theoretical and simulation finding. (Simulation usually show a kink-like transition)
Stability of a tapered FEL
Transverse coherence of tapered FEL

19. Deliverable
HYXSS simulation results: suitable for a few FEL 2011 conference papers
Tapering in self-seeding scheme can be a stand-alone paper  depending on the quality, it can be a journal paper
Commission of LCLS should be at the end of calendar year 2011 (according to P. Emma)