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

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 HXRSS @ LCLS in collaboration with ANL Weekly (1:30 PM Friday) commissioning meeting

Hard X-ray Self-Seeding (HXRSS) 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?)

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

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 HXRSS @ LCLS project (on-going) Extension Taper theory; reaching TW FEL @ LCLS-II (on-going) Monochromator (single crystal) physics (on-going) Seed level: what is a sufficient seed to preserve good coherence (on-going)

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

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

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

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

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

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

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

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

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

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)

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

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

Status and Milestone Experimental tests @ 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 study @ 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

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 HXRSS @ LCLS should be at the end of calendar year 2011 (according to P. Emma)