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Soft X-ray Self-Seeding in LCLS-II J. Wu Jan. 13, 2010

Soft X-ray Self-Seeding in LCLS-II J. Wu Jan. 13, 2010. Originally proposed at DESY [ J. Feldhaus , E.L. Saldin , J.R. Schneider , E.A. Schneidmiller , M.V. Yurkov , Optics Communications, V.140, p.341 (1997) . ] Chicane and gratings in two orthogonal planes x and y. 2.

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Soft X-ray Self-Seeding in LCLS-II J. Wu Jan. 13, 2010

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  1. Soft X-ray Self-Seedingin LCLS-IIJ. WuJan. 13, 2010

  2. Originally proposed at DESY [J. Feldhaus, E.L. Saldin, J.R. Schneider, E.A. Schneidmiller, M.V. Yurkov, Optics Communications, V.140, p.341 (1997) .] Chicane and gratings in two orthogonal planes x and y 2 Schematics of Self-Seeded FEL chicane 1st undulator 2nd undulator grazing mirrors FEL slit SASE FEL Seeded FEL grating electron electron dump

  3. For a Gaussian photon beam Gaussian pulse, at 1.5 Å, if Ipk= 3 kA, Q = 250 pC, sz 10 mm, then transform limit is: sw/w010-6 LCLS normal operation bandwidth on order of 10-3 LCLS electron bunch, double-horn but central part effectively flat top, for flat top Improve longitudinal coherence, and reduce the bandwidth improve the spectral brightness 3 Transform Limited Pulses

  4. Reaching a single coherent spike? LG = 1 m, 20LG= 20 m, for lu= 2 cm, there is ~1000 periods Take 1 nm as example, single spike  1 micron Low charge might reach this, but bandwidth will be broad Narrow band, “relatively long” pulse  Self-Seeding. In the following, we focus on 250-pC case with a “relatively” long bunch, and look for “narrower” bandwidth and “good” temporal coherence For shorter wavelength (< 1 nm), single spike is not easy to reach, but self-seeding still possible 4 Single Spike vs Self-Seeding

  5. Seeding the second undulator (vs. single undulator followed by x-ray optics) Power loss in monochromator is recovered in the second undulator (FEL amplifier) Shot-to-shot FEL intensity fluctuation is reduced due to nonlinear regime of FEL amplifier Peak power after first undulator is less than saturation power  damage to optics is reduced 5 Two-Stage FEL with Monochromator With thesamesaturated peak power, but with two-orders of magnitudebandwidthreduction, thepeak brightness is increased by two-orders of magnitude

  6. J. Hastings suggested varied line spacing gratings (to provide focusing) as the monochromator for the soft x-ray self-seeding scheme Yiping Feng, Michael Rowen, Philip Heimann (LBL), and Jacek Krzywinski et al. are designing John Arthur, Uwe Bergmann, Paul Emma, John Galayda, Claudio Pellegrini, and Jochen Schneider et al. are giving general advices 6 Monochromator

  7. Performances Optics Specs Feng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al.

  8. Cylindrical horizontal focusing M1 Focus at reentrant point Planar pre-mirror M2 Vary incident angle to grating G Planar variable-line-spacing grating G Focus at exit slit Exit slit S Spherical vertical focusing mirror M3 Re-focus at reentrant point Optics Components Feng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al. electron-beam M3 M1 g M2 re-entrant point source point h G

  9. Optical components Deflecting mirror; Pre-mirror; VLS Grating; Collimation mirror Geometry (Dispersion Plane) Feng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al. M2 Gv M1 M3 ZR w0’’ w0 w0’ rM2G rM3 LM1M2 DLRe-entrant r’G L1 r’M3 rtotal

  10. Might need more than one monochromators Efficiency: Monochromator efficiency Phase space conservation: bandwidth reduced by one to two-order of magnitudes Overall efficiency will be on order of a percent to a few 10-5 (about 0.2 – 0.005 %) Still looking for design to have higher efficiency Use blazed profile -- efficiency increases by x10 Use coating to improve reflectivity 10 Monochromator

  11. S-2-E electron distribution: slice emittance entering the undulator 11 LCLS SASE FEL Parameters Slice Emittance small  Gain Length Short

  12. Peak current ~1 kA Undulator period 5 cm, Betatron function 4 m For 250 pC case, assuming a step function current profile, sz ~ 22 mm Gain length ~ 1.4 m SASE spikes ~ 70 12 6-nm Case: Electron Bunch

  13. S-2-E electron distribution: electron current profile entering the undulator 13 LCLS high-brightness electron beam tail head

  14. 6-nm FEL power along first undulator 14 6-nm SASE FEL Parameters saturation around 28 m with ~5 GW Present LCLS-II plan uses 40 meter long undulators

  15. Effective SASE start up power is 200 W. In a bandwidth of 2.210-5, there is only 0.5 W Use small start up seed power 10 kW… Monochromator efficiency 10% (at 6 nm) Phase space conservation: bandwidth decreases 1 to 2-orders of magnitude (about 70 spikes) Take total efficiency 1.010-3 Need 10 MW on monochromator to seed with 10 kW in 2nd und. 15 6-nm Case - Requirement on Seed Power 10 kW 10 MW

  16. FEL power along 2nd undulator for seed power of: 10 MW (black), 100 kW (red), 10 kW (cyan) 16 6-nm Seeded FEL Parameters Saturation around 18, 25 and 29 m with power ~5 GW

  17. Temporal profile at ~26 m in 2nd undulator for seed of 100 kW (black) and 10 kW (red) 6-nm Seeded FEL Parameters 17 ~35 mm

  18. FEL spectrum at ~26 m in 2nd undulator for seed of 100 kW (black) and 10 kW (red) 18 6-nm Seeded FEL Parameters FWHM 3.110-4

  19. Effective pulse duration 35 mm (sz 10 mm) Transform limited Gaussian pulse  bandwidth is 1.110-4 FWHM (For uniform pulse  1.510-4 FWHM) Here the seeded FEL bandwidth is about twice the transform limited bandwidth 19 6-nm Case - Transform Limit

  20. The second undulator can be APPLE type Linear (black), circular (red), or elliptical polarization Pol. ~ 100% 20 Polarization

  21. Temporal profile in 2nd undulator with seed of 100 kW for planar (black) and circular (red) 6-nm Seeded FEL: Polarization 21 ~35 mm Planar at 26 m; Circular at 18 m

  22. FEL spectrum in 2nd undulator with seed of for planar (black) and circular (red) 22 6-nm Seeded FEL : Polarization FWHM 3.110-4 Planar at 26 m; Circular at 18 m

  23. Peak current ~3 kA Undulator period 5 cm, Betatron function 4 m For 250 pC case, assuming a step function current profile, sz 7 mm. Gain length ~ 2.1 m SASE spikes ~ 160 23 6-Å Case: Electron Bunch

  24. S-2-E electron distribution: electron current profile entering the undulator: compress more 24 LCLS high-brightness electron beam tail head

  25. 6-Å FEL power along the first undulator 25 6-Å SASE FEL Parameters saturation around 32 m with power ~10 GW Present LCLS-II plan uses 40 meter long undulators

  26. 6 Å FEL temporal profile at 30 m in the first undulator: challenge 26 6 Å SASE FEL Properties

  27. 6 Å FEL spectrum at 30 m in the first undulator Spiky spectrum: challenge 27 6 Å SASE FEL Properties

  28. Effective SASE start up power is 1.3 kW. In a bandwidth of 6.610-6, there is only 1.6 W Use small start up seed power 20 kW… Monochromator efficiency ~ 0.2 % (at 6 Å) Phase space conservation: bandwidth decreases 1 to 2-orders of magnitude (~ 160 spikes) Take total efficiency 5.010-5 Need 400 MW on monochromator to seed with 20 kW in 2nd und. 28 6-ÅCase - Requirement on Seed Power 20 kW 400 MW

  29. Power along 2nd undulator for seed power of 20 kW (black) and 10 kW (red) 29 6-ÅSeeded FEL Parameters Saturation around 35 m with power on order of 10 GW

  30. Temporal profile at ~35 m in the 2nd undulator for seed of 20 kW (black) and 10 kW (red) 6-ÅSeeded FEL Parameters 30 ~12 mm

  31. FEL spectrum at ~35 m in the 2nd undulator for seed of 20 kW (black) and 10 kW (red) 31 6-ÅSeeded FEL Parameters FWHM 6.210-5

  32. Effective pulse duration 12 mm, sz ~ 3.5 mm Transform limited Gaussian pulse  bandwidth is 3.210-5 FWHM. (For uniform pulse  4.410-5 FWHM) The seeded FEL bandwidth (6.210-5 FWHM) is less than twice the transform limited bandwidth 32 6-Åcase — transform limited

  33. Self-Seeding Summary at 6 nm and 6 Å

  34. VLS gratings are being studied in more details looking for larger overall efficiency Three dimensional overlap of the electron pulse and the photon pulse Electron chicane will be studied in more detail Statistics of the self-seeded FEL performance Full simulation with monochromator wavefront propagation More detailed study on APPLE undulator possibility as the second undulator to generate narrow bandwidth FEL with variable polarization 34 Ongoing work

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