Soft X-ray Self-Seeding in LCLS-II J. Wu Jan. 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 Schematics of Self-Seeded FEL chicane electron 1 st undulator2 nd undulator SASE FEL grating Seeded FEL grazing mirrors slit electron dump FEL

3 For a Gaussian photon beam –Gaussian pulse, at 1.5 Å, if I pk = 3 kA, Q = 250 pC,  z  10  m, then transform limit is:   /  0  10  –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 Transform Limited Pulses

4 Reaching a single coherent spike? –L G = 1 m, 20L G = 20 m, for u = 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 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 Two-Stage FEL with Monochromator With the same saturated peak power, but with two-orders of magnitude bandwidth reduction, the peak 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 et al. are designing –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 Monochromator

Performances Parametersymbolvalueunit Energy range  200 – 2000eV Pulse length (rms)  34 – 12fs Pulse energyE JJ Peak PowerP input MW E-beam size (rms) s mm Resolving powerR> Throughput  total 0.2 – 0.005% Output peak Power P output kW Time delay TT 10.8 – 9.6ps Optics Specs Feng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al.

Cylindrical horizontal focusing M 1 –Focus at reentrant point Planar pre-mirror M 2 –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 M1M1 M3M3 G   M2M2 electron-beam Optics Components source point re-entrant point Feng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al.

Optical components –Deflecting mirror; Pre-mirror; VLS Grating; Collimation mirror w0’w0’ w0w0 w 0 ’’ M1M1 GvGv M3M3 L1L1 r’Gr’G r M3  L Re-entrant ZRZR r total L M1M2 M2M2 r M2G r’ M3 Geometry (Dispersion Plane) Feng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al. L1L1 L M1M2 r M2G r’Gr’G r M3 r’ M3  L Re-entrant r total 200 eV eV

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 – %) –Still looking for design to have higher efficiency Use blazed profile -- efficiency increases by x10Use blazed profile -- efficiency increases by x10 Use coating to improve reflectivityUse coating to improve reflectivity Monochromator

11 S-2-E electron distribution: slice emittance entering the undulator S-2-E electron distribution: slice emittance entering the undulator 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,  z ~ 22  m Gain length ~ 1.4 m SASE spikes ~ 70 6-nm Case: Electron Bunch

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

14 6-nm FEL power along first undulator 6-nm FEL power along first undulator 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 2 nd und. 6-nm Case - Requirement on Seed Power 10 MW 10 kW

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

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

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

19 Effective pulse duration 35  m (  z  10  m) 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 6-nm Case - Transform Limit

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

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

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

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

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

25 6-Å FEL power along the first undulator 6-Å FEL power along the first undulator 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 6 Å FEL temporal profile at 30 m in the first undulator: challenge 6 Å SASE FEL Properties

27 6 Å FEL spectrum at 30 m in the first undulator 6 Å FEL spectrum at 30 m in the first undulator –Spiky spectrum: challenge 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 2 nd und. 6-ÅCase - Requirement on Seed Power 6-Å Case - Requirement on Seed Power 400 MW 20 kW

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

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

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

32 Effective pulse duration 12  m,  z ~ 3.5  m 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 6-Åcase — transform limited 6-Å case — transform limited

Parameter 6 nm 6 Å unit Emittance mmmm Peak Current 13kA Pulse length rms 3512fs Bandwidth FWHM  5 Limited Bandwidth  5 Seed Power 1020kW Power on Mono 10400MW Mono Efficiency 100.2% Sat. Power 510GW Sat. Length 3035m Brightness Increment 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 Ongoing work