Soft X-ray Self-SeedingSoft X-ray Self-Seedingin LCLS-IIin LCLS-II

J. WuJ. WuJan. 13, 2010Jan. 13, 2010

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Originally proposed at DESY Originally proposed at DESY [[J. Feldhaus, E.L. Saldin, J. Feldhaus, E.L. Saldin, J.R. SchneiderJ.R. Schneider, E.A. Schneidmiller, M.V. Yurkov, Optics , E.A. Schneidmiller, M.V. Yurkov, Optics Communications, V.140, p.341 (1997) .Communications, V.140, p.341 (1997) .]]– Chicane and gratings in two Chicane and gratings in two orthogonalorthogonal planes x and y planes x and y

Schematics of Self-Seeded FELSchematics of Self-Seeded FEL

chicane

electron

1st undulator 2nd undulator

SASE FEL

grating

Seeded FEL

grazing mirrors

slit

electron dump

FEL

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For a Gaussian photon beamFor a Gaussian photon beam

– Gaussian pulse, at 1.5 Å, if IGaussian pulse, at 1.5 Å, if Ipkpk= 3 kA, Q = 250 pC, = 3 kA, Q = 250 pC,

zz 10 10 m, then transform limit is: m, then transform limit is: //00 1010

– LCLS normal operation bandwidth on order of 10LCLS normal operation bandwidth on order of 1033

– LCLS electron bunch, double-horn but central part LCLS electron bunch, double-horn but central part effectively flat top, for flat topeffectively flat top, for flat top

Improve longitudinal coherence, and reduce Improve longitudinal coherence, and reduce the bandwidththe bandwidth improve the spectral improve the spectral brightnessbrightness

Transform Limited PulsesTransform Limited Pulses

18.12ln22/1 FWHM tt

61.1FWHM t

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Reaching a single coherent spike? Reaching a single coherent spike? – LLGG = 1 m, 20L = 1 m, 20LGG= 20 m, for = 20 m, for uu= 2 cm, there is ~1000 periods= 2 cm, there is ~1000 periods

– Take 1 nm as example, single spike Take 1 nm as example, single spike 1 micron 1 micron– Low charge might reach this, but bandwidth will be broadLow charge might reach this, but bandwidth will be broad

Narrow band, “relatively long” pulse Narrow band, “relatively long” pulse Self-SeedingSelf-Seeding..

In the following, we focus on 250-pC case with a In the following, we focus on 250-pC case with a “relatively” long bunch, and look for “narrower” “relatively” long bunch, and look for “narrower” bandwidth and “good” temporal coherencebandwidth and “good” temporal coherence

For shorter wavelength (< 1 nm), single spike is For shorter wavelength (< 1 nm), single spike is notnot easy to reach, but self-seeding still possibleeasy to reach, but self-seeding still possible

Single Spike vs Self-SeedingSingle Spike vs Self-Seeding

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Seeding the second undulatorSeeding the second undulator (vs. single (vs. single undulator followed by x-ray optics)undulator followed by x-ray optics)– Power loss in monochromator is recovered in the Power loss in monochromator is recovered in the

second undulator (FEL amplifier)second undulator (FEL amplifier)– Shot-to-shot FEL intensity fluctuation is reduced Shot-to-shot FEL intensity fluctuation is reduced

due to nonlinear regime of FEL amplifierdue to nonlinear regime of FEL amplifier– Peak power after first undulator is less than Peak power after first undulator is less than

saturation power saturation power damage to optics is reduced damage to optics is reduced

Two-Stage FEL with MonochromatorTwo-Stage FEL with Monochromator

With theWith the samesame saturated saturated peak powerpeak power, but with two-orders of , but with two-orders of magnitudemagnitude bandwidthbandwidth reductionreduction, the, the peak brightnesspeak brightness is is increased increased by two-orders of magnitudeby two-orders of magnitude

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J. Hastings suggested varied line spacing J. Hastings suggested varied line spacing gratings (to provide focusing) as the gratings (to provide focusing) as the monochromator for the soft x-ray self-seeding monochromator for the soft x-ray self-seeding schemescheme– Yiping Feng, Michael Rowen, Philip Heimann Yiping Feng, Michael Rowen, Philip Heimann

(LBL), and (LBL), and Jacek Krzywinski et al. are designinget al. are designing

John Arthur, Uwe Bergmann, Paul Emma, John Arthur, Uwe Bergmann, Paul Emma, John Galayda, Claudio Pellegrini, and Jochen John Galayda, Claudio Pellegrini, and Jochen Schneider et al. are giving general advicesSchneider et al. are giving general advices

Monochromator Monochromator

PerformancesPerformancesParameter symbol value unit

Energy range 200 – 2000 eV

Pulse length (rms)

34 – 12 fs

Pulse energy E 1.2 - 17 J

Peak Power Pinput 10 - 400 MW

E-beam size (rms)

s 50 -15 m

Resolving power R > 20000

Throughput total 0.2 – 0.005 %

Output peak Power

Poutput 10 - 20 kW

Time delay T 10.8 – 9.6 ps

Optics SpecsOptics SpecsFeng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al.

Cylindrical horizontal focusing M1

– Focus at reentrant pointFocus at reentrant point

Planar pre-mirror MPlanar pre-mirror M22

– Vary incident angle to grating GVary incident angle to grating G

Planar variable-line-spacing grating GPlanar variable-line-spacing grating G– Focus at exit slitFocus at exit slit

Exit slit S

Spherical vertical focusing mirror M3Spherical vertical focusing mirror M3– Re-focus at reentrant pointRe-focus at reentrant point

M1M3

G

M2

electron-beam

Optics ComponentsOptics Components

source point

re-entrant point

Feng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al.

Optical components– Deflecting mirror; Pre-mirror; VLS Grating; Collimation Deflecting mirror; Pre-mirror; VLS Grating; Collimation

mirrormirror

w0’w0

w0’’

M1 Gv M3

L1 r’G rM3 LRe-entrant

ZR

rtotal

LM1M2

M2

rM2G r’M3

Geometry (Dispersion Plane)Geometry (Dispersion Plane)Feng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al.

L1 LM1M2 rM2G r’G rM3 r’M3 LRe-entrant rtotal

200 eV 13.761030 4.204372 0.036709 5.981053 0.351780 1.993796 1.656204 27.984945

2000 eV 13.761030 3.901582 0.339127 6.021674 0.311159 3.400840 0.249160 27.984572

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Might need more than one monochromatorsMight need more than one monochromators

Efficiency:Efficiency:– Monochromator efficiency Monochromator efficiency – Phase space conservation: bandwidth reduced by Phase space conservation: bandwidth reduced by

one to two-order of magnitudesone to two-order of magnitudes– Overall efficiency will be on order of a percent to a Overall efficiency will be on order of a percent to a

few few 10105 5 (about 0.2 – 0.005 %)(about 0.2 – 0.005 %)– Still looking for design to have Still looking for design to have higherhigher efficiency efficiency

• Use blazed profile -- efficiency increases by x10Use blazed profile -- efficiency increases by x10• Use coating to improve reflectivityUse coating to improve reflectivity

MonochromatorMonochromator

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S-2-E electron distribution: slice emittance S-2-E electron distribution: slice emittance entering the undulatorentering the undulator

LCLS SASE FEL ParametersLCLS SASE FEL Parameters

Slice Emittance small Gain Length Short

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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 Bunch6-nm Case: Electron Bunch

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S-2-E electron distribution: electron current S-2-E electron distribution: electron current profile entering the undulatorprofile entering the undulator

LCLS high-brightness electron beamLCLS high-brightness electron beam

head

tail

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6-nm FEL power along 6-nm FEL power along first undulatorfirst undulator

6-nm SASE FEL Parameters6-nm SASE FEL Parameters

saturation around 28 m with ~5 GW

Present LCLS-II plan uses 40 meter long undulators

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Effective SASE start up power is 200 W. Effective SASE start up power is 200 W. – In a bandwidth of 2.2In a bandwidth of 2.2101055, there is only 0.5 W, there is only 0.5 W

Use small start up seed power 10 kW…Use small start up seed power 10 kW…– Monochromator efficiency Monochromator efficiency 10% (at 6 nm) 10% (at 6 nm)– Phase space conservation: bandwidth decreases Phase space conservation: bandwidth decreases

1 to 2-orders of magnitude (about 1 to 2-orders of magnitude (about 7070 spikes) spikes)– Take total efficiency 1.0Take total efficiency 1.0101033 Need 10 MW on Need 10 MW on

monochromator to seed with 10 kW in 2monochromator to seed with 10 kW in 2ndnd und. und.

6-nm Case - Requirement on Seed Power6-nm Case - Requirement on Seed Power

10 MW 10 kW

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FEL power along FEL power along 22ndnd undulator undulator for seed power for seed power of: 10 MW (black), 100 kW (of: 10 MW (black), 100 kW (redred), 10 kW (), 10 kW (cyancyan))

6-nm Seeded FEL Parameters6-nm Seeded FEL Parameters

Saturation around 18, 25 and 29 m with power ~5 GW

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Temporal profile at ~26 m in Temporal profile at ~26 m in 22ndnd undulator undulator for for seed of 100 kW (black) and 10 kW (seed of 100 kW (black) and 10 kW (redred))

6-nm Seeded FEL Parameters6-nm Seeded FEL Parameters

~35 m

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FEL spectrum at ~26 m in FEL spectrum at ~26 m in 22ndnd undulator undulator for for seed of 100 kW (black) and 10 kW (seed of 100 kW (black) and 10 kW (redred))

6-nm Seeded FEL Parameters6-nm Seeded FEL Parameters

FWHM 3.1104

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Effective pulse duration 35 Effective pulse duration 35 m (m (zz 10 10 m) m)

Transform limited Gaussian pulse Transform limited Gaussian pulse bandwidth is 1.1bandwidth is 1.1101044 FWHM FWHM

(For uniform pulse (For uniform pulse 1.5 1.5101044 FWHM) FWHM)

Here the seeded FEL bandwidth is about Here the seeded FEL bandwidth is about twicetwice the transform limited bandwidththe transform limited bandwidth

6-nm Case - Transform Limit 6-nm Case - Transform Limit

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The second undulator can be APPLE type The second undulator can be APPLE type – Linear (black), circular (Linear (black), circular (redred), or elliptical ), or elliptical

polarizationpolarization– Pol. ~ 100%Pol. ~ 100%

Polarization Polarization

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Temporal profile in Temporal profile in 22ndnd undulator undulator with seed of with seed of 100 kW for planar (black) and circular (100 kW for planar (black) and circular (redred))

6-nm Seeded FEL: Polarization6-nm Seeded FEL: Polarization

~35 m

Planar at 26 m; Planar at 26 m; CircularCircular at 18 m at 18 m

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FEL spectrum in FEL spectrum in 22ndnd undulator undulator with seed of for with seed of for planar (black) and circular (planar (black) and circular (redred))

6-nm Seeded FEL : Polarization6-nm Seeded FEL : Polarization

FWHM 3.1104

Planar at 26 m; Planar at 26 m; CircularCircular at 18 m at 18 m

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Peak current ~3 kAPeak current ~3 kA

Undulator period 5 cm, Betatron function 4 mBetatron function 4 m

For 250 pC case, assuming a step function For 250 pC case, assuming a step function current profile, current profile, zz 7 7 m.m.

Gain length ~ 2.1 m

SASE spikes ~ 160

6-6-ÅÅ Case: Electron Bunch Case: Electron Bunch

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S-2-E electron distribution: electron current S-2-E electron distribution: electron current profile entering the undulator: compress moreprofile entering the undulator: compress more

LCLS high-brightness electron beamLCLS high-brightness electron beam

head

tail

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6-6-ÅÅ FEL power along the FEL power along the first undulatorfirst undulator

6-Å SASE FEL Parameters6-Å SASE FEL Parameters

saturation around 32 m with power ~10 GW

Present LCLS-II plan uses 40 meter long undulators

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6 Å FEL temporal profile at 30 m in the first 6 Å FEL temporal profile at 30 m in the first undulator: challengeundulator: challenge

6 Å SASE FEL Properties6 Å SASE FEL Properties

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6 Å FEL spectrum at 30 m in the first undulator6 Å FEL spectrum at 30 m in the first undulator

– Spiky spectrum: challenge

6 Å SASE FEL Properties6 Å SASE FEL Properties

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Effective SASE start up power is 1.3 kW. Effective SASE start up power is 1.3 kW.

In a bandwidth of 6.6In a bandwidth of 6.61010-6-6, there is only 1.6 W, there is only 1.6 W

UseUse small start up seed power 20 kW… small start up seed power 20 kW…– Monochromator efficiency ~ 0.2 % (at 6 Monochromator efficiency ~ 0.2 % (at 6 ÅÅ))– Phase space conservation: bandwidth decreases Phase space conservation: bandwidth decreases

1 to 2-orders of magnitude (~ 1 to 2-orders of magnitude (~ 160160 spikes) spikes)– Take total efficiency 5.0Take total efficiency 5.0101055 Need 400 MW on Need 400 MW on

monochromator to seed with 20 kW in 2monochromator to seed with 20 kW in 2ndnd und. und.

6-Å6-Å Case - Requirement on Seed PowerCase - Requirement on Seed Power

400 MW 20 kW

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Power along Power along 22ndnd undulator undulator for seed power of for seed power of 20 kW (black) and 10 kW (20 kW (black) and 10 kW (redred))

6-Å6-Å Seeded FEL ParametersSeeded FEL Parameters

Saturation around 35 m with power on order of 10 GW

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Temporal profile at ~35 m in the Temporal profile at ~35 m in the 22ndnd undulator undulator for seed of 20 kW (black) and 10 kW (for seed of 20 kW (black) and 10 kW (redred))

6-Å6-Å Seeded FEL ParametersSeeded FEL Parameters

~12 m

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FEL spectrum at ~35 m in the FEL spectrum at ~35 m in the 22ndnd undulator undulator for seed of 20 kW (black) and 10 kW (for seed of 20 kW (black) and 10 kW (redred))

6-Å6-Å Seeded FEL ParametersSeeded FEL Parameters

FWHM 6.2105

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Effective pulse duration 12 Effective pulse duration 12 m, m, zz ~ 3.5 ~ 3.5 m m

Transform limited Gaussian pulse Transform limited Gaussian pulse bandwidth is 3.2bandwidth is 3.2101055 FWHM. FWHM.

(For uniform pulse (For uniform pulse 4.4 4.4101055 FWHM) FWHM)

The seeded FEL bandwidth (6.2The seeded FEL bandwidth (6.2101055 FWHM) FWHM) is less than is less than twicetwice the transform limited the transform limited bandwidthbandwidth

6-Å6-Å case — transform limited case — transform limited

ParameterParameter 6 nm6 nm 6 6 ÅÅ unitunit

EmittanceEmittance 0.50.5 0.50.5 mm

Peak CurrentPeak Current 11 33 kAkA

Pulse length rmsPulse length rms 3535 1212 fsfs

Bandwidth FWHMBandwidth FWHM 3131 6.26.2 101055

Limited BandwidthLimited Bandwidth 1515 4.44.4 101055

Seed PowerSeed Power 1010 2020 kWkW

Power on MonoPower on Mono 1010 400400 MWMW

Mono EfficiencyMono Efficiency 1010 0.20.2 %%

Sat. PowerSat. Power 55 1010 GWGW

Sat. LengthSat. Length 3030 3535 mm

Brightness IncrementBrightness Increment 5050 150150

Self-Seeding Summary at 6 nm and 6 Self-Seeding Summary at 6 nm and 6 ÅÅ

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VLS gratings are being studied in more details VLS gratings are being studied in more details looking for larger overall looking for larger overall efficiencyefficiency Three dimensional Three dimensional overlapoverlap of the electron pulse of the electron pulse and the photon pulseand the photon pulseElectron chicane will be studied in more detailElectron chicane will be studied in more detailStatistics of the self-seeded FEL performanceStatistics of the self-seeded FEL performanceFull simulation with monochromator wavefront Full simulation with monochromator wavefront propagationpropagationMore detailed study on APPLE undulator More detailed study on APPLE undulator possibility as the second undulator to generate possibility as the second undulator to generate narrow bandwidthnarrow bandwidth FEL with variable FEL with variable polarizationpolarization

Ongoing workOngoing work