LCLS Laser Heater:LCLS Laser Heater: Physics MotivationPhysics Motivation

Zhirong HuangZhirong Huang

Beam Physics Department, SLACBeam Physics Department, SLAC

The need for a laser heater comes from the fact that the The need for a laser heater comes from the fact that the electron beam from highelectron beam from high--brightness injector has an extremely brightness injector has an extremely small slice energy spread (a few keV).small slice energy spread (a few keV).

Such a Such a ““coldcold’’’’ beam can undergo a beam can undergo a microbunching microbunching instability instability during acceleration and compression that increases during acceleration and compression that increases slice energy spread beyond the FEL tolerance.slice energy spread beyond the FEL tolerance.

Increasing the slice energy spread within the FEL tolerance Increasing the slice energy spread within the FEL tolerance can suppress the instability and make the final bunch can suppress the instability and make the final bunch longitudinal phase space much more manageable.longitudinal phase space much more manageable.

LCLS design calls for a laser heater in the injector.LCLS design calls for a laser heater in the injector.

We will soon commission this device in November. We will soon commission this device in November.

IntroductionIntroduction

ParmelaParmela at 1 at 1 nCnC TTF measurement at 4 TTF measurement at 4 nCnC

simulation·measured

mean

ΔE/E

(sec)

3 keV Huning and Schlarb, PAC03

3 keV (3 keV (rmsrms), accelerated to 14 GeV, & compressed ), accelerated to 14 GeV, & compressed ××5050⇒⇒ 33××1010−−6 6 ××50/14 = 150/14 = 1××1010−−5 5 relative energy spreadrelative energy spread⇒⇒ too small for LCLS FEL (dontoo small for LCLS FEL (don’’t care until t care until 11××1010−−44))

How cold is How cold is photoinjectorphotoinjector beambeam

LCLS slice energy spread<5 keV (measurement limit) before comprLCLS slice energy spread<5 keV (measurement limit) before compression ession

•• Initial density modulation induces energy modulation through Initial density modulation induces energy modulation through longitudinal impedance longitudinal impedance Z(kZ(k), converted to more density modulation ), converted to more density modulation by chicane. Space charge impedance is typically more at fault heby chicane. Space charge impedance is typically more at fault here re than CSRthan CSR

λz

Current1%

10%

λ

z

EnergyImpedance

Gain=10

R56

growth of slice energy spread (and emittance)growth of slice energy spread (and emittance)

Gain mechanismGain mechanism

R. R. AkreAkre, et al., PRST, et al., PRST--AB 11, 030703 (2008)AB 11, 030703 (2008)

OTR12 Spectral AnalysisOTR12 Spectral Analysis•• Use Use ““QBQB”” curve to obtain measured intensity gain by ratio of curve to obtain measured intensity gain by ratio of

COTR (QB peak) to No COTR spectra (QB baseline)COTR (QB peak) to No COTR spectra (QB baseline)

•• Calculated intensity gain with 40 A peak current (BC1 off), 1 Calculated intensity gain with 40 A peak current (BC1 off), 1 μμm norm. emittance and fit to 3 keV slice m norm. emittance and fit to 3 keV slice rmsrms energy spread)energy spread)

0 200 400 600 8000

0.5

1

1.5

2

2.5

3

Wavelength (nm)

Inte

nsit

y (c

ount

s)

COTRNo COTR

0 200 400 600 8000

0.5

1

1.5

2

2.5

3

Wavelength (nm)

Inte

nsit

y (c

ount

s)

COTRNo COTR

OTR12 grating spectraOTR12 grating spectra

main image

D. D. RatnerRatner et. al. FEL2008et. al. FEL2008

400 450 500 550 600 650 7000

5

10

15

20

wavelength (nm)

OT

R12

inte

nsity

gai

n

Measured Gaincalculated gain (3 keV E−spread)calculated gain (2 keV E−spread)calculated gain (4 keV E−spread)

with OTR21 screen insertedwith OTR21 screen inserted(smoothes (smoothes μμ--bunching)bunching)

OTR22 after BC2OTR22 after BC2

with OTR21 screen OUTwith OTR21 screen OUT((μμ--bunching present bunching present –– COTR!)COTR!)

OTR21 in BC2OTR21 in BC2

QM21 = 21 kG

QM21 = 27 kG

QM21 = 34 kG

Scan QM21Scan QM21

OTR21OTR21second dipolesecond dipole

COTRCOTR

CSRCSR

0.725 m0.725 m

xxLocation of sharpest Location of sharpest μμ--bunchingbunching

QM21 = 23 kG

xx--emittance growth associated with QM21 emittance growth associated with QM21 change observed but not wellchange observed but not well--understood yet understood yet

Ti:saphTi:saph760 nm760 nm1.2 MW1.2 MW

Injector at 135 MeVInjector at 135 MeV

‘‘Laser heaterLaser heater’’ suggested by suggested by SaldinSaldin et al.et al.

MicroMicro--Bunching Can be Landau Damped with Laser HeaterBunching Can be Landau Damped with Laser Heater

14 GeV Without Laser Heater14 GeV Without Laser Heater 14 GeV14 GeV WithWith Laser HeaterLaser Heater

0.50.5--m undulatorm undulator

LCLS design study: LCLS design study: Z. Huang Z. Huang et al., PRST 2004, PRST 2004

•• Undulator radiationUndulator radiation

•• FEL interaction: energy exchange between eFEL interaction: energy exchange between e-- and field and field ((vv••EE==vvxx EExx ) can be sustained due to the resonant condition) can be sustained due to the resonant condition

•• Some eSome e-- loss energy, others gain loss energy, others gain energy modulation energy modulation with the magnitudewith the magnitude

⎟⎠

⎞⎜⎝

⎛+=

21

2

2

2Ku

γλλ

θθ=K/=K/γγ

λλuu zz

xx

laser peak powerlaser peak power 8.7 GW8.7 GW laser laser rmsrms spot sizespot size

BeamBeam--Laser Interaction in an UndulatorLaser Interaction in an Undulator

• Laser-electron interaction in an undulator induces rapid energy modulation (at 758 nm), to be used as effective energy spread before BC1 (3 keV 40 keV rms)

• Inside a weak chicane for easy laser access, time-coordinate smearing (emittance growth is completely negligible)

Laser Heater DesignLaser Heater Design

See PRD 1.2See PRD 1.2--004004--r2 by P. Emmar2 by P. Emma

parameter symbol Value range unit

electron energy E 135 120 –

180 MeV

FWHM electron bunch length (duration) Δτe 10 5 –

15 ps

rms

transverse electron beam size σx,y 0.2 0.16 –

0.25 mm

bunch charge Q 1 0.2 -

1 nC

transverse emittance γεx,y 1.2 0.8 –

2 μm

rms

uncorr. energy spread (before heater) σE ~3 - keV

Laser wavelength λL 758 750 -

770 nm

Undulator period λu 5.4§ - cm

Undulator parameter K 1.385§ 1.047 –

2.229 -

Undulator minimum gap G 34§ 25 -

100 mm

Number of undulator periods Nu 9 - -

Chicane magnet eff. length (approx.) LB 18 - cm

Bend angle of each chicane magnet θB 7.52 0 -

7.52* deg

Beam offset in chicane center |ηx

| 35 0 -

35 mm

Laser beam waist rms size (Gaussian mode) σL-x,y 0.18 0.16 –

0.3 mm

Laser beam Rayleigh range LR 50 42 -

1600 cm

Laser pulse energy (nominal/high-setting) uL 44/400 0 -

400 μJ

Laser power (nominal/high-setting) PL 2.2/19 0 -

20 MW

Laser pulse duration (FWHM) ΔτL 20 10 -

20 ps

rms

energy spread generated (nom./high) σE-max 45/130 0 -

130 keV

Required spatial overlap of laser and e-beam |Δx|=|Δy| <0.2 - mm

Parameter TableParameter Table

Non-uniform heating

PP0 0 = 1.2 MW= 1.2 MWσσ

rr = 175 = 175 μμmm

matched spotmatched spotσσ

xx,,yy

≈≈

187 187 μμmm

more uniform heating

spread by laser transverse gradient

PP0 0 = 37 MW= 37 MWσσ

rr == 1.5 mm 1.5 mm

large laser spotlarge laser spotσσ

xx,,yy

≈≈

187 187 μμmm

+60 keV

-60 keV In Chicane

After Chicane

Laser Heater SimulationsLaser Heater Simulations

•• Large laser spot generates Large laser spot generates ““doubledouble--hornhorn”” energy distribution, energy distribution, ineffective at suppressing short wavelength microbunchingineffective at suppressing short wavelength microbunching

•• Laser spot matched to eLaser spot matched to e--beam size creates Gaussianbeam size creates Gaussian--like like energy distribution (more efficient heating)energy distribution (more efficient heating)

Transverse matching requirementTransverse matching requirement

laser spot >> electron spot

laser spot = electron spot

laser spot = 1.5*electron spot