Linac Coherent Light Source Stanford Synchrotron … · Linac Coherent Light Source Stanford...

LCLS Laser Heater ReviewLCLS Laser Heater ReviewMarch 1, 2004March 1, 2004

ZhirongZhirong Huang, SLACHuang, [email protected]@slac.stanford.edu

Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center

Suppression of MicrobunchingInstability in the LCLS

Z. Huang, M. Borland (ANL), P. Emma, J. WuC. Limborg, G. Stupakov, J. Welch

Suppression of MicrobunchingInstability in the LCLS

Z. Huang, M. Borland (ANL), P. Emma, J. WuC. Limborg, G. Stupakov, J. Welch

Motivation

CSR/LSC Instability

Cure (laser heater)

Motivation

CSR/LSC Instability

Cure (laser heater)LCLSLCLS




IntroductionIntroductionIntroductionFEL instability in the undulator requires a very bright

electron beam (small emittance and energy spread)

This beam interacting with self-fields in the accelerator can be sensitive to other “undesirable” instabilities

Bunch compressors designed to increase the peak current give rise to a microbunching instability that may degrade the beam quality significantly

Increasing the local energy spread within the FEL tolerance can damp the instability without degrading the XFEL performance




How cold is the photoinjector beam?How cold is the photoinjector beam?Parmela at 1 nC TTF measurement at 4 nC

simulation·measured

mean

∆E/E

(sec)

3 keV Huning and Schlarb, PAC03

Local rms energy spread ~ 3 keV, could be less!




Microbunching instabilityMicrobunching instability• Initial density modulation induces energy modulation through long. impedance Z(k), converted to more density modulation by a chicane growth of local energy spread/emittance!

λλ

t

Energybi bf >> bi or G= bf/ bi >> 1Ζ(k) ∆Ε

R56

Current modulation

1% 10%Gain=10

t




LCLS accelerator systemsLCLS accelerator systems

Linac 1

BC1 BC2

SC wiggler at 4.5 GeV

DL2DL1

End of injector

Linac 2 Linac 3Laser heaterat 135 MeV

• At the end of injector, e-beam carries some residual density modulations which can be amplified in the downstream accel.

• Sources of impedance: CSR in dipoles, longitudinal space charge (LSC) and linac wakefields in linacs

• Landau damping options: a SC wiggler before BC2 at 4.5 GeV or a laser heater before DL1 at 135 MeV




• FEL parameter ρ ~ 5×10-4, not sensitive to energy spread until σδ ~ 1×10-4

• 3 keV initial energy spread after compression = 90 keV,corresponding to σδ < 1×10-5 at 14 GeV

M. Xie’s fitting formulaγε = 1 µmIp = 3.4 kAβ = 25 m quantum

diffusion

can increase σδ by a factor of 10 without FEL degradation

FEL limit

0 1 2 3 4Σ∆f�10

4

4

5

6

FELPow

erGainLe

ngth�m�

Heating within FEL toleranceHeating within FEL tolerance




energy profileenergy profile long. spacelong. space temporal profiletemporal profile

micro-bunching

micro-bunching

σσδδ ≈≈ 33××1010−−66

230 fsec230 fsec

CSR microbunchingCSR microbunching

SCSC--wiggler wiggler damps damps

bunchingbunchingσσδδ ≈≈ 33××1010−−55




• SC wiggler increases σδ 10 times at 4.5 GeV (BC2), suppresses the CSR gain

Landau damping wigglerLandau damping wiggler

100 200 300

0

50

100

Gain

elegant HWiggler onL

Theory HWiggler onL

elegant HWiggler offL

Theory HWiggler offL

Initial modulation wavelength (µm)

• Linac wakefields contribution (2X) included




LSC instabilityLSC instability

LSC-induced energy modulation can accumulate in the linacand can dominate the microbunching instability

SaldinSchneidmillerYurkov




SDL RF zero-phasing measurementSDL RF zero-phasing measurement• RF zero-phasing method can be used to extract LSC-induced energy modulation in the linac (Shaftan/Huang)

25 – 35 keV at 200 A




Space charge oscillationSpace charge oscillationCurrent modulation Energy modulation

• Space charge oscillation in the photoinjector (see C. Limborg’s talk). Initial density modulation can be reduced at the expense of increased energy modulation (which is also bad)




LSC impedanceLSC impedance

• Free-space longitudinal space charge impedance

• At end of photoinjector, beam density modulation freezes and energy modulation accumulates

rb

λ

1/γ

• Evaluate microbunching gain relative to density modulation at injector end, which may over/under estimate gain relative to the drive laser modulation




• 3 keV energy spread is too small to suppress high-frequency LSC instability, which can even amplify shot noise

λc≈13 µm from 3 keV• SC wiggler does not affect BC1 gain, but can smear out microbunching after BC2

0 50 100 150 200Λ0 �Μm�

10

100

1000

10000

Total

Gain

SC wiggler on

SC wiggler off

20 40 60 80 100Λ0 �Μm�

0

25

50

75

100

BC1Gain

Microbunching gain including LSCMicrobunching gain including LSC




• High BC1 gain significant energy modulation in Linac-2, which can be temporally smeared in BC2 and becomes effective energy spread ( SC wiggler too late)• Slice energy spread at undulator (14 GeV), seeded with 1% initial density modulation (see J. Wu’s talk)

Growth of energy spreadGrowth of energy spread

20 30 40 50 60Λ0 �Μm�

5

10

15

Σ∆f��10

4�

FEL limitElegantTheory

Need < 0.1% initial density modulation or suppress theBC1 gain effectively




Laser HeaterLaser Heater

2 cm2 cm

10 cm10 cm

10 cm10 cm 50 cm50 cm

~120 cm~120 cm

θθ ≈≈ 5.75.7ºº

• Laser-electron interaction in an undulator induces rapid energy modulation (at 800 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)

10 period undulator




Non-uniform heating

PP0 0 = 1.2 MW= 1.2 MWσσrr = 175 = 175 µµmmmatched spotmatched spotσσxx,,yy ≈≈ 187 187 µµmm

more uniform heating

spread by laser transverse gradient

PP0 0 = 37 MW= 37 MWσσrr == 1.5 mm 1.5 mm large laser spotlarge laser spotσσxx,,yy ≈≈ 187 187 µµmm

+60 keV

-60 keV In Chicane

After Chicane




• Large laser spot generates “double-horn” energy distributioin, ineffective at suppressing short wavelength microbunching• Laser spot matched to e-beam size creates better heating

�50 0 50�Γ0mc2 �keV�

0

0.005

0.01

0.015

V��ke

V��

1�

large laser spot

matched laser spot

50 100 150 200Λ0 �Μm�

0

5

10

15

BC1Gain Elegant �matched laser spot�

Theory �matched laser spot�Elegant �large laser spot�Theory �large laser spot�

when

when




Control of longitudinal phase spaceControl of longitudinal phase space• Large laser spot yields large total gain, which is still capable to amplify <0.1% initial density modulation• For larger initial density modulation (1%), gain saturates due to longitudinal phase space distortion, but energy spread increase

50 100 150 200Λ0 �Μm�

0

100

200

300

Total

Gain Elegant �matched laser spot�

Theory �matched laser spot�Elegant �large laser spot�Theory �large laser spot�

50 100 150 200Λ0 �Μm�

2

4

6

Σ∆f��10

4�

FEL limitHeater �matched laser spot�Heater �large laser spot�No laser heater

• Matched laser spot minimizes instability effects

gain saturation




Microbunching instability driven by LSC, CSR and machine impedance can be a “nightmare” for LCLS

SummarySummary

The photoinjector beam is too “cold” in energy spread, “heating” within the FEL tolerance (~10X) can control the instability

SC wiggler is too late for the LSC instability occurred in the lower energy end of the linac (L1, BC1 and L2)

A laser heater with a laser spot matched to the transverse e-beam size is most effective in suppressing microbunching

It adds safety margin to the project as well as flexible control of slice energy spread



Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center15.0 15.0 ÅÅ

SaturationSaturationPowerPower

SaturationSaturationLengthLength

P0 = 65 MWw0 = 400 µmPP00 = 65 MW= 65 MWww00 = 400 = 400 µµmm



Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center1.5 1.5 ÅÅ

SaturationSaturationPowerPower

SaturationSaturationLengthLength

P0 = 28 MWw0 = 400 µmPP00 = 28 MW= 28 MWww00 = 400 = 400 µµmm

Linac Coherent Light Source Stanford Synchrotron … · Linac Coherent Light Source Stanford...

Documents

Transcript of Linac Coherent Light Source Stanford Synchrotron … · Linac Coherent Light Source Stanford...