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Bates XFEL Linac and Bunch Compressor Dynamics

1. Linac Layout and General Beam Parameter 2. Bunch Compressor

– System Details (RF, Magnet Chicane)– Linear bunch compressing – Wake field and CSR – Various Effects (Chirp Phase, Source..)– Further optimization and S2E Simulation

3. SummaryFuhua Wang , Dong Wang

MIT-Bates LaboratoryPresentation to MIT X-ray laser Accelerator Science Advisory

CommitteeSeptember 18-19, 2003

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• Beam From the RF Injector: 20 ps, Charge 0.2, 1 nC.

Slice emittance 0.6,1.0 um, Slice p ~ 5KeVTwo operation mode: 0.2nC for 0.1-.2ps, 1nC for 1ps.

• Linac RF: TESLA 9cell cavity, 8 cavity cryomodule. • Two (Four bends) Bunch Compressors with adjustable

R56.• Experiment Station Energy: 1,2,4 GeVWhat make this linac different from other FEL linac driver?Seeding , HGHG operations requires high energy and timing

stability of beam, high compression ratio, extraction at several energies.

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Linac Layout

• BC: Bunch Compressor Chicane• SW: Switchyard• Linac Section (TESLA Cryomodules)• 3rd H: Third Harmonic Linearizerp: Bunch total momentum spanp: slice momentum spread

BC1

R56max -122mm

Lb 20 ps

p ~4%

SW2

SW1

RF Gun p=5KeV

Chirp

3rd H

230 MeV2 GeV1 GeV561 MeV 4 GeV

96 MeV 200 MeV

BC2

R56 -45mm

Lb 1- 4ps

p 1.4%

Lb .2- 1ps

p 0.2%

2 Cryomodules 3 C. modules

12 C. modules

6 C. modules

4

5

6

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2. Bunch Compressor2.1 System Details

• RF Chirp : position – energy correlated

• 3rd Harmonic RF Section for RF nonlinear distortion correction

Cavity is at decelerating phase, Vh=V0/h2 .

Use 3rd harmonic reasons :

Technical and lower wake field (W2, W 3 ).

...)sin()(2

1)cos()sin(

...)sin(2

1)cos()sin(

2

002

00000

hhhhhhh VhVhVV

VVVV

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• First magnet chicane bending angles is adjustable. For 0.1-0.2 ps bunch length operation, chose R56=-122mm .

The large R56 reduces the required energy chirp.

But with issues:

more rf nonlinearity, more CSR effects, more sensitive to phase jitter?

• Chicane locations : 200 MeV and 561 MeV ( initial optimization by P.Emma, April 2003).

• Necessary of second order corrections : sextupole etc ?

Bates Energy Compressor (reverse of bunch compressor)

installed Q and S corrections later for first and 2th order system error corrections.

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Magnet Chicane parameters

Bending Angle (degree) 8.78 5.33

B field (kG) 5.11 8.70

Drift (b1-b2,b3-b4) (m) 2.423 2.447

Drift (b2-b3) (m) 0.5 0.5

R56 (mm) -122 -45

Beam parameters at last bend magnet of chicane

x 0.111 1.299

x (m) 9.22 10.759

E (MeV) 200 561

p/p =Total bunch momentum span 0.04 0.014

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2.2 Linear Compressing (0.2 ps , no wake fields , CSR etc.)

Start: Hard-edge, 0.2nC, 20 ps p=5KeV Q Variation (rms) ~ 0.2-0.3%

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Before Chirp, Chirp phase -21.40

Before 3rd harmonic linearizer

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After 3rd harmonic Linearizer

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After First Chicance, Lb~2ps

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After second chicane.Lb~ 0.2 ps.

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2.3 Wakes and CSR

Wakes only(4GeV) Wakes + CSR(4GeV)

Bunch length increased to ~0.4 ps

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Adjusting Chirp phase to compensate Wakefield & CSR effects ?

Chirp phase adjustment: -21.400=> -21.550

Increased Peak current ~25%Bunch Length down to ~0.3ps

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But same Chirp phase (-21.550) without Wakes & CSR –> over chirped

<= ~20fs,15kA peak

This can’t be real.

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Coherent Synchrotron Radiation & beam emittance growth

• Synchrotron radiation will be coherent if

>>Lbunch .

• Radiation by the tail will catch up with the head and modulate energy.

• Analytic emittance growth by P.Emma (‘Stead state’ CSR)Assuming ‘stead-state’ CSR, the incremental rms coherent energy spread at each dipole magnet slice(Lb) is (Ya. S. Derbenev):

Emittance growth:

3/43/222.0)(

z

be LNrs

)69)1((36

22.01

:Bend ChiacneLast

)(,)(

2

2223

2

4

5222

0

2616

2'

2'

220

20

2

'

BBz

B

N

e

xx

xxxx

LLLNr

dsds

dsRds

ds

dsR

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Bates Xfel-Linac Bunch Compressor : CSR effects on beam emittance

BC1 0.2nC BC2 0.2nC BC1 1nC BC2 1nC ?BC1 1nC R56=-122mm

re= 2.82E-15 2.82E-15 2.82E-15 2.82E-15 2.82E-15

Lb magnet (m) 0.2 0.2 0.2 0.2 0.2

(rad.) 0.1583 0.0931 0.0931 0.0931 0.1583

alpha 0.111 1.299 0.111 1.299 0.111

beta(m) 9.922 10.759 9.922 10.759 9.922

sigma-z(m) 6.00E-04 6.00E-05 1.20E-03 3.00E-04 3.00E-04

sigma-z(ps) 2.00 0.20 4.00 1.00 1.00

N 1.26E+09 1.26E+09 6.30E+09 6.30E+09 6.30E+09

n(mrad) 6.00E-07 6.00E-07 8.00E-07 9.00E-07 6.00E-07

gamma 391.39 1097.85 391.39 1097.85 391.39

0 1.0091 1.0059 1.0001 1.0013 1.0654

Analytic emittance growth estimation

Slice analysis of emittance growth will be performed for more careful study.

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CSR may do more damage to emittance ?

Linear SystemWith Wakes and CSR(No Twiss matching)

E=4 GeV, Chirp phase =-21.40

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2.4 Various effects

• Chirp phase. Sensitive to less than 0.050

-21.450 -21.650-21.550

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-21.450 -21.650-21.550

• Chirp phase sensitive (continue)

Bunch length (Peak current A)

~ 0.4 ps (~700) ~0.3 ps(~900) ~ 0.1ps(~2200!)

p/p

~ 0.15% ~0.2% ~0.1%

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• Injector Bunch Electron Distribution Effect. (Example of Gaussian beam …)

Linear, Chirp phase -21.40 +Wake, CSRSee big energy spike

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Further optimization and S2E Simulation

More work could be done to reduce CSR and optimize compressing process, like adjusting initial bunch density, chicane parameters and the optics.

Example: lower charge, same peak current shorter pulse

0.1nC, 20 ps. Final: 50fs bunch length, ~1000A peak…

Emittance distortion comparable to above mentioned 0.2nC, 20 ps case.

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About S2E Simulation

• Start to End Simulation• Codes: PARMELA(LANL) photo injector

ELEGANT (ANL) Linac + SwitchyardGINGER(LBL) FEL

Plan: Integrate PARMELA out(beam distribution) to ELEGANT simulation. For better simulation in linac. And down to FEL get responses.

Essential for: • Design optimization. • Beam diagnostics, controls and manipulation.• System requirements (error simulations, tolerances).•

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3. Summary

• Preliminary linac optics and bunch compressor design. Beam parameters close to design requirements.

• Tough requirements to Chirp phase.

• S2E simulation required for system optimization and define tolerances

• Much work needed to reduce nonlinear effects and there is still room to work on it!