Dispersion Effects on SASE at FLASH - DESY · 2008. 11. 24. · FEL Beam Dynamics / 24-11-08 Eduard Prat, DESY SASE energy vs electron energy Measurements 490 492 494 496 498 500

FEL Beam Dynamics / 24-11-08 Eduard Prat, DESY

Dispersion Effects

on SASE at FLASH

Eduard PratFEL Beam Dynamics Meeting24 of November of 2008, Hamburg

SASE dependence on e- trajectorySASE vs e- energy for different dispersion scenariosSASE spectrum dependence on dispersion

Measurements done 11 & 15 October 2008 (total=1shift)Simulations done with Genesis 1.3


SASE dependence on electron trajectoryMeasurements

-100 -50 0 50 1000

0.5

1

H19SEED kick [μrad]

SA

SE

pow

er [a

.u.]

-100 -50 0 500

0.5

1

V19SEED kick [μrad]

SA

SE

pow

er [a

.u.]

meas day1meas day2

day1: 450MeVday2: 495MeV

• FWHM: 100-105 µrad (x) / 90 µrad (y) (x>y because e- wiggling motion in the undulator is in x?)• Good reproducibility between different days

Measurements are averaged over 100 shots SASE energy taken by MCP detector (maximum around 20 µJ)


SASE dependence on electron trajectory Simulations vs measurements

-5 0 5

x 10-5

0

500

1000

1500

s[m]

I[A]

Current

-5 0 5

x 10-5

0

1

2

3x 10-6

s[m]

ε u [m

]

Emittance

-5 0 5

x 10-5

960

965

970

975

980

985

s[m]

γ

Energy

-5 0 5

x 10-5

0

0.5

1

1.5

2

s[m]

Δγ

Energy Spread

SASE simulationsBunch length = 100e-4mGaussian current profile (Imax=1.5 KA)Energy chirp of ±1%Rest of conditions constant along the bunch: εu=2.2 μm, matched optics

Good agreement

-150 -100 -50 0 50 100 1500

0.2

0.4

0.6

0.8

1

kick [μrad]

SA

SE

pow

er [a

.u.]

meas2meas1SASE sim.SS. sim.

Idea: finding a “reasonable” input e- beam for Genesis with an orbit sensitivity as in reality.

Steady state simulations (SS): 1 single λTime-dependent: bandwidth


SASE energy vs electron energyMeasurements

490 492 494 496 498 500 502 504 5060

0.2

0.4

0.6

0.8

1

electron energy [MeV]S

AS

E in

tens

ity [a

.u.]

initial caseextra dxdx correcteddx/dy corrected

FWHM goes from 0.82 to 1.72% after correcting dispersion in both planes ☺

5mm11mmdx/dy corrected

31mm12mmdx corrected

28mm48mmextra dx

30mm22mminitial case

dydxrms dispersion

1 2 3 4 5 6-100

-50

0

50

100

# BPM

Dx

[mm

]

1 2 3 4 5 6-60

-40

-20

0

20

40

# BPM

Dy

[mm

]



Energy change by varying ACC456 gradientMeasurements averaged over 100 points


SASE energy vs electron energySimulations

1 2 3 4 5 6-0.04

-0.02

0

0.02

0.04

# BPM

DX

[m]

1 2 3 4 5 6-0.1

-0.05

0

0.05

# BPM

DY

[m]

initial case (meas.)initial case (prediction)after corr. (meas.)after corr. (prediction)

0.4 mrad-2.9 mraddy’

5 mm47 mmdy

1.9 mrad6.0 mraddx’

14 mm12 mmdx

After correctionInitial situation

From the dispersion measurement, the dispersion functions at the entrance of the undulator can be derived:D(s) = D(s0)*R11(s)+D’(s0)*R12(s)

-Orbit changes according to dispersion functions-Optics changes (magnets were not scaled)

Same beam conditions as before

No dispersion generated inside the undulator

Considered effects:


SASE energy vs electron energySimulations vs measurements

Good agreement


SASE spectrum vs dispersionMeasurements

25.5 26 26.5 2780

100

120

140

160

180

200

wavelength [nm]

inte

nsity

[a.u

.]

qecol=84.49Aqecol=84.98Aqecol=85.18Aqecol=85.49Aqecol=86.49Aqecol=87.49Aqecol=88.48A

Widest spectrum and maximum power without dispersion Effects depend on the dispersion sign ↑QECOL ↓ λ(but not symmetrically): ↓QECOL ↑ λ

0 5 10 15 20 25-4

-2

0

2

4

#BPM

x [m

m]

0 5 10 15 20 25-2

0

2

4

#BPM

y [m

m]

•Every measurement averaged over 200 shots•Dispersion generated by varying QECOL current•Centroid orbit along the undulator kept constant


Dispersion generated when changing Q3/5ECOL current

1 2 3 4 5 6-0.04

-0.02

0

0.02

# BPM

DX

[m]

measurementsprediction

4.7 mraddx’

0.7 mmdx

Dispersion functions at the undulator entrance

Rms dispersion in the undulator: 15 mm

Decrease of 0.5 A decrease (0.6% of the current)

No dispersion generated inside the undulator


SASE spectrum vs dispersionSimulations

Effect of the dispersion to SASE spectrum depends on the initialcorrelation between transverse coordinates and energy. In general we have the following relation:

Introducing dispersion changing Q3/5ECOL current, a linear component ηu is added

-1 0 1

x 10-4

0

500

1000

1500

s[m]

I[A]

Current

-1 0 1

x 10-4

960

965

970

975

980

985

s[m]

γ

Energy

SASE simulations

Similar beam conditions as beforeBunch length increased to 300e-4mCurrent increased for the head and the tail

Analysis restricted to horizontal offset x and the impact of DxConsidered dispersions:[0 +5cm -5cm]



-1 0 1

x 10-4

-6

-4

-2

0

2

4

6x 10-4

s [m]

x [m

]

Horizontal offset

-1 0 1

x 10-4

960

970980

s [m]

γ

Energy

2.56 2.58 2.6 2.62 2.64

x 10-8

0

0.2

0.4

0.6

0.8

1

λ [m]

inte

nsity

[a.u

.]

Radiation spectrum

no disp+5cm-5cm

no disp+5cm-5cm

The spectrum becomes narrowerCentral wavelength does not change

No initial correlation x-energy / no off-set



-1 0 1

x 10-4

-4

-2

0

2

4

6

8x 10-4

s [m]

x [m

]

Horizontal offset

-1 0 1

x 10-4

960

970980

s [m]

γ

Energy

2.58 2.6 2.62 2.64

x 10-8

0

0.2

0.4

0.6

0.8

1

λ [m]

inte

nsity

[a.u

.]

Radiation spectrum

no disp+5cm-5cm

no disp+5cm-5cm

Central wavelength depends on the dispersion sign

No correlation x-energy / non-zero off-set (+250 μm)



-1 0 1

x 10-4

-1

-0.5

0

0.5

1x 10-3

s [m]

x [m

]

Horizontal offset

-1 0 1

x 10-4

960

970980

s [m]

γ

Energy

2.58 2.6 2.62

x 10-8

0

0.2

0.4

0.6

0.8

1

λ [m]

inte

nsity

[a.u

.]

Radiation spectrum

no disp+5cm-5cm

no disp+5cm-5cm

Effect to the spectrum width depends on the final correlation (can be narrower or wider)

Central wavelength does not change

Initial linear correlation x-energy



-1 0 1

x 10-4

-10

-8

-6

-4

-2

0

2

4x 10-4

s [m]

x [m

]

Horizontal offset

-1 0 1

x 10-4

960

970980

s [m]

γ

Energy

2.56 2.58 2.6 2.62 2.64

x 10-8

0

0.2

0.4

0.6

0.8

1

λ [m]

inte

nsity

[a.u

.]

Radiation spectrum

no disp+5cm-5cm

no disp+5cm-5cm

The spectrum becomes narrowerCentral wavelength depends on the

dispersion sign

Initial quadratic correlation x-energy


SASE spectrum vs dispersionMeasurements

25.5 26 26.5 2780

100

120

140

160

180

200

wavelength [nm]

inte

nsity

[a.u

.]

qecol=84.49Aqecol=84.98Aqecol=85.18Aqecol=85.49Aqecol=86.49Aqecol=87.49Aqecol=88.48A

Linear + quadratic initial u-u’/energy correlation?Simulations are ongoing…

2.56 2.57 2.58 2.59 2.6 2.61 2.62 2.63 2.64

x 10-8

0

0.2

0.4

0.6

0.8

1

λ [m]

inte

nsity

[a.u

.]

Radiation spectrum

no disp+5cm-5cm


Summary – Next steps

Summary

Dispersion correction improves sensitivity of SASE to electron energy

Introducing dispersion narrows the SASE spectrum and changes the central wavelength. An initial quadratic correlation u/u’-energy explains qualitatively the measurements.

Next steps

Fully simulate SASE wavelength vs dispersion (compare required initial u-u’/energy correlation with s2e simulations / measurements)

Repeat the measurements: see reproducibility, try to introduce dispersion with steerers in the MATCH section instead of QECOL.

Documents

Dispersion Effects on SASE at FLASH - DESY · 2008. 11. 24. · FEL Beam Dynamics / 24-11-08 Eduard Prat, DESY SASE energy vs electron energy Measurements 490 492 494 496 498 500