modified beam parameter range

W. Decking (DESY)2nd XFEL Machine Advisory Committee

Meeting05.05.2010

Modified Beam Parameter Range

2

05.05.2010, 2nd XFEL MACW. Decking, DESY, XFEL Machine Layout Coordinator

Post Linac Beam Lines

2PITZ 2009 results: Nominal 1 nC measurements

x=0.76 mm mrad

y=1.26 mm mrad

min. xy = 0.98 um

100% RMS emittance

(no charge cut in data analysis)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

378 380 382 384 386 388 390 392 394

Imain, A

mm

mra

d

XemitYemitXYemit

y=1.26 um

x=0.76 um

• Q = 1 nC• Gun phase: +6o off-crest• Booster phase: on-crest• Laser temporal profile: 2.1/23.1/2.4 ps• Laser spot size = 0.36 mm

0.4

0.6

0.8

1

1.2

1.4

1.6

7.9

.09

0:0

0

8.9

.09

0:0

0

9.9

.09

0:0

0

10

.9.0

9 0

:00

11

.9.0

9 0

:00

12

.9.0

9 0

:00

13

.9.0

9 0

:00

14

.9.0

9 0

:00

em

itt,

mm

mra

d

EmX

EmY

EmXY

Long term(~4 days): ~6-8.5% (stdev)

~16-21% (peak-to-peak)

Solenoid Scan

3



3PITZ 2009 results: Emittance vs. bunch charge

BSA laser spot size 1 nC 0.5 nC 0.25 nC 0.1 nC1.8 0.44 1.541.5 0.36 0.89..1.26 0.83 0.651.2 0.29 1.08 0.73 0.63 0.441 0.26 1.57 0.7 0.56 0.42

0.8 0.21 0.81 0.47 0.380.5 0.15 1.31 0.55 0.340.2 0.05 0.57

XY-emittance

0.0 0.1 0.2 0.3 0.4 0.5

Laser spot size (mm)

Study of emittance vs.BSA size and charge

gun of +6 deg off-crest, booster on-crest

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

BSA size (mm)

Em

it-X

Y (

um

)

1 nC

0.5 nC

0.25 nC

0.1 nC

4



4

PITZ 2009 results: Emittance vs. bunch charge (with charge cut)

2D scaled emittance

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90 100

charge cut, %

mm

mra

d

XYemit (0.1nC)

XYemit (0.25nC)

XYemit (0.5nC)

XYemit (1nC)

XYemit (1nC, meas1-4)

Study of emittance vs.BSA size and chargegun of -6 deg off-crest, booster on-crest

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

BSA size (mm)

Em

it-X

Y (

mm

-mra

d)

1 nC

0.5 nC

0.25 nC

0.1 nC

5% cut ~0.4 um

(comparable to LCLS result)

10% cut ~0.7-0.8 mm-mrad

(remove non-lasing electrons)

→ beyond XFEL requirement

5



Parameters at Gun

Charge nC 1 0.5 0.25 0.1 0.02

RMS Bunch Length fs 8000 6000 5400 4800 4500 simulation

Peak Current A 49.8 33.2 18.4 8.3 1.8

Slice Emittance m 1 0.7 0.5 0.32 0.2measurement of projected

Slice Energy Spread keV 2 2 1 0.6 0.8 simulation

Compression factor 100 120 162 301 1128

Peak Current A 5000 4000 3000 2500 2000

6



Bunch Compression Scheme

14 GeV

7



Micro-bunching Instability Longitudinal space charge induced growth of initial current fluctuations Damping by large uncorrelated energy spread Smaller initial current -> smaller instability growth Laser heater scaled to provide same energy spread after final

compression Keep final current ripple < 200 A => initial energy spread ≤ 20 KeV

after BC0

after BC1

after BC2

before undulator

Example working point with realistic LH and 10 KeV energy spread

8



Parameters after Laser Heater – Constant Energy Spread

Charge nC 1 0.5 0.25 0.1 0.02


Peak Current A 49.8 33.2 18.4 8.3 1.8



Slice Energy Spread after LH keV 20 16.7 12.3 6.7 1.78

Constant Energy Spread after Compression

9



Wash Out of Initial Current Ripple

Longitudinal space charge washes out initial density ripple

Effect decreases with decreasing peak current Add uncorrelated energy spread to counteract instability

=> ×2 final energy spread at 20 pC

10



Parameters after Laser Heater – Increased Energy Spread

Charge nC 1 0.5 0.25 0.1 0.02


Peak Current A 49.8 33.2 18.4 8.3 1.8






Increased Energy Spread after Compression

11



Parameters at Undulator

Charge nC 1 0.5 0.25 0.1 0.02

Compression factor 100 120 162 301 1128

Peak Current A 5000 4000 3000 2500 2000 Goal

Slice Energy Spread MeV 0.2 0.2 0.2 0.2 0.9 No Laser Heater

Slice Energy Spread after LH MeV 2.0 2.0 2.0 2.0 2.0


Slice Energy Spread after LH MeV 2.0 2.2 2.5 2.9 4.1

Increased Energy Spread after Compression

12



Emittance Degradation

Charge nC 1 0.5 0.25 0.1 0.02

Slice Emittance at Gun m 1 0.7 0.5 0.32 0.2

from measurement of projected

Compression Factor 100 120 162 301 1128

Emittance Degradation 5.0 10.0 20.0 30.0 100.0

from simulation and LCLS experience

Slice Emittance at Undulator 1.05 0.77 0.60 0.42 0.40

13



Final Parameter Set

Gun Charge nC 1 0.5 0.25 0.1 0.02

Slice Emittance m 1 0.7 0.5 0.32 0.2

Peak Current A 49.8 33.2 18.4 8.3 1.8

Slice Energy Spread after LH keV 20.0 18.2 15.3 9.8 3.6

Undulator


Peak Current kA 5.0 4.0 3.0 2.5 2.0

Bunch Length (RMS) fs 80 50 33 16 4

Slice Emittance m 1.05 0.77 0.60 0.42 0.40

Slice Energy Spread MeV 2.0 2.2 2.5 2.9 4.1

14



FLASH S2E Simulations

15



FLASH S2E Simulations

Charge nC 1 0.5 0.25 0.1 0.02

Gu

n

Slice Emittance (RMS) m 0.8 to 1 0.6 to 0.7 0.4 to 0.5 0.25 to 0.3 < 0.1

Peak Current A 50.0 35.0 20.0 8.0 1.6

Slice Energy Spread (RMS) KeV 0.8 to 1 0.7 0.6 to 0.7 0.2 to 0.3 0.1 to 0.15

Un

du

lator


Peak Current kA 2.4 2.4 2.5 2.0 1 to 1.5

Emittance Degradation 20 20 30 60 150

Slice Emittance (RMS) m 1 to 1.3 0.7 to 0.9 0.5 to 0.7 0.4 to 0.5 0.3 to 0.4

Slice Energy Spread (RMS) MeV 0.1 to 0.2 0.1 to 0.2 0.25 0.2 to 0.4 0.25

Slice Energy Spread (RMS)Initial × Compression MeV 0.05 0.07 0.10 0.07 < 0.1

Radiation pulse (FWHM) fs 70.00 30.00 17.00 7.00 2.00

16



Other Issues

Impact of reduced emittance on baseline layout Emittance measurement optimized for 1.4 m => reduced

resolution Impact of reduced charge on baseline layout

Nominal charge range 0.1 to 1 nC with diagnostics resolution optimized at 1 nC

Impact of reduced energy on baseline layout Less chirp compensation from linac wake field

17



Summary

New baseline parameter set established

Detailed analysis follows, taking into account tolerance considerations, complete set of self- and external fields etc.

Charge nC 1 0.5 0.25 0.1 0.02

Peak Current kA 5.0 4.0 3.0 2.5 2.0

Bunch Length (RMS) fs 80 50 33 16 4

Slice Emittance m 1.05 0.77 0.60 0.42 0.40

Slice Energy Spread MeV 2.0 2.2 2.5 2.9 4.1

modified beam parameter range

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