XFEL Bunch Compression System Set Up

1
XFEL Bunch Compression System Set Up

• Commissioning Workshop Zeuthen 2005
• M. Dohlus and T. Limberg

2
Lay Out

• Short look at the European XFEL BC design and
  instrumentation
• Principle of simple bunch compression set up
  method
• Example with rectangular bunch, results for
  parabolic and gaussian bunches
• Set up with multiknobs
• First Sensitivity calculations

3
European XFEL Bunch Compression System after
Optimization with the 2nd BC Stage at 2.5 GeV
4
Diagnostics for the BC (D. Noelle)
500 MeV

• Charge
• 2 Toroids upstream and downstream the BC (1)
• Energy
• 2 special BPMs in the chicane (Flat Chamber) (100
  µm)
• Incoherent Synchrotron Radiation (Position -gt
  rel. Energy, Shape -gt Energy Spread)
• Energy Spread
• Screen/ Wire Scanner (in the dispersive Part of
  the BC) (10 µm)
• Shape of the incoherent Synchrotron Radiation
  Signal
• Long. Distribution
• Either Transverse Mode Cavity or Optical Replica
  System (20 fs)
• Emitance and Optics
• Fodo Channel with 4 Screen/ Wirescanner
  Combinations ( lt 10 µm)
• Slice Parameters in BC1
• Transverse Lola with Off Axis Screens
• Longitudinal Lola with Screen in the
  Commissioning Dump

5
Preliminary Lay Out of Bunch Length Measurement
Downstream 2nd BC Stage
Optical Replica Method (E. Saldin, E.
Scheidmiller, M. Yurkov)
6
Principle Set Up Scheme for BC System I

• Get beam through BC chicanes, stabilize beam
  position in 1st chicane with injector linac RF
  amplitude feedback
• Measure bunch current distribution
• Set linac phase slightly off-crest (-10 to -15
  degrees), set 3rd harmonic RF phase to 180
  degrees (decelerating)
• Adjust injector linac phase to reach 1 kA peak
  current after 1st chicane
• Minimize asymmetry of distribution with 3rd
  harmonic RF amplitude
• Iterate linac phase and 3rd harmonic RF amplitude
  adjustment 2-3 times
• Check beam shape after 2nd chicane, adjust peak
  current with R56

7
First step Adjust Injector Linac RF Phase to get
Ip1kA
Rectangular Charge Distribution q1 nC I50
A Initial Energy 4 MeV Uncorrrelated Energy
Spread 5 keV
8
Next Adjust 3rd harmonic RF Amplitude to make
Profile symmetric
9
Re-Adjust Linac Phase to reach 1kA
10
Touch-up with 3rd harmonic Amplitude (balancing
the shoulders of the profile)
11
Last Touch with Linac Phase, Slight Increase of
2nd Chicane R56 Done.
1 kA
5 kA
After 2nd Chicane
After 1st Chicane
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End Results of Set Up for a Gaussian Charge
Distribution
After 1st Chicane
After 2nd Chicane
Gaussian Charge Distribution q1 nC I50
A Initial energy 4 MeV Uncorr. Energy Spread 5
keV
5 kA
1 kA
13
Results for a Parabolic Charge Distribution
After 2nd Chicane
After 1st Chicane
Parabolic Charge Distribution q1 nC I50
A Initial energy 4 MeV Uncorr. Energy Spread 5
keV
5 kA
1 kA
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Setup Using Multiknobs

• Make knobs to change independently the first,
  second and third derivative of the combined
  accelerating voltage of Injector Linac and 3rd
  harmonic RF, using linac and 3rd harmonic phase
  and 3rd harmonic amplitude.
• V(s) V1cos(k1sj1) V3cos(k3s j3)
• DV g s x1 1010 s2 x2 1012 s3
  o(s4)
• Use gradient knob for peak current, 2nd
  derivative to balance beam distribution in the
  center region and 3rd derivative knob for
  adjusting the tails.
• Linac Amplitude is still used to keep beam energy
  constant.

15
First step Adjust gradient knob to get Ip0.5
kA and use 2nd derivative knob to tune symmetry
Parabolic bunch q1 nC I50 A Initial Energy 4
MeV Uncorr. Energy Spread 5 keV
0.5 kA
0.1 kA
16
Next iterate, use 3rd derivative knob for
precision adjustment
17
2nd Iteration
18
Reaching perfection with 3rd derivative
knob (not essential for usable beam after 2nd
chicane)
19
Increase R56 of 2nd Chicane to achieve 5 kA peak
current
1 kA
20
Sensitivity Scans for a Parabolic Bunch
Peak Current after 2nd Chicane
Peak Current after 1st Chicane
Peak Current after 2nd Chicane
Peak Current after 1st Chicane
Booster Linac Phase
Injector Linac Phase
Magnet Strength 1st Chicane
3rd harmonic RF Phase
3rd harmonic RF Amplitude
Magnet Strength 2nd Chicane
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Sensitivity Scans for a Parabolic Bunch
Peak Current after 2nd Chicane
Peak Current after 1st Chicane
Incoming Charge (Ip const.)
Incoming Ip (Charge const.)
Incoming Ip (Length const.)
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Sensitivities in Numbers

• Criterium Ipeak changes from 5 kA to 6 kA