Magnetic Compression of High Brightness Beams: Survey of Experimental Results

1
Magnetic Compression of High Brightness
BeamsSurvey of Experimental Results

• Scott G. Anderson
• ICFA Sardinia
• July 2002

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Magnetic Compression

• Motivation increase brightness, need sub-ps
  bunches
• Problems 6D phase space deterioration caused by
  collective effects
• Acceleration fields Coherent Synchrotron
  Radiation (CSR)
• Velocity fields Space-charge
• Experiments
• CTF, TTF, SDL, APS, UCLA,
• Features of the data
• Phase space dilution emittance growth, momentum
  spectrum
• Phase space filamentation both longitudinal and
  transverse
• Comparisons with theory/simulation
• Simulations reproduce rms quantities, but not
  intricate phase space structures seen in expt.

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Operating Principle of Magnetic Compression

• Acceleration ahead of crest of rf wave chicane
  dipoles
• acts as lens drift.

accelerating wave
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CTF II Emittance Measurements

• Large bend plane emittance growth observed as a
  function of compression
• Only CSR and/or space-charge were reasonable
  sources of De
• PARMELA predicts 10 of measured De
• CSR-TRACK predicts 60 of measured De

from H. H. Braun, et al., Phys. Rev. Lett. 84,
658 (2000).
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CTF II Emittance and Momentum Distribution
Measurements

• from H. H. Braun, et al., Phys. Rev. ST Accel.
  
• Beams 3, 124402 (2000).

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CTF II Emittance versus Horizontal Size
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TTF
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TTF
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SDL

• Strong micro-bunching with compression source
  not agreed upon.

10
APS?
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UCLA Experiment

• Lower energy (lt 12 MeV) space-charge may play
  significant role in compression
• This allows/requires emittance measurement using
  slits
• Transverse phase space is directly measured

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Interferometer Data
Normalized Signal
Delay Arm Position
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Emittance Versus PWT Phase
Sharp increase is a consistent feature in data
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Bifurcation of Transverse Phase Space
sz 4 ps
sz 0.6 ps
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Varying Phase or Field

• Emittance growth and phase space structure is a
  function of compression.

Emittance mm mrad
Pulse Length psec
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Emittance Growth Vs Beam Size
Emittance Growth mm mrad
sx mm
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Simulation

• Different codes model different processes
  (acceleration fields versus velocity fields.)
• Codes employed
• TREDI Solves Lienard-Wiechert potentials.
• PARMELA Provides input distributions for
  TREDI. Point-to-point space charge for
  comparison.
• ELEGANT CSR only calculation.
• Simulations indicate that for this experiment,
  acceleration fields do not contribute much
  emittance growth, the space charge fields are the
  dominant effect.

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Simulation
Emittance mm mrad
PWT Phase deg.

• Simulation is difficult. Number of
  macro-particles is low because of time-intensive
  space-charge calculations.
• Sharp emittance increase when bifurcation begins
  is missing in simulations.

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Heuristic Model

• To analyze the effect of space-charge in the
  compressor, we model the beam as a series of
  longitudinal slices.
• Since the beam energy spread is heavily
  correlated to slice position, we assume that
  there is no energy spread (no dispersion) within
  a single slice.
• Space-charge forces push a slice based on the
  fields at its centroid due to the other slices.
• Use standard envelope equations to evolve the
  sizes of single slices.

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Configuration Space gymnastics in the Model
(no space-charge)
Beam folds over in configuration space.
Configuration Space
Long. Phase Space
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Space-charge in the model
ellipsoid edge

• In simple model integrate ? space-charge force in
  last magnet to get Dx between slices
• Model predicts size dependence
• In simulation use 3D ellipsoidal fields

Cartoon of config. space evolution.
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Simple calculation with the model
Emittance mm mrad
x mrad
s/Rq
s0

• Kick applied between two slices in the last
  magnet.

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Slice Model Simulation
Configuration space
Trace space bifurcation
Input size dependence
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Bifurcation in PARMELA
z phase space
Energy distribution
Config. space
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Summary of UCLA Experiment

• Features of the data
• Trace space bifurcation
• Emittance growth inversely proportional to beam
  size
• Simulation shows that space-charge is the
  dominant effect
• Slice model simulation, and PARMELA/TREDI
  simulations show same features as data, but not
  as pronounced. Possible pre-existing structure
  in phase space and/or CSR combines with
  space-charge effects to accentuate behavior seen
  in data.
• Blue statements seem applicable to other
  experimental data as well!