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Magnetic Compression of High Brightness Beams: Survey of Experimental Results

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Title: Magnetic Compression of High Brightness Beams: Survey of Experimental Results Author: Scott Anderson Last modified by: Scott Anderson Created Date – PowerPoint PPT presentation

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Title: Magnetic Compression of High Brightness Beams: Survey of Experimental Results


1
Magnetic Compression of High Brightness
Beams Survey of Experimental Results
  • Scott G. Anderson
  • ICFA Sardinia
  • July 2002

2
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.

3
Operating Principle of Magnetic Compression
  • Acceleration ahead of crest of rf wave chicane
    dipoles
  • acts as lens drift.

accelerating wave
4
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).
5
CTF II Emittance and Momentum Distribution
Measurements
  • from H. H. Braun, et al., Phys. Rev. ST Accel.
  • Beams 3, 124402 (2000).

6
CTF II Emittance versus Horizontal Size
7
TTF
8
TTF
9
SDL
  • Strong micro-bunching with compression source
    not agreed upon.

10
APS?
11
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

12
Interferometer Data
Normalized Signal
Delay Arm Position
13
Emittance Versus PWT Phase
Sharp increase is a consistent feature in data
14
Bifurcation of Transverse Phase Space
sz 4 ps
sz 0.6 ps
15
Varying Phase or Field
  • Emittance growth and phase space structure is a
    function of compression.

Emittance mm mrad
Pulse Length psec
16
Emittance Growth Vs Beam Size
Emittance Growth mm mrad
sx mm
17
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.

18
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.

19
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.

20
Configuration Space gymnastics in the Model
(no space-charge)
Beam folds over in configuration space.
Configuration Space
Long. Phase Space
21
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.
22
Simple calculation with the model
Emittance mm mrad
x mrad
s/Rq
s0
  • Kick applied between two slices in the last
    magnet.

23
Slice Model Simulation
Configuration space
Trace space bifurcation
Input size dependence
24
Bifurcation in PARMELA
z phase space
Energy distribution
Config. space
25
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!
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