FOM calculations

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Workpackage 2.2 Longitudinal Bunch Diagnostics

• Reports from
• Oxford University (George Doucas)
• Dundee University (Allan Gillespie)

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Bunch profile diagnostics with coherent
Smith-Purcell radiation

• Wade Allison , Victoria Blackmore, George Doucas,
  Colin Perry
• and M.F. Kimmitt
• (Dept. of Physics, Univ. of Essex)

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Basic principle

• A bunch of N electrons passing near a periodic
  structure of period l (e.g. a grating) produces
  radiation, typically in the far-infrared. (SP
  radiation)
• If the bunch length is short compared to the
  wavelength, then emission is coherent and the
  radiated energy depends on the longitudinal
  (time) profile of the bunch f(t) and the square
  of the no. of electrons
• So, measurements of SP over a broad range of
  wavelengths can determine the bunch profile f(t).
• In principle, any EM phenomenon that depends on
  q2 can be used but SP is cheap and provides
  angular dispersion of the wavelength according.

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Main features

• Radiation is in the far-infrared or sub-mm
  region.
• Has worked well at low energies (5MeV) and 14ps
  bunches, using liquid-He cooled detectors.
• ..but need to go to shorter lengths, higher
  energy, and, if possible, room-temperature
  detectors for a simpler system.
• Currently working at FELIX, 50MeV and 1-3ps
  bunches, using 0.5 and 1.0mm gratings.
• Using an 11-detector array of commercially
  available pyroelectric (PE) detectors.

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Detector array view of grating
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Status

• After last run (Jan. 2005).
• Pyroelectric detectors have worked very well.
• Fast electronics for PE detectors have also
  worked well.
• But.
• Significant background of unknown origin,
  observed even with grating fully retracted from
  beam (diffraction radiation upstream??)

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Status

• So..
• Carousel with 3 different gratings blank
  to investigate background. (already made)
• More efficient lightcollection system gtgtgtWinston
  cone designed
• Better filters (band-pass) for efficient
  background rejection gtgtgt strong overlap with
  astronomers and remote sensing groups.
  (collaborating with RAL)

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Next steps (2005-2006)

• Pending a more accurate determination of
  magnitude of background component, no definitive
  statement on bunch profile, but...
• First indications are that the bunch is
  non-Gaussian, with about 90 of particles inside
  1.5ps.
• This is a tentative conclusion, to be confirmed
  through the use of new filters and new optical
  system.
• Continue runs at FELIX but, at the same time,
  plan for tests in the GeV regime (SLAC).
• Continue with theoretical work on radiative
  processes, signal amplitude and data analysis
  (reconstruction determining, then inverting the
  Fourier Transform of bunch shape)

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Dundee Diagnostics Group
Electro-optic characterisation of ultra-short
relativistic electron bunches

• Allan Gillespie, Jonathan Phillips - University
  of Dundee
• Allan MacLeod - University of Abertay Dundee
• Steve Jamison - University of Strathclyde (Dundee
  Group)
• Collaborators at FELIX Rijnhuizen
• Lex van der Meer, Giel Berden, Britta Redlich

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Range of activities of group

• EO experiments at FELIX facility, the Netherlands
• CSR/edge radiation investigations at FELIX
• Preliminary FROG experiments carried out at FELIX
• EO experiments on gas jet configurations for
  laser-plasma wakefield accelerators (RAL,
  MPQ Garching, Jena)
• Planning EO diagnostics for ERL/P accelerator at
  Daresbury
• EO variant experiments at DESY (with FELIX group)
• Planning EO experiments at Endstation A at SLAC
  (plus JLab, KEK?)

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Re-cap Single-shot measurement of Coulomb field
electron bunch
Spectral decoding OR Temporal decoding
propagating electric field
polariser
chirped probe beam
EO crystal
Effective polarisation rotation proportional to
Coulomb field
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New Temporal Decoding method proves superior
cross correlator
Chirped probe focussed on ZnTe in beamline
(sampling field at single point)
Probe expanded and collimated forcross-correlator
(for temporal-spatial mapping)
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50MeV electron beam measurements FELIX facility,
Rijnhuizen, Netherlands
CCD camera
short pulse
chirped probe
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Electro-optic Signal vs Coulomb Field
crossed-polariser configuration ? EO signal ?
Coulomb Field2
Q300pC, E50MeV
Berden, Jamison, et al., Phys. Rev. Lett. 93
114802 (2004)
Measurements have been made with 150pC lt Q lt
300pC
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Real-time monitoring and bunch profile
modification
? 450 fs FWHM Coulomb field on sub-peak
Bunch profile can be modified by changing the
buncher and accelerator phases
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Time-resolution capability.
300 fs
material properties limit ability to recover true
electric field.
(FWHM)

• Alternative materials (GaP, GeSe,)
• Calibration in 300-100 fs region
• ..?

30 fs
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EO measurement of bunch timing jitter
Bunch timing jitter bunch duration
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Synchronisation and bunch timing jitter
Probe laser synchronised to RF

• 100fs laser-RF synchronisation
• Jitter measurement also subject to laser
  beamline path length changes (active
  stabilisation?)

Bunch sampling rate lt 1 kHz
(could envisage ways for rapid sampling of a pair
of bunches)
Additional timing jitter measurement (or
synchronisation?) between photoinjector laser
and EO probe laser?
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Electro-optic detection of edge radiation..
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Electro-optic detection of edge radiation..
e-
THz
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Summary of EO detection methods at FELIX
FELIX longitudinal profile measurements
real-time bunch profile adjustment bunch
timing jitter charge dependence FELIX
CSR measurements (edge radiation)
non-invasive synchronization measurements ?
further data analysis necessary, but promising
technique solution to bunch halo effects
on EO detector?
Good progress so far on both methods


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EO beamline section
Beam profile monitor
Beam position monitor
e-
CSR diagnostics
EO diagnostics table
LC-ABD WP2.2 Daresbury ERL/P electron bunch
measurements
Laser room
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Overall Summary..
Capabilities of EO detection

• Demonstrated profile measurement of
  650fs FWHM bunches, 300pC
• Current time resolution is 300fs
• Challenges remain in getting sub-200fs
  resolution - materials, EO response,
  deconvolution, ...
• Techniques are applicable to CSR measurements
  - non-invasive nature may be very useful
  (e.g. beam halo)
• Many variations of EO detection possible
  scope for addressing specific diagnostic
  requirements