Electro-Optic Beam Diagnostic at BNL DUV-FEL

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Electro-Optic Beam Diagnostic at BNL DUV-FEL

• Henrik Loos
• for
• National Synchrotron Light Source
• Brookhaven National Laboratory
• Presented at ICFA Mini-Workshop XFEL 2004

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Outline

• DUV-FEL accelerator facility
• Coulomb field measurement
• THz CTR pulse characterization
• Issues for ultrafast electro-optic measurement
• Summary and outlook

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DUV-FEL Facility
50 m
Radiator (NISUS) Wiggler L 10 m, lw 3.89
cm B 0.31 T, K 1.126 HGHG 100 µJ _at_266
nm 3rd harm. 1 µJ _at_89 nm
Energy 170 MeV
Charge 300 pC
Normalized emittance 4 mm mrad
Compressed bunch length 0.3-0.6 ps rms
Energy spread 0.01 rms
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Electro-Optic Bunch Diagnostic

• Uses Pockels-effect to detect electric field E of
  Coulomb field or THz radiation with fs laser.

• Birefringence in lt110gt cut ZnTe with E-field and
  laser polarization ? to 001 axis

• Detect change in laser polarizationwith l/4
  waveplate and analyzer.
• Signal asymmetry A between linear polarization
  states gives phase change.

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Experimental Setup

• Constants l 800 nm n0 2.83r41 4
  pm/V e 10l 0.5 mm
• Dj 90o at 170 kV/cm

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Single Shot Time Calibration

• TiSa chirped to 6 ps.
• e-beam 1ps FWHM.
• Laser delay changed from 0.5 to 1.0 mm
• Strong modulation in spectrum from uncoated ZnTe
  crystal.
• Average of 50 single shot spectra.
• Charge from Coulomb field lower than real
  charge of 250 pC.

Ds 0.5 mm, Q 130 pC
Ds 0 mm, Q 80 pC
Ds -0.5 mm, Q 125 pC
Ds -1.0 mm, Q 130 pC
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Time resolution

• Minimum THz pulse length with 6 ps, 6 nm chirped
  laser
• Distance e-beam/laser (850 µm)
• Monochromator (1800/mm) grating is 30 fs.
• Coherence length in ZnTe (500 µm thick) is 200
  fs.
• Measured length of 1.6 ps dominated by spectral
  distortion and confirmed by simulation.

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Jitter Measurement

• Single shot enables jitter measurement.
• Spectral distortions do not affect centroid
  position.
• 50 shots 25 s.
• Jitter e-beam/seed laser 170 fs.
• Jitter low-level RF/TiSa 200 fs.
• Energy jitter after bend magnet equals 1 ps rf
  phase jitter? mostly rf amplitude jitter.
• Use for feedback on laser phase.

Head Tail
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THz Pulse Field Characterization

• 80 µJ CTR pulse observed at DUV-FEL.
• E-beam 700 pC, 100 MeV, 150 (???) fs rms.
• Measure spatial-temporal electric field
  distribution with EO sampling.
• Understand relay and focusing of CTR.
• Compare with CTR simulation code.
• Compare with bolometer measurement.

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Electro-Optic THz Radiation Setup
Electron Beam
Vacuum Window
Paraboloid
f 7.5
f 1.5
Delay
ZnTe
Analyzer
Polarizer
CCD
TiSa Laser
Coupling Hole, 2 mm
l/4
Lens
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Signal and Reference
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Image Processing for Field Measurement

• Use compensator waveplate to detect sign of
  polarization change.
• Reference IR (left) and Signal IS (right)
  obtained simultaneous.
• Rescale and normalize both.
• Calculate asymmetry A of Signal.
• Subtract asymmetry pattern w/o THz.

A 2IS/IR - 1
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Time Dependent Measurement

• Use mildly compressed bunch of 500 fs rms and
  300 pC to get both 0-phasing and electro-optic
  measurement.
• Temporal scan by varying phase of accelerator RF
  to both sample and cathode laser.
• Approximately equivalent to varying delay between
  both lasers but much faster and computer
  controlled.
• Measured to be 1.2 ps/degree.

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Measured THz Field Movie
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Transverse-Temporal Distribution

• Take horizontal slice through images.
• Asymmetry of 1 equals 170 kV/cm electric field
  strength.
• Charge 300 pC.
• Saturation and over-rotation at higher
  compression.
• Needs crystal 500 mm.

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Simulation of CTR Propagation

• Decompose radiating part of coulomb field in
  Gauss-Laguerre modes.
• Calculate transmission amplitude and phase
  through experiment for THz spectral range.
• Use bunch form factor to reconstruct radiation
  field in time and space.
• Example 300 pC, 300 fs

30 mm
20 ps
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Focus Distribution of THz

• Focus spot size3 mm diameter.
• Single cycle oscillation.
• 300 fs rms length.
• Electric field strength more than 300 kV/cm at
  300 pC charge.
• Pulse Energy 4 mJ.70 mJ (700 pC, 150 fs)

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Simulation vs. Experiment

• Simulation gives 2 times more field.
• Tighter focus in simulation.
• Up to 50 kV/cm measured.

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Single Cycle THz Pulses

• Pulse energy from field 60 nJ.
• Pulse energy with Joule-meter 170 nJ.
• Pulse energy from simulation 800 nJ.
• Good match of temporal and spectral properties.
• Factor 2 and 4 difference in field and energy.
• Measured 80 mJ to have 1 MV/cm field in focus.

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THz Spectrum

• Present intensity limited by geometric apertures.
• Low frequency cutoff at 15 cm-1 or 0.5 THz.

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Potential Ultrafast EO-Detection

• Intense ultrafast THz source.? Modulated
  electron beam (_at_DUV-FEL).? High pulse energy CTR
  (C...R).
• Broadband, uniform response EO-material.? EO-Poly
  mer Composites.
• Time domain laser pulse measurement.? Amplified
  fs-laser (injector drive laser).? Spectral
  phase measurement.? FROG, SPIDER.? Not limited
  by laser pulse length.

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Modulated Beam Studies

• 100 fs e-beam structures from modulated drive
  laser.
• Measured with longitudinal tomography.
• Use to test electro-optic resolution, can be
  further compressed.

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Broadband Electro-Optic Materials

• EO-polymers have 20x larger EO-coefficient than
  ZnTe.
• No phonon resonances in far-IR.
• Phase mismatch.
• Lifetime weeks.
• 10 µm sufficient.
• Cooling?

A.M. Sinyukov, L.M. Hayden, to be published
20 DCDHF-6-V/20 DCDHF-MOE-V/60 APC
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Measuring the Spectral Phase SPIDER
Spectral Phase Interferometry for Direct
Electric-Field Reconstruction (Walmsley group,
Oxford)
400 nm
800 nm
800 nm
Mix 2 replicas from EO-modified pulse with
original streched pulse.
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Summary and Outlook

• Simple single shot chirped EO setup sufficient
  for jitter measurement.
• Jitter of 170 fs equal to low-level rf/laser
  jitter and estimates from HGHG.
• Enables noninvasive laser/e-beam
  synchronization-feedback.
• Ultrafast EO measurement requires time-domain
  method.
• High intensity THz pulses up to 1 MV/cm field
  strength from CTR.
• CTR simulation, pulse energy and electro-optic
  measurement in resonable agreement.
• Extract THz to accessible user station for
  various applications.
• Use time-domain single-shot EO method and apply
  to THz from modulated electron beam.

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Acknowledgements
SDL/DUV-FEL Team
G.L. Carr J. Greco H. Loos J.B. Murphy
J. Rose T.V. Shaftan B. Sheehy Y. Shen
B. Singh X.J. Wang Z. Wu L.H. Yu
In future at SLAC
This work was supported by DOE Contracts DEAC No.
DE-AC02-98CH10886 and AFOSR/ONR MFEL Program No.
NMIPR01520375.