1. 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

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

3. DUV-FEL Facility 50 m Radiator (NISUS) Wiggler: L = 10 m, lw = 3.89 cm
Adjustable
Chicane
177
MeV
RF zero Phasing
Photoinjector
CTR Monitor
Normal incidence
77 MeV
FEL seed
at 800 nm
Modulator
Undulator
NISUS pop-in
monitors
FEL Measurements
Energy, Spectrum, Synchronization
and Pulse Length Measurements
at 266 nm
Ion Pair Imaging
Experiment
at 88 nm
Nisus Wiggler
30 mJ
Ti:Sapphire
Amplifier
Dispersion
Magnet
Trim Chicane
Radiator (NISUS) Wiggler:
L = 10 m, lw = 3.89 cm
B = 0.31 T, K = 1.126
HGHG: 100 nm
3rd harm. 1 nm
Energy
170 MeV
Charge
300 pC
Normalized emittance
4 mm mrad
Compressed bunch length
ps rms
Energy spread
0.01 % rms

4. Electro-Optic Bunch Diagnostic
Uses Pockels-effect to detect electric field E of Coulomb field or THz radiation with fs laser.
Birefringence in <110> cut ZnTe with E-field and laser polarization  to [001] axis
Delay
Multi-Shot
Single-Shot
ZnTe
Laser
e-Beam
E-Field
Detect change in laser polarization with l/4 waveplate and analyzer.
Signal asymmetry A between linear polarization states gives phase change.

5. Experimental Setup
Electrons
Laser
Constants: l = 800 nm n0 = 2.83 r41= 4 pm/V e = 10 l = 0.5 mm
Dj = 90o at 170 kV/cm

6. Single Shot Time Calibration
Ti:Sa 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

7. 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.

8. Jitter Measurement Head Tail 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/Ti:Sa 200 fs.
Energy jitter after bend magnet equals 1 ps rf phase jitter  mostly rf amplitude jitter.
Use for feedback on laser phase.
-600
-400
-200
200
400
600 5 10
Delay (fs) s
= 167 fs -5 15 20 25
Time (ps)
Pulse # (s)
Head Tail

9. 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.

10. Electro-Optic THz Radiation Setup
Vacuum Window
Electron Beam
Paraboloid
f = 7.5”
f = 1.5”
Delay
Polarizer
ZnTe
Analyzer
CCD
Ti:Sa Laser
Coupling Hole, 2 mm
l/4
Lens

11. Signal and Reference
OAP
ZnTe BS
l/4
Pol.
Ref
Signal
Camera

12. 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.
Horizontal (mm)
Vertical (mm) -2 -1 1 2
Pixels
100
200
300
400
500
600
A = 2IS/IR - 1

13. 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.

14. Measured THz Field Movie

15. 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.
Time (ps)
Horizontal pos. (mm) -1 1 2
-0.5
0.5
Image asymmetry

16. 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

17. Focus Distribution of THz
Focus spot size 3 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)

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

19. 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.

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

21. Potential Ultrafast EO-Detection
Intense ultrafast THz source.  Modulated electron beam  High pulse energy CTR (C...R).
Broadband, uniform response EO-material.  EO-Polymer Composites.
Time domain laser pulse measurement.  Amplified fs-laser (injector drive laser).  Spectral phase measurement.  FROG, SPIDER.  Not limited by laser pulse length.

22. 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. -3
-1.5
1.5 3
-50
-25 25 50
Time (ps)
Energy (keV)
70 MeV
180 pC -3
-1.5
1.5 3
-20 20 D
E (keV)
200
Current (A)
Time (ps)

23. 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?
* 20% DCDHF-6-V/20% DCDHF-MOE-V/60% APC
A.M. Sinyukov, L.M. Hayden, to be published

24. 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.

25. 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.

26. 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. NMIPR