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

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

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 µJ @266 nm 3rd harm. 1 µJ @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

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.

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

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

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.

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

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.

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

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

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

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.

Measured THz Field Movie

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

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

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)

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

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.

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

Potential Ultrafast EO-Detection Intense ultrafast THz source.  Modulated electron beam (@DUV-FEL).  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.

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)

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

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.

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.

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.