1. Electro-Optic Bunch Profile Monitors DA Walsh, SP Jamison, WA Gillespie, MA Tyrk, R Pan, T Lefevre

2. Electro Optic Encoding Coulomb field of relativistic bunch probe laser non-linear crystal Standard Description Coulomb field flattens,and represents charge distribution Pockels effect induces polarization ellipticity Technique borrowed from THz electo-optic sampling where (t probe << t THz ) Spectral Decoding o Chirped optical input o Spectral readout o Use time-wavelength relationship ~1ps Temporal Decoding o Long pulse + ultrashort pulse gate o Spatial readout (cross-correlator crystal) o Use time-space relationship ~150fs

3. Benefits of EO techniques Electro optic techniques Scale well with high beam energy – Particle methods get impractical (size, beam dumps) Non-destructive – Bunches can still be used – Live feedback CLIC targets Bunches will be ~150fs rms Diagnostic target ~20fs rms We aim to improve on the resolution and robustness of EO diagnostics

4. Physics of EO encoding More Rigorous Description – nonlinear frequency mixing Coulomb spectrum shifted to optical region Coulomb pulse temporally replicated in optical pulse envelopeoptical field S.P. Jamison Opt. Lett. v31 no.11 p1753 This is not true for short bunches! Standard Description Pockels effect induces polarization ellipticity which is detected as leakage through crossed polarisers Theory borrowed from THz electo-optic sampling where (t probe << t THz ) For short pulses extra frequencies are generated, confusing the retrieval processes

5.  (2) (  thz,  opt )  opt +  coulomb Generation of Optical Sidebands  coulomb  opt  opt -  coulomb  opt EO crystal Sum over all values of  coulomb Intensity ν few mm tens μm λt 800nm Coulomb field Optical field ~50fs circa 20nm Direct measurement Direct measurements of  coulomb (CTR) tricky – bandwidth of many octaves! A rather complex system is being developed to perform this single shot – difficult to calibrate Long wavelength components don’t propagate Spectrum is related to the time profile via a Fourier transform this is already a potentially useful diagnostic! Optical up-conversion measurement Relative bandwidth has shrunk (few %) Long wavelength components shifted to the optical and easy to propagate Single shot spectrum easily obtained Consider single frequency probe and coulomb fields Consider a single frequency probe and short coulomb field “pulse”

6. Development and Benchmarking Δν <50GHz (Δ t >9ps) Femtosecond laser based test bed Femtosecond laser pulse spectrally filtered to produce narrow bandwidth probe Auston switch THz source mimics coulomb field. Field strengths up to 1 MV/cm. Has a well characterised spectral and temporal profile. Investigation of measurement thresholds / signal to noise ratios Required to define system requirements

7. Early test results Testing has revealed complications re. beam crossing-angle tolerance. May have implications for all electro optic sampling systems! Thorough investigation is underway. 1.5mm 150μm System installed at Daresbury to attempt single shot CTR spectrum measurement Successful observation of sidebandsTHz pulse measured via EO sampling ZnTe Probe Sum Freq. THz Diff Freq. Detection

8. Complete Characterisation Problem: Unknown phase Retrieval of temporal profile via spectrum alone still requires assumptions regarding the spectral phase. Many pulse profiles fit a given spectrum. Solution: Frequency Resolved Optical Gating (FROG) This is a standard and robust method for characterising laser pulses. Unambiguously retrieves spectral intensity and spectral phase from the spectrogram SpectrumSpectral Phase Need to know: Baltuska, Pshenichnikov, and Weirsma, J. Quant. Electron., 35, 459 (1999). Considerations: Self-gating avoids timing issues (no need for a fs laser) Single shot measurement possible but requires minimum pulse energy of > 10 nJ 4.5 fs pulse

9. Non-collinear Chirped Pulse Amplification Problem: Up-conversion is relatively weak – our calculations suggest energies of a few nJ. Signal needs amplifying without loss of information. Solution: Non-collinear Chirped Pulse Amplification (NCPA) ~800nm femtosecond signal Stretcher Compressor Stretching factor 10 3 or more to prevent saturation, damage, NL effects Amplified pulse then recompressed BBO Routinely used to produce “single-cycle” optical pulses Amplification with robust nanosecond pulse lasers High gains of 10 7 or more Gain bandwidths >100nm (50THz) Preservation of phase of pulse is possible Few ns, ~10mJ pump pulse @ 532nm Beams ~1.5mm diameter Gain >1000x (~300MW/cm 2 )

10. Envisaged Spectral Up-conversion Characterisation System (2) Amplification Stretcher Compressor Single Shot FROG NL crystal (4) Calculate properties at NL crystal (to remove remaining spectral amplitude and any residual phase distortion) (1) up-convert Coulomb field 50ps 60mJ 1064nm Nd:YAG (doubled) Spectrally filtered Ti:Sapphire THz Source (Spectral intensity and phase distortions can be both modelled and measured)

11. (4) Calculate properties at NL crystal (to remove remaining spectral amplitude and any residual phase distortion) Envisaged Spectral Up-conversion Characterisation System (2) Amplification Stretcher Compressor Single Shot FROG NL crystal (1) up-convert Coulomb field Commercial nanosecond Nd Laser Integrated frequency conversion (OPO) In beam pipe

12. Way Forward & Challenges Continue test program: Characterising the amplification stage in the next few months Validate understanding of amplification and stretching/compression This will allow us to calculate required nanosecond laser parameters Design, build and validate appropriate single-shot FROG apparatus (or buy off-the-shelf if pulse energy is sufficient) In order to exceed bandwidths of ~8THz material issues need to be addressed: Common EO materials have phonon absorptions that distort/absorb signal Investigate possibility of compositing pulse data from different materials Speculatively consider novel materials: poled polymers (robust?) meta materials (possible?)

13. Investigation of thin film ‘meta- materials’ (silver nanoparticles embedded in glass matrix) for novel EO bunch profile systems Experimental characterisation of those materials as novel EO detectors Observed dichroic effect resulting in separation of Surface Plasmon Resonance (SPR) band in first stages of experiments Second-harmonic generation to be investigated in next few months Talisker picosecond laser system Materials and Photonic Systems

14. CTF3 Spectral Decoding EO Diagnostic P: Polarizer H: Half wave plate Q: Quarter wave plate : Mirror with actuators : Finger camera Grating Laser: Wavelength: 780 nm Duration: 100 fs Repetition: 37.4815 MHz Pulse energy: 4 nJ Crystal: Thickness: 1mm Separation: 5-10 mm System Schematics Chamber 1 Chamber 2 1.Planned EO monitor simulated and resolution calculated 2.Overall EO system for CTF3 designed 3.Laser laboratory is functioning 4.Laser system designed and functioning 5.All cables and optical fibres installed 6.All motors/actuators are installed and tested in the machine 7.Optical synchronization system designed 8.Transfer lines for laser and OTR photons installed 9.Finger cameras are installed. 10. Two vacuum chambers are installed 11. Optical lines are being aligned. Califes requirements relaxed compared to CLIC – only need 1ps resolution Spectral decoding provides simple and robust diagnostic system Implementation of frequency doubled, amplified fibre laser to improve reliability

15. Laser ↔ e-bunch synchronization 74.9 MHz 37.5 MHz Non-standard design, providing higher pulse energy Laser parameters Wavelength: 780 nm & 1560 nm Duration: <120 fs Repetition: 37.5 MHz @ 780 nm Pulse energy: 4 nJ @ 780 nm Laser head and controller Synchronization box PD in RF 1 RF 2 Phase out PID in Piezo control Motor control Laser and synchronization system --Streak camera measuring the arriving time of laser and beam --Phase shifter control laser arriving time for scanning

16. Summary We have conceived of a novel, high resolution Electro- Optic bunch length diagnostic Mechanisms to test and characterise this system are being put in place – results in next few months Ways to overcome material limitations are being investigated Spectral decoding at CALIFES to be commissioned within the next two months