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THE DRIVE LASER: EXPERIENCE AT SPARC

THE DRIVE LASER: EXPERIENCE AT SPARC. Carlo Vicario for SPARC collaboration. Summary. SPARC laser system: layout and performances Laser-to-gun optical transfer line: grazing vs normal incidence Laser-to-RF synchronization measurements Longitudinal pulse shaping: experience using DAZZLER

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THE DRIVE LASER: EXPERIENCE AT SPARC

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  1. THE DRIVE LASER: EXPERIENCE AT SPARC Carlo Vicario for SPARC collaboration

  2. Summary • SPARC laser system: layout and performances • Laser-to-gun optical transfer line: grazing vs normal incidence • Laser-to-RF synchronization measurements • Longitudinal pulse shaping: experience using DAZZLER • Emissive properties of the photocathode

  3. SPARC laser: layout and system’s performances

  4. SPARC Laser beam requirements

  5. Ti:Sa CPA laser system by Coherent + pulse shaper

  6. pumps oscillator amplifiers UV stretcher Harmonics generator Coherent Laser System

  7. Laser layout: oscillator Ti:Sa CW oscillator (Mira) is pumped by 5 W green laser (Verdi). The oscillator head can be locked to and external master clock (synchrolock).

  8. Half-wave plate Dazzler Laser layout: time pulse shaper To obtain the desired square profile a manipulation of the spectral phase and/or amplitude has to be applied. The most popular techniques are the AODPF and the SLM in 4f configuration. They work at low energy level. New UV Dazzler S. Coudreu Opt. Lett. 31, (2006), 1899

  9. Laser layout: CPA

  10. BLUE Filter IR UV BBO1 BBO2 λ/2 Laser layout: THG The third harmonic generator consists of by two type-I BBO crystals, of 0.5 and 0.3 mm thickness. The overall efficiency is about 8% and the energy jitter is 5% rms In the THG the optics can be damaged by the IR high peak power (self focusing effects).

  11. Laser layout: UV stretcher The UV stretcher consists of a pair of parallel gratings. It introduces a negative GVD proportional to d, and allows output pulse length between 2 and 20 ps. Efficiency of the grating is about 65%, the overall energy losses are more than 80%

  12. 30 cm lens CCD 4350 g/mm grating UV beam Laser system layout: spectral and time diagnostics • Diagnostics routinely used to monitor time/spectral features of SPARC laser : • Ir+ blue commercial spectrometers resolution > 0.3 mn • ps resolution streak camera • UV home-built spectrometer with 0.05 nm resolution 10 mn bandwidth • UV home-built multi-shot cross-correlator resolution (IR pulse FWFM)

  13. UV spectral-temporal measurements When a large linear chirp α is applied, as in our case, the spectral profile brings to a direct reconstruction of the intensity temporal profile The UV spectrometer as single-shot time profile diagnostics.

  14. Optical transfer line I • The optical transfer line transports the laser beam to the cathode 10 m away. • The transverse profile is selected by an iris and then imaged on the cathode. • The energy losses are mainly introduced by the grating used to compensate the grazing incidencedistortions. • Good pointing stability has been observed (~50 μm). IRIS laser

  15. Laser grazing incidence The laser beam is injected onto the cathode surface at grazing incidence angle (72°) Beam exit Photocathode • Advantages: • No mirror close to the beam axis for normal incidence (no wakefield) • Higher QE • Disadvantages: • A circular beam becomes an ellipse on the cathode • Time slew: the side closer to the laser entry emits earlier than the other side

  16. H posirion mm Compensation scheme A grating with a proper g/mm can be employed to diffract the beam at 72° and be positioned parallel to the cathode. A lens is needed to counterbalance the chromatic dispersion at the image plane. Advantages: • Circular beam at cathode (>98%) • Front tilt compensation(< 200 fs) • Work for different spot sizes. • Drawbacks: • High energy losses: 65% • Sensitive to lens position (±1 mm) • Difficult to be measured • Structures in the spot Simulated spot and front at cathode C. Vicario et al, EPAC06

  17. Normal incidence setup • We change the TL normal incidence to get benefits in term of energy budget and spot uniformity. • With this geometry the cathode’s QE is half respect to the grazing incidence case.

  18. Transverse profile at the virtual cathode • Transverse spot features • Sharp edges • High spatial frequencies • The beam transverse profile strongly influenced the e-beam brightness • Refractive beam shaper + spatial filtering is going to be implemented

  19. System critical performances • Reliability • Laser failures (mainly electronics breaks) cover 20% time • Damages on optics especially in THGis not improbable • Laser spot • Flash lamp pump non-homogeneities worsen the Ti:SA mode • Laser drifts due to the temperature • The energy decay with time observed is due divergence changing of the flash pumped Nd:YAG.

  20. Laser to RF phase noise measurements

  21. Motivations Laser phase stability is mandatory for stable machine operation. For SPARC phase 1 is requires < 2ps rms, other application demands for more challenging level of synchronization. Coherent Synchrolock

  22. Laser to RF phase-noise measurements

  23. Phase noise at oscillator level Statistics on the laser relative phase Stdev=0.35 deg FFT of the relative phase

  24. High energy UV @ 10 Hz 2856 MHz cavity RF to Laser synchronization: measurements on 10 Hz UV pulses On time scale of few minutes the phase jitter is within σRMS=0.48 RF deg. Investigation of the causes of the slow drift (temperature?) and active RF phase shift compensation.

  25. Longitudinal pulse shaping: experience using DAZZLER

  26. Dazzler experience I: experiment at Politecnico in Milan Input spectrum The dazzler was studied as a stand-alone system. The time profile was measured with a SH cross-correlator. The shaped profile was imposed by producing a square spectrum and add even terms polynomial phase. Phase applied Amplitude filter Two passed in the AO crystal Single pass in the AO crystal + 60 cm SF56 efficiency 0.5 efficiency 0.25 C. Vicario et al, EPAC04

  27. Dazzler experience at SDL-BNL • The experiment was in the framework of a INFN/LCLS/SDL-BNL collaboration. • The motivations were: • Study the effects of CPA on the shaped pulse • red shift, saturation effect, gain function of wavelength • Study the effects of shaped pulse on the CPA • Quantify the distortion introduced by the Harmonic conversion • Eventually e-beam characterization

  28. 130 fs, 6-8 nm bandwidth CPA 15 mJ 10 - 20 psec w-tripler 266 nm 0.1 mJ Dazzler Ti:Sa Oscillator Dazzler experience at SDL-BNL Reduction of the e-beam transverse emittance could be observed due to this shaping of the laser. H. Loos et al, PAC05

  29. 0.1 0.5 1 DAZZLER experience at SPARC: short amplified IR pulse The UV spectral shape as function of the input IR pulse length Measured (solid) and simulated (dashed) harmonics spectra IR pulse length [ps] C. Vicario et al, Opt. Lett, 31,2006, 2885 A large enough pulse width (≥0.6 ps) is needed to preserve the square spectrum throughout the third harmonic generation

  30. Equations for SH with vs linear chirp Equation for the SH generation for the complex fields Ai,j In the frequency domain we can integrate A2 and obtain the output intensity Hp: Phase matching, not depletion regime and negligible velocity dispersion The output spectrum is the convolution product Similar consideration can be extended to the THG

  31. -5.8 0 5.8 Effects of non-linear crystal tilt If the non-linear crystal is tilted by an angle θfrom the phase matching condition, the output spectra are distorted Simulated and measured SH spectra vs the tilt of the crystal. SH crystal tilt θ [mrad] The crystal tilt act as a frequency shift and therefore it introduces an asymmetry in the output spectrum.

  32. The UV temporal and spectral profile • Using a chirped IR pulse (with 0.5 ps duration) and a square-like infrared spectral intensity we obtained a square-like UV shape. • The measured UV rise time appears to be too long, 2.5-3 ps.

  33. Simulated UV intensity profile • Ingredients to achieve this profile: • 1 Perfect square IR spectrum 12 nm • Limitation form Dazzler resolution • and amplifier distortions • 2 Long IR input 10 (ps) • Harmonics efficiency prop I(t) • 3 140 um thick SHG crystal instead of 500um • 40 um thick THG crystal instead of 300 um • Harmonic efficiency prop. L2 • 4 Perfect alignment and time overlap 1 ps rise time We can obtain more sharp edges clipping the spectrum tails where it is spatially dispersed!

  34. Modified UV stretcher to obtain sharper rise time M. Danailov et al, FEL06

  35. Preliminary measuremnts: time and spectral intensity UV cross-correlation UV spectrum converted in time (blue) Calculated cross-correlation between the measured IR pulse length and the UV (red)

  36. Modified stretcher: considerations • The spectral measurements indicate rise time less than 1 ps can be obtained. New diagnostics is required to measure such feature directly in time. • The energy losses due to the filtering is about 20%. • The alignment is quite long and tedious. • Distortions of the transverse profile and aberrations have been observed. Investigations are going on.

  37. Cathode laser cleaning Laser cleaning of the single crystal copper cathode was operated moving the laser across the surface step 100 μm. The optical energy was 10 μJ focused over 100 μm diameter, at 72° deg incidence. The cleaning ware performed in presence the moderate field 40 MV/m. Improvement in term of beam brightness due to more transversely uniform e-beam. QE map before and after laser cleaning at low field Vacuum during the cleaning

  38. Conclusive remarks • SPARC laser performances are satisfying but the system requires constant maintenance • Critical points: flash-pumped Nd:YAG, high peak power • Normal incidence is advisable in particular for large bandwidth lasers • Synchronization level can be improved • Uniform transverse laser intensity and constant QE is critical for e-beam quality • Pulse shaping research is still facing the rise time problem. Balance between uniform transverse profile and flat top pulse in time and is still an open issue • Cathode laser cleaning proved to be reliable technique

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