Phin activities at LNF

1. Giancarlo Gatti Phin activities at LNF

2. Outline SPARC project overview SPARC drive laser system IR shaper comparison for flat top laser pulse IR/UV-pulse shaping Laser to RF synchronization schemes Trasverse shaping considerations Conclusive remarks

3. : MiUR Strategic Research Programs Goals: High brightness linac to drive advanced FEL experiments And further experiments

4. SPARC commissioning phase 1: low energy e-beam characterization

5. Sparc phase 2 After gun commissioning in 2006: -Commissioning of downstream linac in progress. -Variable gap undulators installed and ready..... Last minute breaking news: First beam inside undulators

6. Photocathode drive laser Demands: Solutions: • Emittance minimization • Flexibility (Long./Trasv.) • Stable reliable operation • High current (100 A) from Cu cathode Synchronization rf laser < 1ps rms stable laser performance Uniform time and and transverse laser profile: 6-12 ps duration, rise time, 2 mm hard edge High energy laser (50-500 uJ) pulses at 266 nm

7. SPARC laser system: topology time shaper Ti:Sa laser composed by: 12 nm bandwidth oscillator IR pulse shaper CPA amplifiers third harmonic generator UV stretcher (used as shaper) Transport to the cathode • Proc. PAC 2007 TUPMN040

8. Pulse shaper pumps oscillator amplifiers UV stretcher Harmonics generator SPARC laser system • The laser delivers 5-12 ps, 100mJ pulses at 266 nm with a rep. rate of 10 Hz. • Energy jitter (5% rms), pointing stability (<50mm) and synchronization respect to the RF (<2ps rms) • Several subsystems have been integrated: IR pulse shaper, tranverse unifom pulse selection and imaging system to the cathode.

9. Laser temporal pulse shaping M. Petrarca S. Cialdi C. Vicario • Outline: • IR shaper comparison (a.o. Filter, LCM) • Overall system performances • UV shaper

10. Into amplifier From oscillator Telescopes Grating 15 cm Grating Half Half - - wave wave plate plate Dazzler Dazzler Lens Lens Phase mask Tests of two IR pulse shapers LC-SLM DAZZLER Into amplifier 60 cm From oscillator

11. IR programmable pulse shapers LC-SLM DAZZLER Acoustic grating Fast mode Slow mode Acousto-optic interaction in a TeO2 crystal

12. The UV time profiles the two shapers:DAZZLER LC-SLM DAZZLER LC-SLM Rise and fall time ~ 2.6 ps Rise and fall time ~ 2.1 ps The spectral shape after the UV stretcher is very similar to the temporal profile • Opt. Lett. 31, 19 (2006) 2885-2887

13. IR shapers Features: DAZZLER LC-SLM • Compact • Easy alignment • Simultaneously phase & amplitude modulation • Losses within 50% • Resolution = 0.3 nm • Slow optimization • Not-compact • Not easy alignment • Phase only modulation • Losses within 50% • Resolution<0.1 nm • Fast optimization

14. THG distortions: main limitation for fast rise time 0.1 0.5 1 The UV spectral shape as function of the input IR pulse length Measured (solid) and simulated (dots) 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 THG

15. UV pulse shaper The UV stretcher was designed to perform several tasks 1. Lengthen the laser pulse proportional to bandwith up to 20 ps. 2. In the Fourier plane an amplitude filter, such as an iris, can be applied to cut the tails of an almost square spectrum produced bu the DAZZLER or LC-SLM, the obtained spectrum profile is transferred into the time profile by the stretcher 3. A on-line spectrometer is integrated. 3 2 1 Appl. Opt. 46, 22 (2007) 4959

16. Picture of the UV shaper Spectrom CCD Filter plane Focusing lenses input output Grating pair

17. Simulation experiment Cross-correlated UV profile 1 FWHM 15 ps rise time 1.5 ps FWHM 10 ps: experim. vs simulation

18. Measured UV profile 2 for several pulse length UV shaper allows for fast rise time despite of different chirp factors in the stretcher

19. Shaping without the IR filter When a proper sharp cut is applied to the natural UV Gaussian laser spectrum, a flat top profile in time can be produced. Results are comparable to the two stages pulse shaping. Price to pay 20% higher energy losses Cheaper and simpler respect to the other IR pulse shapers The rise and fall time are reduced to ~1.5 ps (limited by the bandwidth)

20. No IR shapers: simulations IR shape No IR shape spectra Time shape

21. Experimental results No-cut Cut applied Spectra Time Measured (red) and Simulated (black) Appl. Opt. 46, 22 (2007) 4959-4962

22. Exotic applications: UV multipeaks generation With a grid in the fourier plane we obtained 4 peaks pulse (FEL microbunching enhancement)

23. Laser to RF synchronization M. Bellaveglia, S. Gallo, C. Vicario

24. Synchronize the laser and RF Laser to RF synchronization is needed to have photoinjector optimal and stable operation Photoelectron gun phase < 1 deg rms for emittance compensation Velocity bunching, pulse compression and laser acceleration demands for a tighter specification (100 fs) Pulse selection amplification Laser oscillator THG+ stretcher Cavity length control Measure Δf Laser to the cathode RF chain Master clock

25. Phase noise at oscillator level Measurements set up and results 350 fs rms

26. UV time jitter: measure at 10 Hz HV Photo diode Pill box cavity Laser mixer m-wave reference 0.67 ps rms over 6 hours Phase noise detection acquisition Active feedback RF phase shifter Time of arrival jitter estimated with the RF deflector is 390 fs

27. Toward next step: Laser-driven RF To reduce the time jitter we can synthesize the RF frequency from a photodiode excited by the oscillator pulses. The value measured can be affected by the apparatus resolution, shortly more detailed characterization This technique is applicable to lock for 1 laser system All-optical synchronization system and clock distribution to go at sub 100 fs level

28. Sparc P1 highlights Square laser at the cathode Opt. Lett. 31, (2006) 2885 Appl. Opt. 46, 22 (2007) 4959 REV. SC. INSTR. 77, 093301 2006 First ever emittance oscillation B=6*1013 A/m*rad PRL 99, 234801 (2007) PRST- AB 11, 032801 (2008)

29. E:beam experimental results current 100 A energy‏ 5.5 MeV 29 Duration, risetime 5, 1.5 ps rms spot size 0.45 mm RF phase +18° The flat top pulse shape allowed the observation of the double-minimum emittance evolution at SPARC (only predicted by the theory). UV LASER • PRL 99, 234801 (2007)

30. Gaussian vs flat beam:comparison 30

31. Recent advances: Trasverse shaping Main demands: -Squared beam -Power efficiency Final choice: Telescope system to map Gaussian into flat top Pros: above 70% efficiency (up to 95%) Cons: exact matching of TEM00 gaussian alignment stability filtering cuts real efficiency 2 kinds of commercial refractive Systems probed.

32. INPUT BEAM Our choice: No spatial filter OUTPUT BEAM

33. Trasportation up to cathode Shape is preserved through relay imaging trasport (10 mt.)

34. Conclusions Extensive development on laser pulse shaping has been done at LNF-SPARC within the PHIN collaboration The two stages pulse shaping for 1.5 ps rise time Demonstration of UV-only pulse shaping Work on faster rise time and reduced losses Sub ps synchronization with upgrade at < 200 fs has been demonstrated E-beam results encouraging the search of flat top pulse, definitive comparison at higher energy Complete shaping in the next future when charge issues will be overcome.