1. Laser pulse shaping for high- brightness photoinjector Carlo Vicario for SPARC collaboration

2. C.Vicario Care Meeting, LNF Nov 15 2006 2 Outlines The SPARC project. SPARC laser system: layout and performances Laser-to-RF synchronization measurements Time pulse shaping using the DAZZLER Conclusive remarks.

3. C.Vicario Care Meeting, LNF Nov 15 2006 3 The SPARC photoinjector Sparc is an R&D program conceived to produce high current (100 A)and low emittance e-beam (2mm-mrad). A 150 MeV photoinjector has been designed to drive a SASE-FEL. To minimize the non-linear space charge forces, and therefore the emittance, a square time profile from the photocathode drive laser is required.

4. C.Vicario Care Meeting, LNF Nov 15 2006 4 The SPARC Collaboration

5. C.Vicario Care Meeting, LNF Nov 15 2006 5 The SPARC photoinjector At LNF 14.5 m1.5m 20º 1.5 m 10.0 m 6.0 m RF sections Undulator GunSolenoids GUN PARAMETERS LINAC PARAMETERS FEL PARAMETERS Frequency:2856 MHz Frequency: 2856 MHz Wavelength: 530 nm Peak Field:120 MV/m Accelerating Field: 25-12.5-12.5 MV/m Coop. Length: 300 mm Solenoid Field:0.27 Tesla Solenoid Field: 0.1 Tesla Beam Energy:5.6 MeV Beam Energy: 155 MeV Charge:1.1 nC Laser:11.5 ps x 1 mm (Flat Top with <1 ps rise time) Therm. emitt.0.3 mm Machine parameters

6. C.Vicario Care Meeting, LNF Nov 15 2006 6 The SPARC Emittance Meter Rev.Sci.Instr. Vol.77, Issue 8 - 2006

7. C.Vicario Care Meeting, LNF Nov 15 2006 7 Reconstruction of the beam envelope The emittance-meter moves and stops in several position when the CCD collects several images and a program calculate the RMS parameters and the error bars

8. SPARC laser system

9. C.Vicario Care Meeting, LNF Nov 15 2006 9 Laser beam requirements Laser central wavelength266.7[nm] Laser pulse lenght FWHM2-12 [ps] Electron charge1 [nC] RMS energy jitter (UV)< 5% [rms] Laser pulse rise time 1 [ps] Laser pulse longitudinal ripples<30% ptp Transverse intensity profileTop hat Laser spot radius 1.1 (mm) RMS rf to laser time jitter< 2ps Centroid pointing stability 50 μm Spot ellipticity on cathode (1-a/b) <10%

10. C.Vicario Care Meeting, LNF Nov 15 2006 10 Ti:Sa CPA laser system and time pulse shaper Time and spectral diagnostics

11. C.Vicario Care Meeting, LNF Nov 15 2006 11 Sparc Laser System oscillator pumps amplifiers Harmonics generator UV stretcher Pulse shaper

12. C.Vicario Care Meeting, LNF Nov 15 2006 12 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). pulse duration130 fs Central wavelength800mn bandwidthup to 12 nm rep. rate79.3 MHz pulse’s energy10 nJ

13. C.Vicario Care Meeting, LNF Nov 15 2006 13 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. We tested the AOPDF and experiment with SLM is going to start. Dazzler Half-wave plate For more details see talk by Cialdi in phin parallel session

14. C.Vicario Care Meeting, LNF Nov 15 2006 14 Laser layout: CPA Rep. rate10 Hz spatial mode~Gaussian output pulse’s energy, power < 50 mJ, 0.5 TW IR amplitude jitter3%

15. C.Vicario Care Meeting, LNF Nov 15 2006 15 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 10% and the energy jitter is 5% rms Laser layout: THG IR BLUE UV λ /2 BBO1 BBO2 Filter

16. C.Vicario Care Meeting, LNF Nov 15 2006 16 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 lengths between 2 and 20 ps. Efficiency of the UV grating is about 65%, the overall energy losses are more than 80%

17. C.Vicario Care Meeting, LNF Nov 15 2006 17 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 FWHM)

18. C.Vicario Care Meeting, LNF Nov 15 2006 18 UV spectral-temporal measurements The UV spectrometer can be used as a single-shot time profile diagnostics. To produce a flat time profile a square-like spectrum is required When a large linear chirp α is applied, as in UV stretcher, the spectral profile at 266 nm gives a direct reconstruction of the intensity profile in time See talk by Petrarca in Phin parallel session

19. C.Vicario Care Meeting, LNF Nov 15 2006 19 Optical transfer line to the cathode The optical transfer line transports the laser beam to the cathode 10 m away. The laser impinge on the cathode from a mirror in vacuum at normal incidence The transverse profile is selected by an iris and then imaged on the cathode. Good pointing stability has been observed (~50 μm).

20. Laser to RF phase noise measurements

21. C.Vicario Care Meeting, LNF Nov 15 2006 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. C.Vicario Care Meeting, LNF Nov 15 2006 22 Laser to RF phase-noise measurements

23. C.Vicario Care Meeting, LNF Nov 15 2006 23 Phase noise at oscillator level Statistics on the laser to RF Relative phase FFT of the relative phase Stdev=0.34 ps

24. C.Vicario Care Meeting, LNF Nov 15 2006 24 RF to Laser synchronization: measurements on 10 Hz UV pulses 2856 MHz cavity High energy UV @ 10 Hz On time scale of 30 minutes the phase jitter is within σ RMS = 0.47 ps. Investigation of the causes of the slow drift (temperature?) and active RF phase shift compensation.

25. Longitudinal pulse shaping: experience using DAZZLER

26. C.Vicario Care Meeting, LNF Nov 15 2006 26 Dazzler experience The dazzler was studied as a stand-alone system at politecnico in Milan. The shaped profile was imposed by producing a square spectrum and add even terms polynomial phase. The distortion introduced by the amplification and the THG has been investigated in collaboration with LCLS and SDL at Brookhaven Nat. Lab. Time distribution at oscillator level C. Vicario et al, EPAC04 Time distribution after the UV conversion H. Loos et al, PAC05

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

28. C.Vicario Care Meeting, LNF Nov 15 2006 28 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.

29. C.Vicario Care Meeting, LNF Nov 15 2006 29 Modified UV stretcher to obtain sharper rise time

30. C.Vicario Care Meeting, LNF Nov 15 2006 30 Preliminary measurements: UV time and spectral intensity UV cross-correlation with 0.5 ps IR probe UV spectrum converted in time (blue) Calculated cross-correlation between the measured IR pulse length and the UV (red) The rise time is 1.5 ps

31. C.Vicario Care Meeting, LNF Nov 15 2006 31 Modified stretcher: considerations The spectral measurements indicate rise time of less than 1.5 ps can be obtained. New diagnostics is required to measure such feature directly in time. From simulations, assuming the actual UV bandwidth (1.2 nm) rise time of 1.2 ps is the best result achievable. The energy losses due to the filtering is about 20%. To mitigate distortions and aberrations on the transverse laser profile longer focal lengths is advisable.

32. C.Vicario Care Meeting, LNF Nov 15 2006 32 Conclusive remarks Synchronization level is satisfying but feedback to compensate the long term drift should be implemented. Uniform transverse laser intensity is critical for e- beam quality. Pulse shaping researchs is still facing the rise time problem. The modified UV stretcher can be used to produce sharper pulse edges. Systematic measurements on the e-beam generated using the flat top laser profile are going on.

33. C.Vicario Care Meeting, LNF Nov 15 2006 33 Care publications 2006 Published Articles High-power third-harmonic flat pulse laser generation, S. Cialdi, M. Petrarca, C. Vicario, Opt. Lett.,31, 2885 (2006) and Virt. J. of Ultrafast Scie. (2006). Rectangular pulse formation in a laser harmonic generation, S.Cialdi, F. Castelli, I. Boscolo, Appl. Phys. B 82, 3 (2006) 383- 389 A train of micro-bunches for PWFA experiments produced by RF photoinjectors,. M. Boscolo, M. Ferrario, C. Vaccarezza, I. Boscolo, F. Castelli, S. Cialdi. Int. J. Mod. Phys. B (2006)

34. C.Vicario Care Meeting, LNF Nov 15 2006 34 Proceedings and reports 1.M. Boscolo, M. Ferrario, C. Vaccarezza, I. Boscolo, F. Castelli, S. Cialdi, “Laser comb: simulations of pre-modulated e beams at the photocathode of a high brightness rf photoinjector, Edimburgh, EPAC 2006 2.M. Petrarca, P. Musumeci, M. C. Mattioli, C. Vicario, G. Gatti, A. Ghigo, Production of Temporally fla-top UV laser pulses for SPARC photoinjector, Proc. of EPAC 2006, Edinburgh, Scotland, THPCH153 3.C. Vicario, M. Bellaveglia, D. Filippetto, A. Gallo, G. Gatti, A. Ghigo, P. Musumeci, M. Petrarca, Commissioning of the laser system for SPARC photoinjector Proc. of EPAC 2006, Edinburgh, Scotland, THPCH151 Physics degree thesis –Compressione di un impulso laser Nd:YAG con fibra in un sistema 4f- asimmetrico, Valeria Brizzolara, 27/Ott/2006 Care publications 2006