Laser pulse shaping for highbrightness photoinjector Carlo Vicario

Laser pulse shaping for highbrightness photoinjector Carlo Vicario for SPARC collaboration C. Vicario Care Meeting, LNF Nov 15 2006

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

The SPARC photoinjector • Sparc is an R&D program conceived to produce high current (100 A)and low emittance e-beam (2 mm-mrad). • A 150 Me. V 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. C. Vicario Care Meeting, LNF Nov 15 2006 3

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

The SPARC photoinjector At LNF Gun Solenoids RF sections 1. 5 m Undulator 20º 1. 5 m 10. 0 m 6. 0 m 14. 5 m Machine parameters GUN PARAMETERS LINAC PARAMETERS Frequency: 2856 MHz Peak Field: 120 MV/m Accelerating Field: 25 -12. 5 MV/m Solenoid Field: 0. 27 Tesla Solenoid Field: 0. 1 Tesla Beam Energy: 5. 6 Me. V Beam Energy: 155 Me. V Charge: 1. 1 n. C Laser: 11. 5 ps x 1 mm (Flat Top with <1 ps rise time) Therm. emitt. 0. 3 mm C. Vicario Frequency: 2856 MHz Care Meeting, LNF Nov 15 2006 FEL PARAMETERS Wavelength: 530 nm Coop. Length: 300 mm 5

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

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 C. Vicario Care Meeting, LNF Nov 15 2006 7

SPARC laser system C. Vicario Care Meeting, LNF Nov 15 2006

Laser beam requirements C. Vicario Laser central wavelength 266. 7[nm] Laser pulse lenght FWHM 2 -12 [ps] Electron charge 1 [n. C] RMS energy jitter (UV) < 5% [rms] Laser pulse rise time 1 [ps] Laser pulse longitudinal ripples <30% ptp Transverse intensity profile Top hat Laser spot radius 1. 1 (mm) RMS rf to laser time jitter < 2 ps Centroid pointing stability 50 μm Spot ellipticity on cathode (1 -a/b) <10% Care Meeting, LNF Nov 15 2006 9

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

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

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 duration 130 fs Central wavelength 800 mn bandwidth up to 12 nm rep. rate 79. 3 MHz pulse’s energy 10 n. J C. Vicario Care Meeting, LNF Nov 15 2006 12

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

Laser layout: CPA C. Vicario Rep. rate 10 Hz spatial mode ~Gaussian output pulse’s energy, power < 50 m. J, 0. 5 TW IR amplitude jitter 3% Care Meeting, LNF Nov 15 2006 14

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 10% and the energy jitter is 5% rms BLUE Filter IR UV BBO 1 C. Vicario λ/2 BBO 2 Care Meeting, LNF Nov 15 2006 15

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% C. Vicario Care Meeting, LNF Nov 15 2006 16

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

UV spectral-temporal measurements 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 • 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 C. Vicario Care Meeting, LNF Nov 15 2006 18

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). C. Vicario Care Meeting, LNF Nov 15 2006 19

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

Motivations Laser phase stability is mandatory for stable machine operation. For SPARC phase 1 is requires < 2 ps rms, other application demands for more challenging level of synchronization. Coherent Synchrolock C. Vicario Care Meeting, LNF Nov 15 2006 21

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

Phase noise at oscillator level Statistics on the laser to RF Relative phase Stdev=0. 34 ps FFT of the relative phase C. Vicario Care Meeting, LNF Nov 15 2006 23

RF to Laser synchronization: measurements on 10 Hz UV pulses High energy UV @ 10 Hz 2856 MHz cavity On time scale of 30 minutes the phase jitter is within σRMS=0. 47 ps. Investigation of the causes of the slow drift (temperature? ) and active RF phase shift compensation. C. Vicario Care Meeting, LNF Nov 15 2006 24

Longitudinal pulse shaping: experience using DAZZLER C. Vicario Care Meeting, LNF Nov 15 2006

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 Time distribution after the UV conversion C. Vicario et al, EPAC 04 C. Vicario H. Loos et al, PAC 05 Care Meeting, LNF Nov 15 2006 26

DAZZLER experience at SPARC: amplified IR short pulse The UV spectral shape as function of the input IR pulse length 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 A large enough pulse width (≥ 0. 6 ps) is needed to preserve the square spectrum throughout the third harmonic generation C. Vicario Care Meeting, LNF Nov 15 2006

The UV temporal and spectral profile • Using a chirped IR pulse (with 0. 5 ps duration) and a squarelike infrared spectral intensity we obtained a square-like UV shape. • The measured UV rise time appears to be too long, 2. 5 -3 ps. C. Vicario Care Meeting, LNF Nov 15 2006 28

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

Preliminary measurements: UV time and spectral intensity UV cross-correlation with 0. 5 ps IR probe The rise time is 1. 5 ps C. Vicario UV spectrum converted in time (blue) Calculated cross-correlation between the measured IR pulse length and the UV (red) Care Meeting, LNF Nov 15 2006 30

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. C. Vicario Care Meeting, LNF Nov 15 2006 31

Conclusive remarks • Synchronization level is satisfying but feedback to compensate the long term drift should be implemented. • Uniform transverse laser intensity is critical for ebeam 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. C. Vicario Care Meeting, LNF Nov 15 2006 32

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) 383389 • 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) C. Vicario Care Meeting, LNF Nov 15 2006 33

Care publications 2006 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, THPCH 153 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, THPCH 151 Physics degree thesis – Compressione di un impulso laser Nd: YAG con fibra in un sistema 4 fasimmetrico, Valeria Brizzolara, 27/Ott/2006 C. Vicario Care Meeting, LNF Nov 15 2006 34