C. Vicario LCLS ICW SLAC Oct. 9-11, THE DRIVE LASER: EXPERIENCE AT SPARC Carlo Vicario for SPARC collaboration

C. Vicario LCLS ICW SLAC Oct. 9-11, 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

C. Vicario LCLS ICW SLAC Oct. 9-11, SPARC laser: layout and system’s performances

C. Vicario LCLS ICW SLAC Oct. 9-11, SPARC 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%

C. Vicario LCLS ICW SLAC Oct. 9-11, Ti:Sa CPA laser system by Coherent + pulse shaper

C. Vicario LCLS ICW SLAC Oct. 9-11, Coherent Laser System oscillator pumps amplifiers Harmonics generator UV stretcher

C. Vicario LCLS ICW SLAC Oct. 9-11, 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 duration130 fs Central wavelength800mn bandwidthup to 12 nm rep. rate79.3 MHz pulse’s energy10 nJ

C. Vicario LCLS ICW SLAC Oct. 9-11, 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. Dazzler Half-wave plate New UV Dazzler S. Coudreu Opt. Lett. 31, (2006), 1899

C. Vicario LCLS ICW SLAC Oct. 9-11, Laser layout: CPA laser pump 11KHz, 7 W, 100 ns laser pump210 Hz, 560 mJ, 7 ns rep. rate10 Hz spatial mode~Gaussian output pulse’s energy, power < 50 mJ, 0.5 TW IR amplitude jitter3%

C. Vicario LCLS ICW SLAC Oct. 9-11, 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). Laser layout: THG IR BLUE UV λ /2 BBO1 BBO2 Filter

C. Vicario LCLS ICW SLAC Oct. 9-11, 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%

C. Vicario LCLS ICW SLAC Oct. 9-11, Laser system layout: spectral and time diagnostics 30 cm lens 4350 g/mm grating CCD UV beam 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)

C. Vicario LCLS ICW SLAC Oct. 9-11, UV spectral-temporal measurements The UV spectrometer as single-shot time profile diagnostics. 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

C. Vicario LCLS ICW SLAC Oct. 9-11, 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 incidence distortions. Good pointing stability has been observed (~50 μm). laser IRIS

C. Vicario LCLS ICW SLAC Oct. 9-11, Laser grazing incidence Photocathode Beam exit The laser beam is injected onto the cathode surface at grazing incidence angle (72°) Advantages: 1.No mirror close to the beam axis for normal incidence (no wakefield) 2.Higher QE Disadvantages: 1.A circular beam becomes an ellipse on the cathode 2.Time slew: the side closer to the laser entry emits earlier than the other side

C. Vicario LCLS ICW SLAC Oct. 9-11, Compensation scheme Advantages:  Circular beam at cathode (>98%)  Front tilt compensation (< 200 fs)  Work for different spot sizes. H posirion mm Simulated spot and front at cathode C. Vicario et al, EPAC06 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. Drawbacks:  High energy losses: 65%  Sensitive to lens position (±1 mm)  Difficult to be measured  Structures in the spot

C. Vicario LCLS ICW SLAC Oct. 9-11, 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.

C. Vicario LCLS ICW SLAC Oct. 9-11, 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

C. Vicario LCLS ICW SLAC Oct. 9-11, System critical performances Reliability –Laser failures (mainly electronics breaks) cover 20% time –Damages on optics especially in THG is 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.

C. Vicario LCLS ICW SLAC Oct. 9-11, Laser to RF phase noise measurements

C. Vicario LCLS ICW SLAC Oct. 9-11, 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

C. Vicario LCLS ICW SLAC Oct. 9-11, Laser to RF phase-noise measurements

C. Vicario LCLS ICW SLAC Oct. 9-11, Phase noise at oscillator level Statistics on the laser relative phase FFT of the relative phase Stdev=0.35 deg

C. Vicario LCLS ICW SLAC Oct. 9-11, RF to Laser synchronization: measurements on 10 Hz UV pulses 2856 MHz cavity High energy 10 Hz On time scale of few minutes the phase jitter is within σ RMS = 0.48 RF deg. Investigation of the causes of the slow drift (temperature?) and active RF phase shift compensation.

C. Vicario LCLS ICW SLAC Oct. 9-11, Longitudinal pulse shaping: experience using DAZZLER

C. Vicario LCLS ICW SLAC Oct. 9-11, Dazzler experience I: experiment at Politecnico in Milan 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. Single pass in the AO crystal + 60 cm SF56 Two passed in the AO crystal C. Vicario et al, EPAC04 efficiency 0.5 Input spectrum Phase applied Amplitude filter efficiency 0.25

C. Vicario LCLS ICW SLAC Oct. 9-11, 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 Dazzler experience at SDL-BNL

C. Vicario LCLS ICW SLAC Oct. 9-11, Dazzler experience at SDL-BNL H. Loos et al, PAC05 Dazzler CPA  tripler Ti:Sa Oscillator 15 mJ psec 266 nm 0.1 mJ 130 fs, 6-8 nm bandwidth Reduction of the e-beam transverse emittance could be observed due to this shaping of the laser.

C. Vicario LCLS ICW SLAC Oct. 9-11, DAZZLER experience at SPARC: short amplified IR pulse A large enough pulse width (≥0.6 ps) is needed to preserve the square spectrum throughout the third harmonic generation IR pulse length [ps] Measured (solid) and simulated (dashed) harmonics spectra C. Vicario et al, Opt. Lett, 31,2006, 2885 The UV spectral shape as function of the input IR pulse length

C. Vicario LCLS ICW SLAC Oct. 9-11, Equations for SH with vs linear chirp In the frequency domain we can integrate A 2 and obtain the output intensity Hp: Phase matching, not depletion regime and negligible velocity dispersion Equation for the SH generation for the complex fields A i,j The output spectrum is the convolution product Similar consideration can be extended to the THG

C. Vicario LCLS ICW SLAC Oct. 9-11, Effects of non-linear crystal tilt SH crystal tilt θ [mrad] If the non-linear crystal is tilted by an angle θ from the phase matching condition, the output spectra are distorted The crystal tilt act as a frequency shift and therefore it introduces an asymmetry in the output spectrum. Simulated and measured SH spectra vs the tilt of the crystal.

C. Vicario LCLS ICW SLAC Oct. 9-11, 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, ps.

C. Vicario LCLS ICW SLAC Oct. 9-11, Simulated UV intensity profile Ingredients to achieve this profile: 1Perfect square IR spectrum 12 nm Limitation form Dazzler resolution and amplifier distortions 2Long IR input 10 (ps) Harmonics efficiency prop I(t) um thick SHG crystal instead of 500um 40 um thick THG crystal instead of 300 um Harmonic efficiency prop. L 2 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!

C. Vicario LCLS ICW SLAC Oct. 9-11, Modified UV stretcher to obtain sharper rise time M. Danailov et al, FEL06

C. Vicario LCLS ICW SLAC Oct. 9-11, 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)

C. Vicario LCLS ICW SLAC Oct. 9-11, 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.

C. Vicario LCLS ICW SLAC Oct. 9-11, 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

C. Vicario LCLS ICW SLAC Oct. 9-11, 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