laser pulse shaping for high- brightness photoinjector carlo vicario for sparc collaboration

Laser pulse shaping for high-brightness photoinjector

Carlo Vicario

for

SPARC collaboration

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

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

The SPARC Collaboration


5

The SPARC photoinjector At LNF

14.5 m1.5m

20º

1.5 m

10.0 m 6.0 m

RF sections

Undulator

Gun Solenoids

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


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The SPARC Emittance Meter

Rev.Sci.Instr. Vol.77, Issue 8 - 2006


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Reconstruction of the beam envelope

The emittance-meter moves and stops in several position when the CCD The emittance-meter moves and stops in several position when the CCD collects several images and a program calculate the RMS parameters and collects several images and a program calculate the RMS parameters and the error barsthe error bars

SPARC laser system


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Laser beam requirements

Laser central wavelength 266.7[nm]

Laser pulse lenght FWHM 2-12 [ps]

Electron charge 1 [nC]

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 < 2ps

Centroid pointing stability 50 μm

Spot ellipticity on cathode (1-a/b) <10%


10

Ti:Sa CPA laser system and time pulse shaper

Time and spectraldiagnostics


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Sparc Laser System

oscillatorpumps

amplifiers

Harmonics generator

UV stretcher

Pulse shaper


12

Laser layout: oscillatorTi: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 800mn

bandwidth up to 12 nm

rep. rate 79.3 MHz

pulse’s energy 10 nJ


13

Laser layout: time pulse shaperTo obtain the desired square profile a manipulation of the spectral phase and/or amplitude has to be applied. The most popular techniques arethe AODPF and the SLM in 4f configuration. We tested the AOPDF and experiment with SLM is going to start.

Dazzler

Half-wave plateGratingGrating

LensLens

f f f f

Mask

GratingGrating

LensLens

f f f f

Mask

For more details see talk by Cialdi in phin parallel session


14

Laser layout: CPARep. rate 10 Hz

spatial mode ~Gaussian

output pulse’s energy, power

< 50 mJ, 0.5 TW

IR amplitude jitter 3%


15

The third harmonic generator consists ofby 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/2BBO1 BBO2

Filter


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Laser layout: UV stretcherThe 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%

50 112.5 175 237.5 3002

5.25

8.5

11.75

15

grating spacng mm

outp

ut p

ulse

leng

th p

s

output pulse length vs grating distance [ps/mm-nm]

mmlg /4300


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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)


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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


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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).

0 1500 3000 4500 6000

0

1000

2000

In

te

nsity a

.u

.

m

01

50

03

00

04

50

0

0

10

00

20

00

Intensity a.u.

m

S

Laser to RF phase noise measurements


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MotivationsLaser phase stability is mandatory for stable machine operation.For SPARC phase 1 is requires < 2ps rms, other application demands formore challenging level of synchronization.

Coherent Synchrolock


22

Laser to RF phase-noise measurements


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Phase noise at oscillator levelStatistics on the laser to RFRelative phase

FFT of the relative phase

Stdev=0.34 psStdev=0.34 ps


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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 σRMS=0.47 ps.

Investigation of the causes of the slow drift (temperature?) and active RF phase shift compensation.

Longitudinal pulse shaping: experience using DAZZLER


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

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

0.10.5

1

IR p

ulse len

gth

[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


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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

Modified UV stretcher to obtain sharper rise time

dtt inout

]/[35.0 cmmnps

mmlgcmf /430020


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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)

10 9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 101

2

3

4

5

time ps

5

1

crrrx

0.068

max crrr x( )( )

4.6 ST1

max ST1

1010 x 21.8 ST0

The rise time is 1.5 ps


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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.


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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

Care publications 2006Published 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)


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Proceedings and reports1. 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

laser pulse shaping for high- brightness photoinjector carlo vicario for sparc collaboration

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