C. Vicario LCLS ICW SLAC Oct. 9-11, 2006. THE DRIVE LASER: EXPERIENCE AT SPARC Carlo Vicario for...

C. Vicario LCLS ICW SLAC Oct. 9-11, 2006.

THE DRIVE LASER: EXPERIENCE AT SPARC

Carlo Vicario

for

SPARC collaboration

2C. Vicario

LCLS ICW SLAC Oct. 9-11, 2006.

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


SPARC laser: layout and system’s performances

4C. Vicario


SPARC 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%

5C. Vicario


Ti:Sa CPA laser system by Coherent + pulse shaper

6C. Vicario


Coherent Laser System

oscillatorpumps

amplifiers

Harmonics generator

UV stretcher

7C. Vicario


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

8C. Vicario


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. They work at low energy level.

Dazzler

Half-wave plateGratingGrating

LensLens

f f f f

Mask

GratingGrating

LensLens

f f f f

Mask

New UV Dazzler S. Coudreu Opt. Lett. 31, (2006), 1899

9C. Vicario


Laser layout: CPAlaser pump 1 1KHz, 7 W, 100 ns

laser pump2 10 Hz, 560 mJ, 7 ns

rep. rate 10 Hz

spatial mode ~Gaussian

output pulse’s energy, power

< 50 mJ, 0.5 TW

IR amplitude jitter 3%

10C. Vicario


The third harmonic generator consists ofby 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/2BBO1 BBO2

Filter

11C. Vicario


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%

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

12C. Vicario


Laser system layout: spectral and time diagnostics

30 cm lens

4350 g/mmgrating

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)

13C. Vicario


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

)(~

)(

)(

)()(

2

2

2

2

2

2

tItI

teA

eAtI

ti

ti

14C. Vicario


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).

lase

r

IRIS

15C. Vicario


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

16C. Vicario


Compensation scheme

Advantages: Circular beam at cathode (>98%) Front tilt compensation (< 200 fs) Work for different spot sizes.

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-0.05

0.00

0.05

0.10

0.15

0.20

transform limited bw=1.6nm

horiz position [nm]

time

arr

iva

l [p

s]

H posirion mm

Simulated spot and front at cathode

T

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 measuredStructures in the spot

17C. Vicario


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.

18C. Vicario


Transverse profile at the virtual cathode

0 1500 3000 4500 6000

0

1000

2000

In

te

nsity a

.u

.

m

0150

0300

0450

0

0

100

0

200

0

Intensity a.u.

m

S

•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

19C. Vicario


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.


Laser to RF phase noise measurements

21C. Vicario


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

22C. Vicario


Laser to RF phase-noise measurements

23C. Vicario


Phase noise at oscillator levelStatistics on the laser relative phase

FFT of the relative phase

Stdev=0.35 degStdev=0.35 deg

24C. Vicario


RF to Laser synchronization: measurements on 10 Hz UV pulses

2856 MHz cavity

High energy UV @ 10 Hz

On time scale of few minutes the phase jitter is

within σRMS=0.48 RF deg.

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


Longitudinal pulse shaping: experience using DAZZLER

26C. Vicario


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

27C. Vicario


• 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

28C. Vicario


Dazzler experience at SDL-BNL

H. Loos et al, PAC05

Dazzler

CPA

tripler

Ti:Sa Oscillator

15 mJ10 - 20 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.


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

0.10.5

1

IR p

ulse len

gth

[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

30C. Vicario


Equations for SH with vs linear chirp

kzj

g

eAAjt

A

Vz

A

112

2

2 1

2

1

~

1

~22

2 sin)(

AAddcI

)(22

1)()((

111

~

1

~

)2/(

)()( 11

j

j

eS

deSSAA

2)()()(

2)(

1

~ jeSA

In the frequency domain we can integrate A2 and obtain the output intensityHp: Phase matching, not depletion regime and negligible velocity dispersion

)2/( 1

Equation for the SH generation for the complex fields Ai,j

The output spectrum is the convolution product

Similar consideration can be extended to the THG

31C. Vicario


Effects of non-linear crystal tilt

2

1122

2 sin)( AAddcI

-5.8 0

5.8

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 shiftand therefore it introduces an asymmetry in the output spectrum.

Simulated and measured SH spectravs the tilt of the crystal.

32C. Vicario


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.

33C. Vicario


Simulated UV intensity profile

Ingredients to achieve this profile: 1 Perfect square IR spectrum 12 nm

• Limitation form Dazzler resolution and amplifier distortions

2 Long IR input 10 (ps)• Harmonics efficiency prop I(t)

3 140 um thick SHG crystal instead of 500um 40 um thick THG crystal instead of 300 um

• Harmonic efficiency prop. L2

4 Perfect alignment and time overlap

1 ps rise time1 ps rise time

We can obtain more sharp edges clipping the spectrum tails where it is spatially dispersed!

34C. Vicario


Modified UV stretcher to obtain sharper rise time

dtt inout

]/[35.0 cmmnps

mmlgcmf /430020 M. Danailov et al, FEL06

35C. Vicario


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)

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

36C. Vicario


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.

S

37C. Vicario


Cathode laser cleaningLaser 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 transverselyuniform e-beam.

QE map before and after laser cleaningat low field

Vacuum during the cleaning

38C. Vicario


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

C. Vicario LCLS ICW SLAC Oct. 9-11, 2006. THE DRIVE LASER: EXPERIENCE AT SPARC Carlo Vicario for...

Documents

Transcript of C. Vicario LCLS ICW SLAC Oct. 9-11, 2006. THE DRIVE LASER: EXPERIENCE AT SPARC Carlo Vicario for...