-care 2008- cern 2-5 december 2008 giancarlo gatti phin activities at lnf
TRANSCRIPT
-Care 2008-CERN 2-5 December 2008
Giancarlo GattiGiancarlo Gatti
Phin activities at LNF
-Care 2008-CERN 2-5 December 2008
Outline
•SPARC project overview
•SPARC drive laser system
• IR shaper comparison for flat top laser pulse
• IR/UV-pulse shaping
•Laser to RF synchronization schemes
•Trasverse shaping considerations
•Conclusive remarks
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: MiUR Strategic Research Programs
Goals: High brightness linac to drive advanced FEL experimentsAnd further experiments
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SPARC commissioning phase 1: SPARC commissioning phase 1: low energy e-beam characterization low energy e-beam characterization
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Sparc phase 2
• After gun commissioning in 2006:
-Commissioning of downstream linac in progress.
-Variable gap undulators installed and ready.....
Last minute breaking news:First beam inside undulators
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Photocathode drive laser
Demands:
Solutions:• Emittance minimization
• Flexibility (Long./Trasv.)
• Stable reliable operation
• High current (100 A) from Cu cathode
Synchronization rf laser < 1ps rms stable laser performance
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High energy laser (50-500 uJ) pulses at 266 nm
Uniform time and and transverse laser profile: 6-12 ps duration, rise time, 2 mm hard edge
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SPARC laser system: topology
Ti:Sa laser composed by:
•12 nm bandwidth oscillator
•IR pulse shaper
•CPA amplifiers
•third harmonic generator
•UV stretcher (used as
shaper)
•Transport to the cathode
•
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Proc. PAC 2007 TUPMN040
time shaper
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SPARC laser system
oscillatorpumps
amplifiers
Harmonics generator
UV stretcher
Pulse shaper
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• The laser delivers 5-12 ps, 100 J pulses at 266 nm with a rep. rate of 10 Hz.
• Energy jitter (5% rms), pointing stability (<50 m) and synchronization respect to the RF (<2ps rms)
• Several subsystems have been integrated: IR pulse shaper, tranverse unifom pulse selection and imaging system to the cathode.
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Laser temporal pulse shaping
M. Petrarca S. Cialdi C. Vicario
Outline:-IR shaper comparison (a.o. Filter, LCM)-Overall system performances-UV shaper
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Tests of two IR pulse shapers10
Dazzler
Half-wave plate
Dazzler
Half-wave plate
From oscillator
Into amplifierTelescopes
Phase mask
Lens
Lens
Grating
Grating
From oscillator Into amplifier
60 c
m
15 cm15 cm
DAZZLERDAZZLERLC-SLMLC-SLM
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IR programmable pulse shapers
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DAZZLERDAZZLER
LC-SLMLC-SLM
Fast mode
Slow mode
Acoustic grating
Acousto-optic interaction in a TeO2 crystal
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The UV time profiles the two shapers:DAZZLER LC-SLM
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Rise and fall time ~ 2.6 ps Rise and fall time ~ 2.1 ps
Opt. Lett. 31, 19 (2006) 2885-2887
LC-SLMLC-SLMDAZZLERDAZZLER
The spectral shape after the UV stretcher is very similar to the temporal profile
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IR shapers Features:
DAZZLERLC-SLM
• Compact
• Easy alignment
• Simultaneously phase &
amplitude modulation
• Losses within 50%
• Resolution = 0.3 nm
• Slow optimization
• Not-compact
• Not easy alignment
• Phase only
modulation
• Losses within 50%
• Resolution<0.1 nm
• Fast optimization
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THG distortions: main limitation for fast rise time
A large enough pulse width (≥0.6 ps) is needed to preserve the square spectrum throughout the THG
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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UV pulse shaper
The UV stretcher was designed to perform several tasks
1. Lengthen the laser pulse proportional to bandwith up to 20 ps.
2. In the Fourier plane an amplitude filter, such as an iris, can be applied to cut the tails of an almost square spectrum produced bu the DAZZLER or LC-SLM, the obtained spectrum profile is transferred into the time profile by the stretcher
3. A on-line spectrometer is integrated.
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1
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Appl. Opt. 46, 22 (2007) 4959
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Picture of the UV shaper
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SpectromCCD
Focusinglenses
Filter plane
input
output Gratingpair
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Cross-correlated UV profile 117
Simulationexperiment
Simulationexperiment
FWHM 10 ps: experim. vs simulation
FWHM 15 ps rise time 1.5 ps
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Measured UV profile 2 for several pulse length
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UV shaper allows for fast rise timedespite of different chirp factors in the stretcher
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Shaping without the IR filter
When a proper sharp cut is applied to the natural UV
Gaussian laser spectrum, a flat top profile in time can be
produced.
Results are comparable to the two stages pulse shaping.
Price to pay 20% higher energy losses
Cheaper and simpler respect to the other IR pulse shapers
The rise and fall time are reduced to ~1.5 ps (limited by the
bandwidth)
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No IR shapers: simulations20
spectra
Time shape
IR shape No IR shape
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Experimental results
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Appl. Opt. 46, 22 (2007) 4959-4962
Spectra
No-cut
TimeMeasured (red) and
Simulated (black)
Cut applied
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Exotic applications: UV multipeaks generation
• With a grid in the fourier plane we obtained 4 peaks pulse (FEL microbunching enhancement)
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Laser to RF synchronization
M. Bellaveglia, S. Gallo, C. Vicario
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Synchronize the laser and RF
• Laser to RF synchronization is needed to have photoinjector optimal and stable operation
• Photoelectron gun phase < 1 deg rms for emittance compensation
• Velocity bunching, pulse compression and laser acceleration demands for a tighter specification (100 fs)
Laser oscillator
Cavity length control
Pulse selection amplification
THG+stretcher
Laser to the cathode
L
cf
2
Measure Δf
Masterclock
RF chain
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Phase noise at oscillator level
Measurements set up and results
350 fs rms
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UV time jitter: measure at 10 Hz
wavereference
Activefeedback
RF phase shifter Time of arrival jitter estimated
with the RF deflector is 390 fs
0.67 ps rms over 6 hours
Pill box cavity
HV Photo diode
mixer
Laser
Phase noise detection
acquisition
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Toward next step: Laser-driven RF
• To reduce the time jitter we can To reduce the time jitter we can synthesize the RF frequency from synthesize the RF frequency from a photodiode excited by the a photodiode excited by the oscillator pulses.oscillator pulses.
• The value measured can be The value measured can be affected by the apparatus affected by the apparatus resolution, shortly more detailed resolution, shortly more detailed characterizationcharacterization
• This technique is applicable to This technique is applicable to lock for 1 laser systemlock for 1 laser system
• All-optical synchronization system All-optical synchronization system and clock distribution to go at sub and clock distribution to go at sub 100 fs level100 fs level
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Sparc P1 highlights
B=6*1013 A/m*rad
First ever emittance oscillation
Square laser at the cathode
PRL 99, 234801 (2007)
Opt. Lett. 31, (2006) 2885 Appl. Opt. 46, 22 (2007) 4959
REV. SC. INSTR. 77, 093301 2006
PRST- AB 11, 032801 (2008)
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E:beam experimental results• The flat top pulse shape allowed the observation of the
double-minimum emittance evolution at SPARC (only predicted by the theory).
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+18°RF phase
0.45 mmrms spot size
5, 1.5 psDuration, rise time
5.5 MeVenergy
100 Acurrent
PRL 99, 234801 (2007)
UV LASER
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Gaussian vs flat beam:comparison
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Recent advances: Trasverse shaping
Main demands:
-Squared beam -Power efficiency Final choice:
Telescope system to map Gaussian into flat top
Pros: above 70% efficiency (up to 95%)Cons: exact matching of TEM00 gaussian alignment stability filtering cuts real efficiency
2 kinds of commercial refractiveSystems probed.
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OUTPUTBEAM
300 400 5000
10
20
30
40
50
60
70
Inte
nsity
(a.
u.)
X (pixels)
IRIS CUT
200 250 300 350 400 450 500 550 6000
10
20
30
40
50
60
70
Inte
nsity
(a.
u.)
X (pixel)
line 150 line 100 line 200
NO IRIS
INPUT BEAM
Our choice:No spatial filter
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0 50 100 150 200 250 300
700
800
900
1000
1100
1200
1300
1400
Inte
nsi
ty (
a.u
.)
Pix
SHAPERON CATHODE
Trasportation up to cathode
Shape is preserved through relay imaging trasport (10 mt.)
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Conclusions
• Extensive development on laser pulse shaping has been Extensive development on laser pulse shaping has been done at LNF-SPARC within the PHIN collaboration done at LNF-SPARC within the PHIN collaboration ▫ The two stages pulse shaping for 1.5 ps rise time▫ Demonstration of UV-only pulse shaping▫ Work on faster rise time and reduced losses
• Sub ps synchronization with upgrade at < 200 fs has been Sub ps synchronization with upgrade at < 200 fs has been demonstrateddemonstrated
• E-beam results encouraging the search of flat top pulse, E-beam results encouraging the search of flat top pulse, definitive comparison at higher energydefinitive comparison at higher energy
• Complete shaping in the next future when charge issues Complete shaping in the next future when charge issues will be overcome.will be overcome.
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