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Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Experience with and expectations for the drive laser for the APS PC gun
Yuelin LiAdvance Photon Source, Argonne National Laboratory
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
LayoutLayout
* Current APS drive laserConfiguration, and features Problems and solutionsCurrent performances, and surprisesSummary
* Expectation for the next drive laserOperation and performance requirementSome commercial systems, new ideasAdaptive emittance optimization loopSummary
* Acknowledgement
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Role of the APS pc gun drive Role of the APS pc gun drive laserlaser
1 nC0.5-10 ps
3 mm mrad200-450 MeV
LEUTL: the free electron laser projectSaturated at wavelength as short as 150 nm
100-200 fs pulses, energy 60-250 J
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
APS pc gun drive laserAPS pc gun drive laser
Single-shot autocorrelator
Scanning Autocorrelator
Spectrometer
Head 1 Head 1
6 Hz, 0.4 mJ @ 263 nm1.8 nm bandwidth1-10 ps pulse durationTime jitter, 2 ps spec (?)
Flash lamp-pumped Nd:Glass Chirped pulse amplification laser
BBO
Imaging aperture
Divergencecontrol
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
FeaturesFeaturesEnvironment Monitoring
Temperature and humidityLaser monitoring
Oscillator Energy (off line), pulse duration (off line), mode,
spectrum, AmplifierCavity buildup, mode, pulse duration, FROG (offline)
UVEnergy, mode, virtual cathode
Laser controlOff line: Pulse duration, divergence, spot size on VCOn line: Pulse energy, trajectory Semi automatic cathode cleaning
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
FROG traces of the laserFROG traces of the laser
Raw Reconstructed
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Semi automatic cathode Semi automatic cathode cleaningcleaning
Before
After
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Problems and solutionsProblems and solutions
Poor beam profilemode inhomogenityhigher order mode
Poor pointing stability50% rms
Poor outputup to 50% rmsnot enough energy
Poor reliabilitymechanical broken rodsoptical damage
Intense maintenance
•Replacing KDP with BBO•Adding pinholes at both end of the cavity•Sealing the transport line•Imaging
•Imaging•Sealing the transport line
•Replacing stretcher-compressor gratings(From originally unknown 1800 l/mm, 76% efficiency to JY 1740 l/mm, 90% efficiency)
•Scheduling flash lamp replacement•Switching cathode from Cu to Mg
•Switching Kigre rods to Schott rods•Adding pinholes to cavity
•Hiring a baby sitter
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
IR Spatial ProfileIR Spatial Profile
x=1.14 mmy=0.79 mm
30-40 mW @ 6 Hz
4
3
2
1
0
50 100
0 1 2 3 4 5 6
50
100
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Virtual cathode imagesVirtual cathode images
4
3
2
1
0
50 100
0 1 2 3 4 5 6
50
100
Size: variableProfile: 30% flat top Pointing stability: ~2%
0 1 2 3 4 5 6
50
100
4
3
2
1
0
50 100
Direct beam
Hard edge image
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Frequency conversionFrequency conversionThe power in the second harmonics at matched phase is (with pump depletion)
I
cnn
dL
L
L
P
P
effNL
NL
22
20
22
2
4
1
tanh
2.01.67491.67490.527FHG
1.91.65511.65511.053SHG
deff (pm/v)nnm
2/14
2/12
)/(
3443
)/(
6913
APL
APL
NL
NL
For BBO, type I critical phase match
At low intensityP4P
4
At high intensityP4P
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Frequency conversionFrequency conversion
Expected Conversion efficiency
MeasuredGreen/IR 53%UV/green 20%
0.0732 Wavelength bandwidth (nm cm)
4.0 51 Temperature bandwidth (K cm)
0.160.53 Angular acceptance (mrad cm)
0.532 m1.064 m
UV/Green
Green/IRO
ld g
rati
ngs
New
gra
ting
s
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
UV Energy stabilityUV Energy stabilityLamps at 10 million shots
-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.150
200
400
600
800
1000
1200
1400
Co
un
ts
E/<E>-1
15.0 15.5 16.0 16.50
50
100
150
200
20020927
<E>=208 J
E/<E>=4.3%
En
erg
y (
J)
Hours
Lamps at 1 million shots
-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.150
500
1000
1500
2000
Co
un
ts
E/<E>-1
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.60
2
4
6
8
10
20021012
<E>=9.68 J
E/<E>=1.7%
En
erg
y (
J)
Hours
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Surprise: Timing stabilitySurprise: Timing stability
119 MHz rf waveform Laser rf waveform
Laser rising edge
171 ps p-p, 19 ps rmsLaser falling edge
56 ps p-p, 7 ps rms
Laser oscillator TBWP GLX-200 oscillator at 119 MHzLock device TBWP CLX-1000 timing stabilizer with spec <2 psRF source Gigatronics 2856 MHz/24 orCrystal oscillator 119 MHz
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Summary on current laserSummary on current laser
* Finally usable in stability and profile
However: it is stretching its limit ….
* Flash lamps age quickly 10 million shots is the margin we use now
3-weeks of 24-7 operation at 6 HzNeeds careful attention for stable operation at the end
* Laser rods break at about 15 million shots or less Time consumingChanges laser characteristics: divergence, mode size, optical path, etc….
• No room for further improvementEnergy stability, reliability, etc..
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Time to dream for a new drive laser
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
The future APS drive laser The future APS drive laser
Role
Primary electron beam source for both LEUTL and APS in routine operation (LEUTL is becoming a user facility)
Key operational requirement
Turn key systemReliable: no break down during normal operationStable over long timeMinimum maintenanceDeliver up to 5 nC per shot for injection to APS
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
The role of cathodesThe role of cathodes
Cathode (Egap+ EA)/eV Q.E. (263nm) LifeLaser energy/nC
Cu 4.5-5.6 eV210-6 (APS)310-5 (Nguyen, LANL)410-5 (GTF)
Long2.4 mJ160 J120 J
Mg3.78 eV
1.310-4 (Spring-8)1.310-3 (APS)310-3 (Nguyen, LANL)
Long36 J3.6 J1.6 J
Cs2Te 3.5 eV5% (Nguyen, LANL)1% (FNAL)
Months
CsI 6.4 eV Est. 10-3 Long
K2CsSb 2.1 eV 10% Hours
Cs3Sb 2.05 eV 6% Unstable
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Laser: Dream vs realityLaser: Dream vs reality
Current Dream Reality
Pulse energy on cathode a <500 J 100 J Pulse repetition rate b 6 Hz 60 Hz Energy stability 2% rms 0.5% rms
5-10% p-p 2% p-p Pulse length 2-10 ps 2-10 ps Pulse shaping Possible Y R&D neededProfile shaping Semi Y R&D neededSpatial homogeneity 50% 10% R&D neededPointing stability 1-5% rms 1% p-p R&D neededTiming jitter to rf 6 ps rms <0.5 ps Demonstrated
(2 ps spec)Advanced features Active hydrothermal control N Y Automatic energy control Possible Y Automatic emittance optimization N YYY R&D needed
a. Based on APS QE for a Mg cathode of about 1.310-3, for 1 nC of charge from the gun.
b. With long life cathode, single pulse per rf cycle. For a SC rf with higher duty factor the requirement is different.
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Commercial Ti:Sa amplifier Commercial Ti:Sa amplifier systemssystemsAdvertised performancesMake and model Rep rate Energy Stability Mode
Clark MXR, CPA-2010 0.2-2 kHz, >0.6 mJ, 1% TM00(Built in active hydrothermal Stabilization)
Spectra Physics, Spitfire 1-5 KHz 0.7 mJ 1% (p-p) TM00, 1.5 diffraction limitSpectra Physics, Hurricane 1-5 KHz 0.7 mJ 1.5% (p-p) TM00, 1.5 diffraction limitFemto Lasers, FemtoPower 1 kHz 0.8 mJ 2% (p-p) TM00, 2 diffraction limitCoherent RegA9000 300 kHz 4 J 2% (p-p) TM00, 2 diffraction limit
Energy stability <1.5% p-p, 0.24% rms @ 1k Hz (http://www.poslight.com/)
Performance ExampleSpectra Physics-Positive Light Hurricane
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
An idea: Gain less amplifierAn idea: Gain less amplifier
Seed: high rep, low energy pulsesf, Es Cavity with length matching
the rep rate of the seed, L=1/f
Low rep outputEout=nEs=Es/loss
AdvantagesUltra stable: Eout=n<Es>Linear device: easier to shapeLow jitter: no jitter between seed and output
Jones and Ye, Opt Lett 27, 1848 (2002)
Example: 300 mW, 100 MHz, for loss=10-4, Eout=30 J @100 kHz
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Commercial lockable oscillatorsCommercial lockable oscillators
Make and model rep rate jitter spec
TBWP TIGER 200/CLX1100 up to 150 MHz <1 psFemto Laser Femto Source/Femtolock up to 150 MHz <1 psSpectra-Physics Lok-to-Clock Tsunami up to 100 MHz <1 psCoherent Mir Synchro-Lock up to 100 MHz <2 ps
Advertised performance
Laser TBWP TIGER 200 oscillator at 119 MHzLocking TBWP CLX1100RF source HP 8665B at 119 MHz
Test results 8 ps
“Subfemtosecond timing jitter between two independent, actively synchronized, mode locked lasers”
Shelton et al, Opt Lett 27, 312 (2002)
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Adaptive pulse manipulationAdaptive pulse manipulation
Single shot Emittance measurement
Transverse profile measurement
Deformable mirror
Laser Electron beam
SLM pulse shaper
Single shot longitudinal profile measurement
Longitudinal and transver profile control
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
Measurement techniquesMeasurement techniques
Laser longitudinal profileFROG is the choice
in IR, green, SHGin UV, Polarization gating
Single-shot emittance measurementMulti slit mask
FROG example
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
SummarySummary
Laser
Laser technology is mature enough for the basic requirement
Advanced features
R&D is needed to adapt existing adaptive pulse shaping and waveform control technologies
Laser Issues for Electron RF Photoinjectors, Oct. 23-25, 2002, SLAC
AcknowledgementAcknowledgement
Gil Travish (former laser commander)Ned Arnold Sandra Biedron Arthur
GrelickMike Hahne Kathy Harkay Rich
KodenhovenRobert Laird John Lewellen Greg
Markovich Stephen Milton Antothny PetrylaSupported by the U. S. Department of Energy, Office
of Basic Energy Sciences
Contract No. W-31-109-ENG-38