a study of the feasibility of a laser system for the clic photo-injector
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Rutherford Appleton Laboratory
A STUDY OF THE FEASIBILITY OF A LASER SYSTEM FOR THE CLIC PHOTO-INJECTOR
Ian Ross, Central Laser Facility, RALSteve Hutchins, CERN
Rutherford Appleton Laboratory
REPETITION TIME
PULSE TRAINDURATION
PULSEENERGY
TIME BETWEEN PULSES
STABILITY
PULSE TRAINS
Rutherford Appleton Laboratory
PHOTO-CATHODE SPECIFICATIONS
CLICCTF3
UV energy per micropulse 5µJ 0.84µJ
Pulse duration ‹10ps ‹10ps
Wavelength ‹270nm ‹270nm
Time between pulses 2.13ns 0.67ns
Pulse train duration 91.6µs 1.4µs
Repetition Rate 100Hz 5Hz
Energy stability ‹ 0.5% ‹ 0.5%
Laser/RF synchronisation ‹1ps ‹1ps
Reliability 109 shots between servicing4 months at 100Hz
Rutherford Appleton Laboratory
LASER SPECIFICATIONS
CLICCTF3
Energy per micropulse 100µJ 17µJ
Total pulse train energy 4.3J 32mJ
Pulse train mean power 47kW 25kW
Laser average power 430W 0.15W
Efficiency of IR to UV 5% 5%
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BASIC LASER SYSTEM
PULSE GENERATOR/OSCILLATOR OPTICS AMP 1 OPTICS AMP 2
TARGET OPTICS HARMONIC FINAL AMP OPTICS CONVERSION
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KEY ISSUES
•0.5% Stability and Controllability
•47kW pulse train power
•430W average power
•1ps synchronisation
•Uniform flat top beam profile
•Design for CLIC useable for CTF3
Rutherford Appleton Laboratory
BASIC DESIGN FEATURES
Stability - CW or QUASI-CW laser DIODE-PUMPING
careful design FEEDBACK system with
rapid response
Pulse train power - diode pump power = COST47kW pump efficiency - AMP. DESIGN
storage efficiency - MATERIAL
Average power - thermal dynamics420W MATERIAL FRACTURE
OPTICAL DISTORTION
Pulse duration (10ps) - MATERIAL
Simple design - few amplifiers with high gain MATERIAL
UV generation - high efficiency gives min. COST
Beam quality - OPTICS
Rutherford Appleton Laboratory
BASIC LASER SYSTEM
PULSE GENERATOR/OSCILLATOR OPTICS AMP 1 OPTICS AMP 2
TARGET OPTICS HARMONIC FINAL AMP OPTICS CONVERSION
cw MLNd:YLF1047nm
5ps
Nd:YLF Nd:YLF
Nd:YLF4ω BBO
262nm
Rutherford Appleton Laboratory
QUESTIONS TO ANSWER
•Is it feasible?
•Is it affordable?
•Where are the uncertainties in the physics/technology?
•What programme will establish confidence and lead to an optimised design?
Rutherford Appleton Laboratory
ND:YLF OSCILLATOR
Available commercially
Expected performance - 10W @ 0.5GHz (CLIC) 1.5GHz (CTF3)
- 5ps @ 1047nm
NUMBER OF AMPLIFIERS
Available input energy per pulse = 10nJRequired output energy per pulse = 100μJ
Required amplifier gain = 10,000
Need to limit gain per amplifier to about 20.
Simplest system has 3 amplifiers with average gain per amplifier of 22.
Rutherford Appleton Laboratory
FINAL AMPLIFIER DESIGN - PHYSICS
Requirements - diode pump power › output power (47kW)
- efficient extraction of diode power - high stability along the pulse train
Simulations carried out for single and double pass amplifiers.
For maximum stability the trick is to operate in quasi-steady-state mode with continuous pulse train input.
Sensitivity to 1% changes in input energy and pump power
0 50 1000.99
1
1.01
(microseconds)
Out
put
Ene
rgy
(rel
)
0 50 1000.99
1
1.01
(microseconds)
Out
put
Ene
rgy
(rel
)
Rutherford Appleton Laboratory
AMPLIFICATION SCHEME TOLERANCES FOR 0.5% STABILITY
AMP 1 X100 AMP 2DOUBLE X20 AMP 3PASS DOUBLE X4
PASS SINGLE PASS
100% 40% 2% 0.5%
QCW PUMP DIODE ARRAY MODULES
0.5%2%40%
Rutherford Appleton Laboratory
FINAL AMPLIFIER DESIGN - LAYOUT
STACKED DIODE ARRAY
WATER COOLINGSILICA TUBE
AMPLIFIER ROD
MIRROR COATINGS
•All rays from diodes (20 x 80deg) collected by rod
•1cm diam. gives excellent absorption tolerant to pump wavelength and polarisation
•un-absorbed fraction reflected back into rod
•water blanket improves coupling efficiency
Rutherford Appleton Laboratory
THERMAL EFFECTS
•Thermal fracture of rod at CLIC 420W - this may be a problem for Nd:YLF
•Thermal lensing in the amplifier rods - Nd:YLF gives both a spherical and a
cylindrical lens which must be compensated
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FOURTH HARMONIC GENERATION
ω ω 2ω 2ω
4ω2ω
BBO BBO η = 50% η = 50%
•Predicts 25% efficiency overall
•Literature reports 25% efficiency
•Design exercise assumed 10% - achievement of say 20% would substantially cut the cost of the laser.
Rutherford Appleton Laboratory
OPTICS DESIGN
REQUIREMENTS
•Stability requires generation of a single mode beam.
•Optics for beam size changes.
•For maximum efficiency the beam profile must be a flat top in both amplifiers and harmonic crystals.
•Beam profile to be flat-top on the photo-cathode.
•Compensation for thermal lensing in amplifiers.
•Minimise the effects of diffraction to keep intensity variations across the beam to less than 2:1.
Rutherford Appleton Laboratory
OSC AMP 1
RELAY CYL PC LENS GATE
APODISER
RELAYCYL RELAYLENS
LENS
AMP 2 AMP 3
CYL LENS
RELAY DELAY PC FHG RELAY LINE PHASE RETARDER
PHOTOCATHODE
OPTICS SCHEME FOR PHOTO-INJECTOR LASER SYSTEM
Rutherford Appleton Laboratory
CONCLUSIONS
•FEASIBLE
•AFFORDABLE Total pump power for CLIC ~75kW @ $7/W gives $0.5M for the diode arrays and a system cost of perhaps $1M
•CRITICAL ISSUES HAVE BEEN IDENTIFIED
•FUTURE PROGRAMME HAS BEEN PROPOSED
Rutherford Appleton Laboratory
FUTURE PROGRAMME
Experimental Programme to:
Resolve uncertainties - Increased confidenceObtain more accurate data - Definitive design
• Oscillator operation at 0.5GHz and 1.5GHz CLIC and CTF3
commercial systems have lower repetition ratesCERN
• Feedback control of a) laser pump diode current b) fast optical gate
needs to be FAST (μs response)ACCURATE (0.1%)
CERN• Amplification - all aspects of design
- thermal effects (lensing/fracture limit) RAL
• Fourth harmonic generation - maximum efficiency?CERN
• Check laser damage thresholds
Rutherford Appleton Laboratory
PROPOSED RAL PROGRAMME
Amplifier development - test as close to design parameters as possible - at minimum cost
STACKED DIODE ARRAY1.5kW each
CYLINDRICALLENS
AMPLIFIER ROD5 X 50 mm
MIRROR COATING
WATERCOOLING
SILICA TUBE
Scaled down version with short length - 4.5kW pumpGives measurable small signal and saturated gain
Good test of: pump efficiencygainsteady state saturated operationextraction of stored energythermal effects
Develop theory and simulations
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