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A STUDY OF THE FEASIBILITY OF A LASER SYSTEM FOR THE CLIC PHOTO-INJECTOR

A STUDY OF THE FEASIBILITY OF A LASER SYSTEM FOR THE CLIC PHOTO-INJECTOR Ian Ross, Central Laser Facility, RAL Steve Hutchins, CERN. PULSE TRAINS. STABILITY. PULSE ENERGY. PULSE TRAIN DURATION. TIME BETWEEN PULSES. REPETITION TIME. PHOTO-CATHODE SPECIFICATIONS CLIC CTF3

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A STUDY OF THE FEASIBILITY OF A LASER SYSTEM FOR THE CLIC PHOTO-INJECTOR

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  1. A STUDY OF THE FEASIBILITY OF A LASER SYSTEM FOR THE CLIC PHOTO-INJECTOR Ian Ross, Central Laser Facility, RAL Steve Hutchins, CERN

  2. PULSE TRAINS STABILITY PULSE ENERGY PULSE TRAIN DURATION TIME BETWEEN PULSES REPETITION TIME

  3. PHOTO-CATHODE SPECIFICATIONS CLIC CTF3 UV energy per micropulse 5µJ0.84µJ Pulse duration ‹10ps‹10ps Wavelength ‹270nm‹270nm Time between pulses 2.13ns0.67ns Pulse train duration 91.6µs1.4µs Repetition Rate 100Hz5Hz Energy stability ‹ 0.5%‹ 0.5% Laser/RF synchronisation ‹1ps‹1ps Reliability 109 shots between servicing 4 months at 100Hz

  4. LASER SPECIFICATIONS CLIC CTF3 Energy per micropulse 100µJ17µJ Total pulse train energy 4.3J32mJ Pulse train mean power 47kW25kW Laser average power 430W0.15W Efficiency of IR to UV 5%5%

  5. BASIC LASER SYSTEM PULSE GENERATOR/ OSCILLATOR OPTICS AMP 1 OPTICS AMP 2 TARGET OPTICS HARMONIC FINAL AMP OPTICS CONVERSION

  6. 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

  7. BASIC DESIGN FEATURES • Stability - CW or QUASI-CW laser • DIODE-PUMPING • careful design • FEEDBACK system with • rapid response • Pulse train power - diode pump power = COST • 47kW pump efficiency - AMP. DESIGN • storage efficiency - MATERIAL • Average power - thermal dynamics • 420W 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

  8. BASIC LASER SYSTEM PULSE GENERATOR/ OSCILLATOR OPTICS AMP 1 OPTICS AMP 2 cw ML Nd:YLF 1047nm 5ps Nd:YLF Nd:YLF 4ω BBO 262nm Nd:YLF TARGET OPTICS HARMONIC FINAL AMP OPTICS CONVERSION

  9. 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?

  10. ND:YLF OSCILLATOR Available commercially Expected performance - 10W @ 0.5GHz (CLIC) 1.5GHz (CTF3) - 5ps @ 1047nm NUMBER OF AMPLIFIERS Available input energy per pulse = 10nJ Required 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.

  11. 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

  12. AMPLIFICATION SCHEME TOLERANCES FOR 0.5% STABILITY QCW PUMP DIODE ARRAY MODULES 40% 2% 0.5% AMP 1 X100 AMP 2 DOUBLE X20 AMP 3 PASS DOUBLE X4 PASS SINGLE PASS 100% 40% 2% 0.5%

  13. FINAL AMPLIFIER DESIGN - LAYOUT • 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

  14. 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

  15. FOURTH HARMONIC GENERATION ω ω 2ω 4ω 2ω 2ω 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.

  16. 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.

  17. OPTICS SCHEME FOR PHOTO-INJECTOR LASER SYSTEM AMP 2 AMP 3 OSC AMP 1 APODISER RELAY CYL PC LENS GATE RELAY CYL RELAY LENS LENS CYL LENS RELAY DELAY PC FHG RELAY LINE PHASE RETARDER PHOTOCATHODE

  18. 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

  19. FUTURE PROGRAMME Experimental Programme to: Resolve uncertainties - Increased confidence Obtain more accurate data - Definitive design • Oscillator operation at 0.5GHz and 1.5GHz CLIC and CTF3 commercial systems have lower repetition rates CERN • 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

  20. PROPOSED RAL PROGRAMME Amplifier development - test as close to design parameters as possible - at minimum cost Scaled down version with short length - 4.5kW pump Gives measurable small signal and saturated gain Good test of: pump efficiency gain steady state saturated operation extraction of stored energy thermal effects Develop theory and simulations

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