A STUDY OF THE FEASIBILITY OF A LASER SYSTEM FOR THE

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 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 Reliab
ility 109 shots between servicing 4 months
at 100Hz
4
LASER SPECIFICATIONS CLIC CTF3 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
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

8
BASIC LASER SYSTEM PULSE
GENERATOR/ OSCILLATOR OPTICS AMP 1
OPTICS AMP 2
cw ML NdYLF 1047nm 5ps
NdYLF
NdYLF
4? BBO 262nm
NdYLF
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
NDYLF OSCILLATOR Available commercially Expecte
d performance - 10W _at_ 0.5GHz (CLIC)
1.5GHz (CTF3) - 5ps _at_ 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
0.5
2
40
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 NdYLF
• Thermal lensing in the amplifier rods
  - NdYLF gives both a spherical
  and a cylindrical lens which must be compensated

15
FOURTH HARMONIC GENERATION
?
? 2?
4? 2?
2?
BBO BBO ? 50 ? 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
  21.

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
  _at_ 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? C
ERN 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 stea
dy state saturated operation extraction of
stored energy thermal effects Develop theory
and simulations