Transcript Next step (after fusion ignition)

A Scalable Design for a High Energy,
High Repetition Rate, Diode-Pumped
Solid State Laser (DPSSL) Amplifier
Paul Mason, Klaus Ertel, Saumyabrata Banerjee, Jonathan Phillips,
Cristina Hernandez-Gomez, John Collier
Workshop on Petawatt Lasers at Hard X-Ray Light Sources
5-9th September 2011, Dresden, Germany
[email protected]
STFC Rutherford Appleton Laboratory,
Centre for Advanced Laser Technology and Applications
R1 2.62 Central Laser Facility, OX11 0QX, UK
+44 (0)1235 778301
Motivation
• Next generation of high-energy PW-class lasers
– Multi-J to kJ pulse energy
– Multi-Hz repetition rate
– Multi-% wall-plug efficiency
• Exploitation
Beamline
Facility
– Ultra-intense light-matter interactions
– Particle acceleration
– Inertial confinement fusion
• High-energy DPSSL amplifiers needed
– Pumping fs-OPCPA or Ti:S amplifiers
– Drive laser for ICF
– Pump technology for HELMHOLTZ-BEAMLINE
HELMHOLTZBEAMLINE
Amplifier Design Considerations
• Requirements
– Pulses from 10’s J to 1 kJ, 1 to 10 Hz, few ns duration, efficiency 1 to 10%
• Gain Medium
Long fluorescence lifetime
Higher energy storage potential
Minimise number of diodes (cost)
Available in large size
Handle high energies
Good thermo-mechanical properties
Handle high average power
Sufficient gain cross section
Efficient energy extraction
Low quantum defect
Increased efficiency & reduced heat load
– Ceramic Yb:YAG down-selected as medium of choice
• Amplifier Geometry
High surface-to-volume ratio
Efficient cooling
Low (overall) aspect ratio
Minimise ASE
Heat flow parallel to beam
Minimise thermal lens
STFC Amplifier Concept
• Diode-pumped multi-slab amplifier
– Ceramic Yb:YAG gain medium
– Co-sintered absorber cladding for ASE suppression
• Distributed face-cooling by stream of cold He gas
– Heat flow along beam direction
– Low overall aspect ratio & high surface area
• Operation at cryogenic temperatures
– Higher o-o efficiency – reduction of re-absorption
– Increased gain cross-section
– Better thermo-optical & thermo-mechanical properties
• Graded doping profile
– Equalised heat load in each slab
– Reduces overall thickness (up to factor of ~2)
~175K
Modelling
• Laser physics
50%
– Assumptions
• Target output fluence 5 J/cm²
• Pump 940 nm, laser 1030 nm
3.8
– Efficiency & gain
• Optimum doping x length product
for maximum storage ~ 50%
• Optimum aspect ratio to minimise
risk of ASE (g0D < 3) of ~1.5
– Extraction
Cr4+:YAG
• Extraction efficiency ~ 50%
• Thermal & fluid mechanics
– Temperature distribution
– Stress analysis
– Optimised He flow conditions
Yb:YAG
Scalable Design
HiPER
HiLASE /
ELI ELI
/ XFEL
Prototype
DiPOLE
~ 1 kJ
~ 100 J
~ 10 J
14 x 14 cm
200 cm2
5 x 5 cm
25 cm2
2 x 2 cm
4 cm2
Aspect ratio
1.4
1.2
1
No. of slabs
10
6
4
1 cm
0.7 cm
0.5 cm
5
3
2
0.33 at.%
0.79 at.%
1.65 at.%
Extractable energy
Aperture
Slab thickness
No. of doping levels
Average doping
level
DiPOLE Prototype Amplifier
• Design sized for ~ 10 J @ 10 Hz
• Aims
–
–
–
–
–
Validate & calibrate numerical models
Quantify ASE losses
Test cryogenic gas-cooling technology
Test (other) ceramic gain media
Demonstrate viability of concept
Yb3+
Cr4+
Ceramic YAG disk with
absorber cladding
• Progress to date
– Cryogenic gas-cooling system commissioned
– Amplifier head, diode pump lasers & front-end
installed
– Full multi-pass relay-imaging extraction
architecture under construction
– Initial pulse amplification tests underway
Diode pump laser
Optical Gain Material
• 4 x co-sintered ceramic Yb:YAG disks
1030 nm
940 nm
Fresnel limit ~84%
55 mm
– Circular 55 mm diameter x 5 mm thick
– Cr4+ absorbing cladding
– Two doping concentrations (1.1 & 2.0 at.%)
Pump
2x2
cm²
Yb3+
PV
0.123
wave
35 mm
Cr4+
Amplifier Head Design
• Schematic
• CFD modelling
Disks
Uniform T across
pumped region ~ 3K
Pump
Vacuum
Pump
vacuum
windows
He flow
40
m3/hr
~ 25 m/s @ 10 bar, 175 K
pressure
windows
Diode Pump Laser
– Ingeneric, Amtron & Jenoptic
Measured
20 mm
• Built by Consortium
• Two systems supplied
– 0 = 939 nm, FWHM < 6 nm
– Peak power 20 kW, 0.1 to 10 Hz
– Pulse duration 0.2 to 1.2 ms
– Uniform square intensity profile
– Steep well defined edges
– ~ 80 % spectral power within  3 nm
– Good match to Yb:YAG absorption
spectrum @ 175K
20 mm
DiPOLE Laboratory
Amplifier
head
2 x 20 kW diode
pump lasers
Cryo-cooling
system
Front-end Injection Seed
• Free-space diode-pumped MOPA design
– Built by Mathias Siebold’s team @ HZDR Germany
• Cavity-dumped Yb:glass oscillator
Amplifier
crystal
– Tuneable 1020 to 1040 nm
•  ~ 0.2 nm
– Fixed temporal profile
• Duration 5 to 10 ns
nsec
oscillator
Booster
pump
diode
– PRF up to 10 Hz
– Output energy up to 300 µJ
• Multi-pass Yb:YAG booster
amplifier
– 6 or 8 pass configuration
– Output energy ~ 100 mJ
Polarisation
switching
waveplate
100 mJ
output
Initial Pulse Amplification Results
• Simple bow tie extraction architecture
– 1, 2 or 3 passes
– Limited by diffraction effects
Seed
Amplified
beam
Pump
Pump
• Injection seed
– Gaussian beam expanded to overfill pump region
– Energy ~ 60 mJ
Spatial Beam Profiles @ 100K, 1 Hz
Gain  8
Gain  6
E = 2.6 J @ 10 Hz
Pulse Energy v. Pump Pulse Duration
• 3 passes @ 1 Hz
Onset of ASE loss
5.9 J
• Relay-imaging multi (6 to 8) pass extraction architecture is
required to allow >10 J energy extraction at 175K
Conclusions
• Cryogenic gas cooled Yb:YAG amplifier offers potential for
efficient, high energy, high repetition rate operation
– At least 25% optical-to-optical efficiency predicted
• Proposed multi-slab architecture should be scalable to
at least 1 kJ generating ns pulses at up to 10 Hz
– Limit to scaling is acceptable B-integral
• DiPOLE prototype amplifier shows very promising results
– Installation of relay-imaging multi-pass should deliver 10 J @ 10 Hz
• Strong candidate pump technology for generating high
energy, ns pulses at ~ 1 Hz for HELMHOLTZ-BEAMLINE
Thank you for your attention!
Any Questions?