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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

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slide1

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

paul.mason@stfc.ac.uk

STFC Rutherford Appleton Laboratory,

Centre for Advanced Laser Technology and Applications

R1 2.62 Central Laser Facility, OX11 0QX, UK

+44 (0)1235 778301

slide2

Motivation

  • Next generation of high-energy PW-class lasers
    • Multi-J to kJ pulse energy
    • Multi-Hz repetition rate
    • Multi-% wall-plug efficiency
  • Exploitation
    • 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

BeamlineFacility

  • HELMHOLTZ- BEAMLINE
slide3

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
    • Ceramic Yb:YAG down-selected as medium of choice
  • Amplifier Geometry
slide4

STFC Amplifier Concept

~175K

  • 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)
slide5

Modelling

Cr4+:YAG

50%

pump region

  • Laser physics
    • Assumptions
      • Target output fluence 5 J/cm²
      • Pump 940 nm, laser 1030 nm
    • 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
      • Extraction efficiency ~ 50%
  • Thermal & fluid mechanics
    • Temperature distribution
    • Stress analysis
    • Optimised He flow conditions

3.8

Yb:YAG

slide7

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
  • 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

Ceramic YAG disk with absorber cladding

Yb3+

Cr4+

Diode pump laser

slide8

Optical Gain Material

Cr4+

Pump2 x 2

cm²

  • 4 x co-sintered ceramic Yb:YAG disks
    • Circular 55 mm diameter x 5 mm thick
    • Cr4+ absorbing cladding
    • Two doping concentrations (1.1 & 2.0 at.%)

55 mm

35 mm

Yb3+

Fresnel limit ~84%

PV

0.123

wave

1030 nm

940 nm

slide9

Amplifier Head Design

  • Schematic
  • CFD modelling

Disks

Uniform T across pumped region ~ 3K

Pump

Pump

He flow

pressure

windows

Vacuum

vacuum

windows

He flow

40 m3/hr ~ 25 m/s @ 10 bar, 175 K

slide10

Diode Pump Laser

  • Built by Consortium
    • Ingeneric, Amtron & Jenoptic
  • 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 absorptionspectrum @ 175K

Measured

20 mm

20 mm

slide11

DiPOLE Laboratory

Cryo-cooling system

Amplifier head

2 x 20 kW diode pump lasers

slide12

Front-end Injection Seed

Amplifier crystal

nsecoscillator

  • Free-space diode-pumped MOPA design
    • Built by Mathias Siebold’s team @ HZDR Germany
  • Cavity-dumped Yb:glass oscillator
    • Tuneable 1020 to 1040 nm
      •  ~ 0.2 nm
    • Fixed temporal profile
      • Duration 5 to 10 ns
    • PRF up to 10 Hz
    • Output energy up to 300 µJ
  • Multi-pass Yb:YAG boosteramplifier
    • 6 or 8 pass configuration
    • Output energy ~ 100 mJ

Booster pump diode

x3 or x4

Polarisation switching waveplate

100 mJ

output

slide13

Initial Pulse Amplification Results

  • Simple bow tie extraction architecture
    • 1, 2 or 3 passes
    • Limited by diffraction effects
  • Injection seed
    • Gaussian beam expanded to overfill pump region
    • Energy ~ 60 mJ

Seed

Amplified

beam

Pump

Pump

slide14

Spatial Beam Profiles @ 100K, 1 Hz

Gain  8

Gain  6

E = 2.6 J @ 10 Hz

slide15

Pulse Energy v. Pump Pulse Duration

Onset of ASE loss

  • 3 passes @ 1 Hz
  • Relay-imaging multi (6 to 8) pass extraction architecture is required to allow >10 J energy extraction at 175K

5.9 J

slide16

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 toat 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
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