cryo-cooled high energy laser amplifiers - stfc.ac.uk · klaus ertel central laser facility stfc...
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Klaus Ertel
Central Laser FacilitySTFC Rutherford Appleton LaboratoryDidcot, OX11 0QX, United Kingdom
Cryo-Cooled High Energy Laser Amplifiers
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Outline
• Basics of laser amplifiers•Motivation for cryo cooling• Common types of cryo cooled systems• The DiPOLE architecture• DiPOLE cryo system
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Laser medium
(Pulsed) Laser AmplifiersLight Amplification by Stimulated Emission of Radiation
Here: solid state• Host, e.g. YAG, sapphire• Active ions, e.g. Yb3+, Ti3+
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Laser medium
(Pulsed) Laser AmplifiersPumping
Pump Light
Pumps source:• Usually other laser or laser diodes• Pulsed operation• Pulse duration ns to ms
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Energy
(Pulsed) Laser AmplifiersAfter Pumping
Energy stored in excited active ions
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Laser medium
Energy
(Pulsed) Laser AmplifiersAmplification (Extraction)
• Pulse generated by oscillator (+ pre-amplifiers)• Usually few ns long (naturally or stretched)
Laser Pulse (low energy)‘seed pulse’
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Laser medium
Energy
(Pulsed) Laser AmplifiersAfter Extraction
Heat
• Stored energy extracted by seed pulse• Process never 100% efficient -> heating of gain medium
Repeat…Amplified Laser Pulse (high energy)
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Laser medium
Energy
(Pulsed) Laser AmplifiersHeating
Heat
• The higher the average power (pulse energy x reprate) the more heat• Heating (+ cooling) leads to temperature gradients• Medium must not become too hot• Gradients must not become too strong
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Thermo-Optic Effects
Heat
Heat
T
Collimated Beam
ThermalFocussing
Other issues• Stress depolarisation, fracturing• Lower + non-uniform amplification (gain)
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Why Cryo Cooling of Laser Medium
To Enable • Higher pulse energy• Higher repetition rate• Higher efficiency
Through • Mitigation of thermal effects
• Improved thermal conductivity• Reduced thermo-optical effect
• Better performance• Higher gain coefficient
• Lower losses• Reduced reabsorption
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Common types: Ti:sapphire
Laser medium• Ti:sapphire crystal• In vacuum chamber• Conduction cooled
Gifford-McMahon cooler
• Broad band amplification• Very short pulses, ~30fs (after compression)• Applications:
• Ultrafast spectroscopy• Plasma physics, extreme conditions• Materials processing
Courtesy of H Kiriyama, JAEA
Courtesy of H Kiriyama, JAEA
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Common types: Yb:YAG• Fairly new material• Quite established for cw and low pulse energies• Needs pumping by laser diodes• Ideally suited for high average powers + high efficiencies• Emerging: high pulse energy systems (multi-J)• Applications:
• Laser pumping (Ti:sapphire…)• Plasma physics / extreme conditions• Practical: materials processing, remote sensing…
• High pulse energies facilitated by cryo cooling through:• Lower losses• Higher gain• Reduced thermo-optic effect
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Yb:YAG
Cr4+:YAG
Diode Pumped Optical Laser Experiment
Co-sintered ceramics Absorber claddingfor ASE management
Yb:YAGSlabs
Window
He Gas in~150 K
Vanes
He Gas out
Pump +Seed
Stronger Doping Towards Middle
14 c
m(1
kJ)
DiPOLE Amplifier Concept
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DiPOLE Cryo-Cooling System
Amp Head
FanHeat Ex-
changer LN2 in
N2 out
ControlValves
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DiPOLE Cryo-Cooling System
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DiPOLE Cryo-Cooling System
Parameter Typical Range/Design
Working temperature 150 K 90 K – 300 K
Heat load <400 W > 1kW
He flow rate 30 g/s < 40 g/s
He pressure 10 bar 2 bar – 20 bar
Stability +/- 0.2 K +/- 0.5 K
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Current + Future systems
DiPOLE Prototype• 100 W laser power• 400 W heat load• 30 g/s flow
HiLASE Contract• 1 kW laser power• 5 kW heat load• 130 g/s flow
XFEL HED Project• Similar HiLASE• Possibly combined
0.5kW / 5kW • OJEU tender soon..
Outlook• More projects on the horizon, other groups active too• Towards more compact, ruggedized systems practical applications• Away from LN2, towards closed-cycle cooling
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Acknowledgement
Tom Bradshaw, Chris Densham, Mike Fitton, Tristan DavenneSTFC Rutherford Appleton Laboratory, Technology Department
Thanks for listening!