C-1 Thermal properties of laser crystal

Report
Thermal properties of
laser crystal
SuperKEKB Injector Laser RF Gun Review
Rui Zhang
ACCL Division V, RF-Gun Group
Feb 20, 2015
Outline
• Introduction to current Yb laser system of SuperKEKB
injector laser RF gun
• Au-Sn soldering Yb:YAG/Cu bonding composite
– AuSn soldering material
– Soldering components
• Thermal effect measurement
• Summary
Thermal properties of laser crystal
Introduction to current laser system
1 nJ @ 1035 nm
Yb-doped fiber
oscillator
51.9 MHz
Synchronization
system
Yb-doped
fiber pre
amplifier
Yb-doped fiber
main amplifier
Transmission
Grating
Stretcher
Yb-doped fiber
pre amplifier
EO Pulse
picker
double
Yb-doped fiber
amplifier
3rd stage
2856 MHz trigger
From Accelerator
RF GUN
SHG
Yb:YAG thindisk 6-pass
amplifier
SHG
Yb:YAG thindisk 5-pass
amplifier
Yb:YAG thindisk 5-pass
amplifier
Yb:YAG thindisk 1-pass
amplifier
Yb:YAG thindisk 2-pass
amplifier
5th stage
4th stage
Introduction to current A1 laser system
AuSn soldering material
Au-Sn phase diagram
• Eutectic point at 278±2 ºC
• L ↔ [ζ + δ]
• Eutectic components:
Au5Sn and AuSn
For AuSn solder material:
• No need for flux
• High thermal conductivity
and low thermal expansion
coefficient
• High resistant to corrosion
and oxidation
• High creep resistance
Comparison between AuSn, InSn, Cu and Yb:YAG
Melting point (ºC)
Thermal conductivity (W/m/K)
Thermal expansion coefficient (10-6/K)
AuSn (80:20)
280
58
16
InSn (50:50)
118
34
20
Copper
396
16.4
Yb:YAG
11
6.7
Au-Sn soldering Yb:YAG/Cu bonding composite
Soldering components
Coated Yb:YAG laser crystal disk
• Diameter: 1/2 inch (12.7
mm)
Under microscope
Top surface
• Thickness: 0.5 mm
• Yb-ions dopant: 10 a.t.%
• Top surface: AR @ 1030
nm and 940 nm
Bottom surface
• Bottom surface: HR @
1030 nm and 940 nm, Crcoating and Au-coating
Au-Sn soldering Yb:YAG/Cu bonding composite
Soldering components
Au-Sn layer
Bottom surface
Copper plate
• Diameter:
(12.7 mm)
1/2
inch
• Thickness: 2.0 mm
• Au-Sn layers on top
surface: Au-80% and
Sn-20%
Au-Sn layer after
soldering
Au-Sn soldering Yb:YAG/Cu bonding composite
Soldering fixtures
• Spring (force constant is 0.05 Kg/mm)
• Spring’s deformation length: 10 mm
• Total pressure: 1.21 kg/cm2
• Vacuum chamber’s vacuum degree: 5.2×10-5
Pa
• Polyimide is used for protesting the AR
coating
Au-Sn soldering Yb:YAG/Cu bonding composite
Yb:YAG/Cu bonding samples analysis
Yb:YAG/Cu bonding
composite soldered at 310 ºC
500 μm
16 μm
Side view. The thickness of bonding
layer is ~16 μm
Damaged
samples
Au-coating on the bottom surface of
Yb:YAG disk was damaged partly
AR-coating on the top surface of
Yb:YAG disk was damaged partly
Au-Sn soldering Yb:YAG/Cu bonding composite
Optical measurement experimental setup
He-Ne laser:
• Wavelength: 632.9 nm
• Power: 1.2 mW
• Mode structure: TEM00>99%
• Enlarged beam diameter: 5 mm
Pump laser:
• Pump LD peak power: 4 kW
• Pulse width: 600 μs
• Repetition rate: 5 Hz
Thermal effect measurement
Measurements for soldering quality
Before soldering
After soldering
Wave front interference
Wave front interference
Beam profile
Beam profile
Thermal effect measurement
Thermal effect measurement 1
Old design (without soldering)
Wave front
without pump
Wave front under 450
mJ pump
Wave front under 630
mJ pump
Wave front under 840
mJ pump
New design
Thermal effect measurement 2
Old design (without soldering)
Wave front
without pump
Wave front under
950 mJ pump
Beam profile
without pump
Beam profile under
950 mJ pump
New design
Summary
• Soldering component
Fundamental soldering technology
– Copper plate with excellent surface quality
– Soldering for bigger size laser active medium
• Thermal effect measurement
Available measuring setup
– Measurement under high repetition rate pump (50 Hz)
The End

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