Thermal properties of laser crystal Rui Zhang ACCL Division V, RF-Gun Group Feb 20, 2015 SuperKEKB Injector Laser RF Gun Review

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 Outline Thermal properties of laser crystal

Yb-doped fiber oscillator Yb-doped fiber pre amplifier Yb-doped fiber amplifier Transmission Grating Stretcher EO Pulse picker double Yb:YAG thin- disk 6-pass amplifier Yb:YAG thin- disk 5-pass amplifier SHG Synchronization system 2856 MHz trigger From Accelerator SHG RF GUN 51.9 MHz nm Yb-doped fiber main amplifier Yb:YAG thin- disk 5-pass amplifier Yb-doped fiber pre amplifier Yb:YAG thin- disk 2-pass amplifier Introduction to current laser system Yb:YAG thin- disk 1-pass amplifier 3 rd stage 4 th stage 5 th stage Introduction to current A1 laser system

AuSn (80:20)InSn (50:50)CopperYb:YAG Melting point (ºC) Thermal conductivity (W/m/K) Thermal expansion coefficient (10 -6 /K) Comparison between AuSn, InSn, Cu and Yb:YAG Au-Sn phase diagram Eutectic point at 278±2 ºC L ↔ [ζ + δ] Eutectic components: Au 5 Sn 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 Au-Sn soldering Yb:YAG/Cu bonding composite AuSn soldering material

Coated Yb:YAG laser crystal disk Diameter: 1/2 inch (12.7 mm) Thickness: 0.5 mm Yb-ions dopant: 10 a.t.% Top surface: 1030 nm and 940 nm Bottom surface: 1030 nm and 940 nm, Cr- coating and Au-coating Top surface Bottom surface Under microscope Soldering components Au-Sn soldering Yb:YAG/Cu bonding composite

Copper plate Diameter: 1/2 inch (12.7 mm) Thickness: 2.0 mm Au-Sn layers on top surface: Au-80% and Sn-20% Bottom surface Au-Sn layer Au-Sn layer after soldering Au-Sn soldering Yb:YAG/Cu bonding composite Soldering components

Spring ( force constant is 0.05 Kg/mm ) Spring’s deformation length: 10 mm Total pressure: 1.21 kg/cm 2 Vacuum chamber’s vacuum degree: 5.2×10 -5 Pa Polyimide is used for protesting the AR coating Soldering fixtures Au-Sn soldering Yb:YAG/Cu bonding composite

Yb:YAG/Cu bonding composite soldered at 310 ºC Side view. The thickness of bonding layer is ~16 μm 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 Damaged samples 500 μm 16 μm Yb:YAG/Cu bonding samples analysis Au-Sn soldering Yb:YAG/Cu bonding composite

Pump laser: Pump LD peak power: 4 kW Pulse width: 600 μs Repetition rate: 5 Hz He-Ne laser: Wavelength : nm Power: 1.2 mW Mode structure: TEM 00 >99% Enlarged beam diameter: 5 mm Optical measurement experimental setup Thermal effect measurement

Before soldering After soldering Wave front interference Beam profile Thermal effect measurement Measurements for soldering quality

Old design (without soldering) New design Wave front without pump Wave front under 450 mJ pump Wave front under 630 mJ pump Wave front under 840 mJ pump Thermal effect measurement 1

Old design (without soldering) New design Wave front without pump Wave front under 950 mJ pump Beam profile under 950 mJ pump Beam profile without pump Thermal effect measurement 2

The End 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)