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Scattering of the ITM and ETM mirrors and constraints for the Wide-Angle Baffles in bKAGRA
Simon ZEIDLER*, Tomotada AKUTSU
NAOJ, AOS
f2f KAGRA meeting at the University of Niigata, March 2017
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● Baffles to be installed close to the ITM and ETM mirrors
● To block scattering from interior
● Simulations to find most effective design
3 km cavity
mirror WAB
recoil mass
Design: cryopayload-group, KEK
Why do we need wide-angle baffles (WAB)
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Simulating back-scattering from the interior of the cryostat
● Simulation tool: “LightTools”
● 108 rays per simulation run
● Mirror’s scattering distribution calculated with multi-layer scattering theories
● Specular reflection not included
● Result: scattering map as a function of L (longitude) and V (latitude)
Back-scattering
V
LSapphire Mirror
Back-scattering
Back-scattering
Main beam
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→ surface roughness (100ppm) + point defects (30ppm)
coating design: Hirose-san
Scattering distribution on coated sapphire mirror
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Results without WAB
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Simulation results of assembly-frame scattering
Pbsc
/Pscat
~ 0.043 %
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Pbsc
/Pscat
~ 0.1 %
Simulation results of inner-shield scattering
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Results with WAB
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Pbsc
/Pscat
~ 0.1 % → ~ 0.013 %
Pbsc
/Pscat
~ 0.043 % → ~ 0.003 %
Change of back-scattered power with WAB
WAB: length – 300 mm diameter – 280 mm radiation disk – 210 mm from
mirrorDesign: Obuchi-san (ATC, NAOJ)
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Impact on strain-noise of KAGRA
Inner Shield
Assembly Frame(no vibration-noise data)
KAGRA Sensitivity
Vibration noise: Ochi-san
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Impact on strain-noise of KAGRA
Inner Shield
KAGRA Sensitivity
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Simulation results with WAB
Inner shield
Assembly frame
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Elongation of WAB brings better results
300 mm → 400 mm
Pbsc
/Pscat
~ 1.5 x 10-7
Pbsc
/Pscat
~ 3.3 x 10-5
Inner shield
Assembly frame
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Elongation of WAB brings better results
400 mm → 546 mm (maximum elongation)
Inner shield
Assembly frame
Pbsc
/Pscat
~ 1.5 x 10-7
Pbsc
/Pscat
~ 10-10
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Strain-noise change for 400 mm WAB
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Strain-noise change for 546 mm WAB
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Elongated WAB in the cryostat
● Separated assembly frame for baffle suspension
● Issue: installation alongside the payload installation
Design: Obuchi-san (ATC, NAOJ)
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● Simulations on scattering inside the cryostats done under conditions close to reality (scattering distribution, interior, etc.)
● Influence of back-scattering on the strain noise highest for “inner shield”
● Need vibration-noise data for assembly frame (ongoing)
● Decreasing of strain noise possible but WAB needs to be longer than initial design
● Coating of baffle (Solblack) not included in simulations
● Influence from Solblack-coating: Pbsc
/Pscat
~ 1.7 x 10-9
Summary
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recoil mass (Ti)
main mirror
Simulation results of recoil-mass scattering
Pbsc
/Pscat
~ 0.027 %