1. Composite mirror suspensions

2. The idea
A fresh approach to the design of low thermal noise cryogenic suspensions for KAGRA’s mirrors

3. Key features: Composite structure Compressive joints Silicon flexures
Machined Sapphire ribbons
Silicon springs
Micron Indium gaskets
Defects etched away

4. Key technologies: Ultrasound machining of sapphire
Magneto-hydro-dynamic polishing
Silicon etching to eliminate defects
Compressed Indium connections

5. Attachment to mirror Mini shelves machined inside the mirror sides
Ultra-sound machining
Advantages:
Minimal volume of disruption of mirror
much less material machined off
shelves in the profile of mirror are less vulnerable
Mechanically more rigid
Elimination of silicate bonding (any bonding)

6. Examples of USM

7. Flexure structure Machined Double arrow structure Thin, etched flexure
Short aspect ratio
Large thermal conductance

8. Flexure structure Etching of the surface
Expected to increase the break point
Expected to increase quality factor.

9. Flexure structure
Contact surface perpendicular to tension eliminates shear stress
Eliminates need for adhesion
Micron thickness Indium gaskets impedes creak
Vanishing thermal resistance

10. Flexure structure Indium gaskets advantages
In case of breakage, disassemble by simply warming the mirror
Easy replacements
Scalable to larger sizes !

11. Chao Shiu laboratory, Taiwan Silicon cantilever with KOH wet etching
4” un-doped double-side polished (001) silicon wafer, 500um thickness
Etching
from residual gas
44.35 mm
10 mm
92 μm or 52 μm
34 mm
0.35mm
500 μm
5.5mm
Amplitude(V)
Original Data
Decay Time = (s)
φmeasurement = 4.3*10-6
Frequency = (Hz)
Silicon cantilever (d=92um)
Silicon cantilever (d=52um)
Amplitude
Frequency=59.04 (Hz)
Decay Time = (s)
φmeasurement = 1.4*10-6
Time(sec)

12. Flexure Q-factor ~106
Extremely large q-factors after dilution for pendulum
Extremely good thermal noise even at room temperature

13. Sapphire ribbon structure
Ultra-sound machined from high quality monocrystal
0.2 micron surface finish from ultra-sound machining
Magneto-hydro-dynamical polishing to reflect phonon and increase thermal conductance

14. Sapphire ribbon structure
Thin aspect ratio possible to reduce violin mode frequency
Problem: rigid transversally
May need cross flexure step

15. Etched Silicon cantilever blades
Etch the bending area
Leave thick section for clamping and for fiber connection
Up to 0.2 GPa measured for polished wafers
Only limited flexure possible

16. Larger stresses possible?
MEM sensors operating at 1.4 GPa, ~ 10 times higher limit!
Is etching eliminating surface defect and therefore causing the larger strength?
if YES, large bands possible!
Lower frequency bounce modes

17. NIKHEF test
Produce a number od samples
Test and see

18. Conductance budget
Preliminary conductance budget from Sakakibara with 1 W load
Thin ribbon responsible for bulk of loss ! ! !
Plenty of space for parametric optimization