1. Mechanical loss measurement for KAGRA sapphire suspension system Takafumi Ushiba Department of Physics, The University of Tokyo The first international meeting on KAGRA KISTI, Daejeon, Korea, June 24-25, 2016

2. KAGRA 3-km arm interferometric gravitational detector. Using underground site Cooling mirrors Mirrors are suspended by the “type-A” suspension system. (room-temperature vibration isolation system + cryogenic payload)

3. Cryogenic payload Four-stage pendulums at the bottom of the type-A suspension system Platform Marionette Intermediate mass Test mass Room-temperature suspension Platform ―Vibration isolation from cryocoolers Marionette ―Rough tilt adjustment Intermediate mass ―Controlling test mass Below the intermediate mass, the test mass and fibers are made of sapphire. →sapphire suspension system

4. Sapphire suspension system Suspension system for test masses. High thermal conductivity Low thermal noise → high mechanical Q-factor Blades are for absorbing fiber-length difference. Ears are for hanging fibers on the mirror. The blade and fiber are bonded by using Indium. The ears are attached to the mirror by using HCB.

5. Mechanical Q-factor measurement

6. What is a mechanical Q-factor? An index of the sharpness of resonances. frequency Displacement

7. Requirements are calculated by Dr. Dan Chen. Measured Not so severe requirement Mechanical loss requirement Not yet

8. Sapphire blade Image of a sapphire blade Thick part ( ～ 4 mm) Thin part ( ～ 2.5 mm) Absorbing fiber-length difference →should be soft mechanically Thinner is better Longer is also better Also should be strong sufficiently The blade is turning-back shape with some wedge due to the space. At the turning point, the blade becomes thick, because the stress is gathering at this part. At the clamping and hanging areas, the blade becomes thin.

9. Purpose of measurement 1. Checking that a Q-factor of a sapphire blade is large enough for the KAGRA suspension system. ―A sapphire Q-factor is very large, but there are some concerns. Complex shape Using some machining process 2. Constructing the measurement setup that can measure over 10 6 of a Q-factor of sapphire blades. ―A quality of all sapphire parts should be checked before installation. →Preparation for high Q-measurement setup is important.

10. Setup diagrum Laser Cryostat Blade QPD Electroad High voltage amplifier Function generator ComputerFilter Signal LabVIEW system SR560 Control Signal AC voltage DC+AC voltage Using lock-in method

11. Setup photo Electroad Blade Clump 70 mm 52 mm The blade is clumped by cupper blocks with M4 screws. The electroad is bonded to the alminum plate with cryogenic glue, and the Al plate is fixed to the Cu block.

12. Resonant search Screen shot of a LabVIEW control window. Resonant frequency: about 471.9 Hz Signal is very clear(10 times larger than the noise floor). 471.9 471.74 472.06

13. Method for Q-measurement fitting Measured values We measure 11 times in order to reduce the statistical error.

14. Result Measurement No.Q-factor 12.55×10 4 21.65×10 4 These measurements were performed at around 8 K. Both Q-factors are averages of 11-times measurement. Measurement No.1 and No.2 is using the same setup, but all parts were removed and rebuilt between No.1 and No.2. Measured Q-value is about 100 times smaller than our requirement.

15. Discussion and future work Possibility Energy loss at the cupper clamp is large. ―Measure different mechanical mode. ―Reclamp the blades and measure, again and again. Intrinsic Q-factor of a blade is small. ―Anneal the blade. ―Use another blade. We don’t know which is a real problem. So, both trials are planned to be performed.

16. Summary We measure a Q-factor of the sapphire blade. Measured value is about 100 times smaller than our requirement. There are several possibilities: ―Clamp loss ―Intrinsic loss Now, we are trying to confirm why a Q-factor is small.