Behavior of an inverted pendulum in the Kamioka mine R. Takahashi (NAOJ), A. Takamori (ERI), E. Majorana (INFN) GWADW 2010 We are investigating behavior of an inverted pendulum (IP) in the Kamioka mine where is the site of LCGT. IP is employed as pre-isolator of the seismic attenuation system (SAS) for LCGT.

2 IP Seismic Attenuation System (SAS) for LCGT

Contents 1.Unsealed IP with a geophone a.Setup of apparatus b.Geophone c.Results d.Ground tilt 2.Sealed IP with a small accelerometer a.Setup of apparatus b.Accelerometer c.Results d.Vertical ground vibration 3.Summary

1. Unsealed IP with a geophone Aug , site One-leg IP Geophones (Horizontal 2, Vertical 2)

1-a. Setup of apparatus Load (50mHz) Geophone (horizontal) Geophone (vertical) “Roughly” enclosed to avoid air flow Flex joint 1. Unsealed IP

1-b. Geophone Vertical Velocity response of 1Hz geophone (L-4C) Ground 1 Ground 2 Differential Sensitivity limit by CMR ： 5x m/Hz It is possible to sense the level less than 1/10 of ground vibration in 0.2-2Hz. 1. Unsealed IP

1-c. Results (1) Ground IP Noise limit Horizontal 1. Unsealed IP The displacement of IP reached the sensitivity limit at 3Hz. The displacement of IP was larger than ground vibration around 0.2Hz.

1-c. Results (2) Ground IP 1st IP 2nd IP 3rd Ring down of IP Q~35 Prediction settled unchanged The excess around 0.2Hz is not due to harmonics of fundamental mode. 1. Unsealed IP What is this gap?

9 We supposed that the excess is due to ground tilt. IP is very sensitive to ground tilt. IP with natural frequency of 50mHz corresponds to 100m pendulum. Actually Virgo group pointed out ground tilt is dominant source of the IP displacement in the SA at frequencies less than 0.1Hz. 1-d. Ground tilt (1) 1. Unsealed IP

Blue line ： Ground tilt supposing that displacement of IP is due to ground tilt Red line ： Ground tilt estimated by differential signal between two vertical geophones The excess around 0.2Hz in IP spectrum is not due to ground tilt. The excess is due to residual air flow? 1-d. Ground tilt (2) 1. Unsealed IP

Jan & Feb , Sealed IP with a small accelerometer Small accelerometer

12 2-a. Setup of apparatus 2. Sealed IP

f 0 =1.98Hz Q=20 60mm 65mm Folded pendulum MakerUFT MaterialA7075 Size60 x 30 x 65 Hinge20  m 2-b. Accelerometer (1) A servo type accelerometer consist of this pendulum and a position sensor. 2. Sealed IP

2-b. Accelerometer (2) Commercial photo sensor 2. Sealed IP

2-c. Results Ground IP Though measured displacement was close to sensitivity limit, displacement of IP was still larger than ground vibration around 0.2Hz. Natural frequency of IP was tuned to 70mHz. Ground IP 2. Sealed IP Prediction (Q=69) Ground IP Notch at 2Hz is due to the resonance of the folded pendulum.

2-d. Vertical ground motion Vertical ground vibration had not correlation with IP displacement. IP resonance is excited by vertical vibration? (out of sensitive zone) 2. Sealed IP Sensitive zone for both sensor

3. Summary We investigated behavior of an IP in the Kamioka mine. We investigated behavior of an IP in the Kamioka mine. The displacement of IP was larger than ground vibration around 0.2Hz. The displacement of IP was larger than ground vibration around 0.2Hz. The excess is not harmonics of fundamental mode of the IP. The excess is not harmonics of fundamental mode of the IP. The excess is not due to ground tilt. The excess is not due to ground tilt. The excess is not due to vertical vibration directly. The excess is not due to vertical vibration directly. Though we could not specify the origin, it might be non- linear vertical coupling. Though we could not specify the origin, it might be non- linear vertical coupling. We need more investigations. We need more investigations.

Long term stability 2/24 16:22 – 2/25 7:53 IP was stable without glitch for 15.5 hours.

IP tilt meter The drift of ~0.5  rad was observed for 15.5 hours. The application of IP to tilt meter may be useful. Since 70mHz IP corresponds to 50m pendulum. Ground tilt→Displacement of IP→Tilt of accelerometer→DC signal of accelerometer

Measured transfer function of IP Measured transfer function from force to displacement of the SAS in TAMA Measured isolation ratio of the SAS prototype (IP was tuned to 220mHz.)

21 Displacement of IP in TAMA Controlled by LVDT with different band width Measured by ACC Broad band: following the ground motion Narrow band: free at > natural frequency (~0.05Hz) IP motion > ground motion at 0.16Hz