Masaki Ando, Akiteru Takamori, Kimio Tsubono Department of Physics, University of Tokyo Earthquake Research Institute, University of Tokyo 1st International LISA-DECIGO workshop (Nov , 2008 ) Development of a Low-Frequency Gravitational-Wave Detector Using Magnetically-Levitated Torsion Antenna Collaborator Koji ISHIDOSHIRO (University of Tokyo)

Outline Ground-based low-frequency GW detector realized by Magnetically-Levitated Torsion Antenna  Background and purpose  Detection principle  key technology (Magnetic Levitation)  Prototype experiments  Summary Table of Content Prototype tests Study basic ideas and fundamental noises

Background and purpose  Low-frequency GWs Large amplitude, Interesting sources Supermassive Black Holes and Inflation of the Universe  GW detectors Ground-based Interferometers Test mass suspended to be free mass No sensitive to GWs under resonant frequency of suspension (~1Hz) Space Interferometers Almost free mass Not easy Background Purpose  Early implement a ground-based low-frequency GWs detector  Detect GWs, or set upper limits

Detection principle Torsion antenna Bar-shaped test mass Tidal force of GWs induce rotation of test mass GW signals are detected from rotation measurement Tidal force of x-polarized GWs Rotation x y z Equation of motion : shape factor typical ~ 1 Free rotation without loss Torsion antenna have fundamentally sensitive to low-freq. GWs

Key Technology Magnetic-Levitation Pinning effect between a Permanent magnet (PM) and a superconductor magnet (SCM) Test mass PM SCM Difficulty to get free rotation without loss Restoring force with loss in its rotational degree freedom In principle  Stable levitation  Free rotation without loss in it rotational degree freedom Fiber suspension

Prototype experiment Superconductor magnet Gd-Ba-Cu-O φ60mm,t20mm Critical temp. 92K Cryo-Cooler Pulse-tube (low-vibration) Lowest Temp. 62K Vacuum Maintained at ~10 -3 Pa by turbo pomp Experimental setup Purpose Study basic ideas and fundamental noises  Practical loss and spring constant  Rotational stability Superconductor magnet 36cm 120cm Optical table Mirror for interferometer Torsion Antenna Permanent magnet Pulse-tube Cryo-cooler Valve unit From Ando ’ s talk at amaldi7 30cm Test mass Superconductor magnet Cryo-Cooler

Measured loss factor and spring constant Loss factor: Rotate the levitated PM + columnar test mass Monitor the rotational speed by a reflective photo sensor ⇒ Exponentially decay -> Loss factor Spring constant: Stop the levitated PM + columnar test mass Monitor the resonance rotational Resonance frequency -> Spring Constant PM Columnar test mass Mark for measurement Top view Bottom view Results Methods Time [sec] Rotation speed Measurement Fitting

Thermal noise limits Loss factor Fundamental torque noise

Measured rotation noises PD BS Laser AOM EOM FI Torsion antenna Measurement Michelson interferometer is used for rotational sensor Rotational noises are calibrated from feedback signals

Measured rotation noises Aluminium Mass ： 145 g Momentum:1.3x10 -3 kg m 2 Permanent magnets for actuator Nd φ1mm,t5mm Mirrors for interferometer Permanent magnet Nd Φ22mm, t10mm Torsion antenna 39cm 30cm

Measured rotation noise Results 3x10 -8 [rad/Hz 1/2 h~3x10 -8 [1/Hz 1/2 For optimal polarized GWs

Noise analysis Comparison with coupled noises from Seismic motion Coupling model 2-d simple rigid-body pendulum Misalignment (1mm) suspension center and gravity center Coupled noises are not negligible More precision analysis is required Worst case

Summary Ground-based low-frequency GW detector Key technology : Magnetic-levitation (pinning effect) Prototype antenna γ=2x [m 2 kg/s], κ=7x10 -8 [Nm/rad] 3x10 -8 [rad/Hz 1/2 Study basic ideas and fundamental noises  Loss and spring constant  Stability of magnetic levitation Thermal noise limits 1.3x [1/Hz 1/2 may be limited by coupled noises from seismic motion More precision noise analysis is progressing  Free rotation without loss  Stable levitation