Masaki Ando, Akiteru Takamori, Kimio TsubonoDepartment of Physics, University of TokyoEarthquake Research Institute, University of Tokyo

1st International LISA-DECIGO workshop (Nov. 12-13, 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 testsStudy basic ideas and fundamental noises

Background and purpose

Low-frequency GWsLarge amplitude, Interesting sources

Supermassive Black Holes and Inflation of the Universe

GW detectorsGround-based Interferometers

Test mass suspended to be free mass

No sensitive to GWs under resonant frequencyof suspension (~1Hz)

Space InterferometersAlmost free mass

Not easy

Background

Purpose Early implement a ground-based low-frequency GWs detector Detect GWs, or set upper limits

Detection principleTorsion antenna

Bar-shaped test massTidal 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-LevitationPinning 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. 92KCryo-Cooler Pulse-tube (low-vibration) Lowest Temp. 62KVacuum Maintained at ~10-3 Pa by turbo pomp

Experimental setup

PurposeStudy basic ideas and fundamental noises Practical loss and spring constant Rotational stability

Superconductormagnet

36cm

120cm

Optical table

Mirrorfor interferometer

Torsion Antenna

Permanentmagnet

Pulse-tubeCryo-cooler

Valveunit

From Ando’s talk at amaldi7

30cmTest mass

Superconductormagnet

Cryo-Cooler

Measured loss factor and spring constant

Loss factor:Rotate the levitated PM + columnar test massMonitor the rotational speed by a reflective photo sensor

⇒Exponentially decay -> Loss factor

Spring constant:Stop the levitated PM + columnar test massMonitor the resonance rotational

Resonance frequency -> Spring Constant

PM

Columnar test mass

Mark for measurement

Top view

Bottom view

Results

Methods

Time [sec]

Rota

tion

sp

eed

MeasurementFitting

Thermal noise limitsThermal noise limits

Loss factor

Fundamental torque noise

Measured rotation noises

PD

BS

Laser

AOM

EOMFI

Torsionantenna

MeasurementMichelson interferometer is used for rotational sensorRotational noises are calibrated from feedback signals

Measured rotation noises

Aluminium Mass ： 145 gMomentum:1.3x10-3 kg m2

Permanent magnetsfor actuatorNd φ1mm,t5mm

Mirrorsfor interferometer

Permanent magnetNd Φ22mm, t10mm

Torsion antenna

39cm

30cm

Measured rotation noiseResults

3x10-8[rad/Hz1/2] @200mHz

h~3x10-8[1/Hz1/2] @200mHz

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=2x10-10[m2kg/s], κ=7x10-8[Nm/rad]

3x10-8[rad/Hz1/2]@200mHz

Study basic ideas and fundamental noises Loss and spring constant

Stability of magnetic levitation

Thermal noise limits 1.3x10-12[1/Hz1/2] @200mHz

may be limited by coupled noises from seismic motion

More precision noise analysis is progressing

Free rotation without loss Stable levitation