Magnetic coupling noise in a
Torsion-bar Antenna for
Gravitational wave Observation
Department of physics,
Graduate school of science,
University of Tokyo
Kenshi Okada
Abstract
Torsion-bar Antenna (TOBA)
→ Sensitive to low-frequency (0.1-1 Hz) GWs on the ground
A sensitivity of h ～ 10-19 at 0.1-1 Hz could be realized using
10m scaled TOBA
Prototype detectors (test mass : 20cm) using superconductive
magnetic levitation are developed
Superconductor
For demonstration and noise evaluation
→ the noise level is limited by
magnetic noise and seismic noise
Next step is to upgrade the detector
(ex. magnetic shield)
Magnet
Magnetic field
Test mass
Torsion-bar Antenna (TOBA)
Low frequency (0.1–1 Hz) GWs are very important targets
for cosmology and astronomy (ex. stochastic GW background)
・ Ground based Laser-interferometric detector
Not sensitive below 10 Hz
・ Space-borne mission
Requires huge resources and time
So we propose...
New type of gravitational-wave detector
for low-frequency (0.1–1 Hz) GW on the ground
Torsion-bar antenna (TOBA)
M. Ando et al, 2010, Phys. Rev. Lett., 105, 161101
Lower cost compared to space crafts
Able to repair and upgrade on the ground
Principle of Torsion-bar Antenna
Interferometric detector
z
Torsion-bar Antenna
GW
z
GW
y
y
mirror
mirror
Laser
mirror
x
mirror
Torsion bar
photodetector
Detect differential length
change
Detect rotation
The rotation is read by
an interferometer
x
Principal sensitivity of TOBA
Sensitivity of 10 m scaled TOBA (example)
M. Ando et al, 2010, Phys. Rev. Lett., 105, 161101
Bar length :10m,
Mass : 7600 kg
Bar Q-value : 105 ,
Temp : 4K
Support Loss : 10-10
Read-out by Fabry-Perot
Interferometer
Laser source :
1064 nm, 10W
Cavity length : 1 cm
Finesse : 100
Suspension of the test mass :
Strong force for suspension and
low support loss are needed
→ Superconducting magnetic levitation
is used in prototype detectors
Overview of prototype detector
Prototype detector using superconducting magnetic levitation
For demonstration and noise evaluation
Test mass
Pulse tube cyrocooler
Support made of low
thermal-conductivity
Inverted T-shape
Bar length : 200 mm
・ Feedback
→ coil-magnet actuator
・ Readout of rotation
→ Michelson interferometer
Wavelength : 1064 nm
Output : 40mW
600 mm
Two prototype detectors are
developed in Tokyo and Kyoto
Superconducting magnetic levitation
Pulse tube cryocooler
Lowest achieved
temperature : 63 K
Superconductor
Pinning effect
Almost zero gradient of
magnetic force around
the rotational axis
Gd-Ba-Cu-O
Φ 60 mm, t 20 mm
Critical temperature
: 92 K
Superconductor
Magnet
Magnetic field
Test mass
Test mass
Magnet
Nd φ22mm×10mm
Mass：131 g
Bar length：20 cm
Moment of inertia
：3.25x10-4 kg m2
ADVANTAGES :
・ Strong levitation force
・ Low resonant frequency
・ Low thermal noise
Prototype detector
Acoustic shield
To prevent the effects of vibration of air to
the Laser and the vacuum chamber
Pulse tube cryocooler
Superconductor
Test mass
Laser
Interferometer
Vacuum chamber
10-3Pa
Current detector noise
GW strain-equivalent noise level
2010 7/20 3:00-4:00
Current best sensitivity：
4×10-9 Hz
-1/2
at 0.5Hz
superconductor
Tidal force of GW
→ rotation
Test mass
Test mass is levitated by magnetic force
→ External magnetic field can be a source of
a detector noise
Origin of the magnetic coupling noise
Magnet
Nd, φ22×10
mZ
Magnetic
moment
M
Test mass
mY
mX
The magnetic moment is supposed
to be almost vertical, but it still
has horizontal magnetic moments
Vacuum chamber
By
N
mx
Test mass
Y
The horizontal magnetic moments
are coupled with the external
magnetic field to make torque on the
test mass
→ Magnetic coupling noise
X
Estimate of the horizontal magnetic moments
The horizontal magnetic moments are evaluated by using
Magnetic sensors and coils
Magnetic sensor
Coil
Magnetoresistive element
Resistance
45 Ω
HMC1002,
Honeywell
Inductance
40 mH
Resolution : 3 ×10-9 T
Coil
Coil
B(f)
Testmass
400 mm
The measured magnetic moments :
m x = (3 ±1)×10-3 A・m2
m y = (6 ±2)×10-3 A・m2
( m z = 2 A・m2 )
Evaluation of the magnetic coupling noise
GW strain-equivalent noise level
Spectra of the external magnetic field
Magnetic coupling noise
Magnetic coupling noise is estimated by
the horizontal magnetic moments and the external magnetic field
Evaluated noises
Magnetic coupling noise
Current noise level is limited by
Thermal noise
Laser frequency
noise
Laser
intensity noise
Seismic noise
magnetic coupling noise
(below 0.4 Hz)
and
seismic noise (above 0.4Hz)
Noise of PD
Observation
5 hours simultaneous observational run
by two detectors (in Tokyo and Kyoto) on July 20, 2010
→ Poster presentation by Ayaka Shoda
Upgrade of the prototype detector
For further feasibility study of 10m scaled TOBA
・ Magnetic shield
→ 60db reduction of the magnetic coupling noise
・ Move to a place with lower seismic noise (ex, Kamioka)
→ 40db reduction of the seismic noise
・ Control of horizontal
fluctuation of test mass
→ 20db reduction of
the seismic noise
・ Larger test mass
(20 cm →40 cm)
→ 20db reduction of thermal noise
→ 6db improvement of sensitivity
10-12 in GW sensitivity around 0.5 Hz is expected
Summary
Torsion-bar Antenna enables us to search for Low-frequency
GWs (0.1 – 1 Hz) on the ground
Goal : sensitivity of h ～ 10-19 at 0.1-1 Hz (10m scaled TOBA)
Prototype detectors using superconductive magnetic
levitation are developed
Feasibility and noise characteristics are studied
→ Current sensitivity is h ～ 10-9 around 0.5 Hz
and limited by magnetic noise and seismic noise
Upgrade of the prototype is planned
The expected sensitivity : h ～ 10-12 around 0.5 Hz
End
low-frequency Gravitational wave
Low frequency (0.1 – 1 Hz) gravitational waves are very important
target for cosmology and astronomy
• A stochastic GW background
• Mergers of massive black holes
• Pulsers
(0.1 Hz-1 kHz)
Laser-interferometric detector
Space-borne mission
LISA（ESA, NASA）
After 2018
LIGO, VIRGO,
GEO,
TAMA300, LCGT
DECIGO(JAPAN)
ＬＩＧＯ
Not sensitive below 10 Hz
Hopefully around 2027
ＬＩＳＡ
Sensitive in low-frequency
But we need much time, costs, risks..
Low-frequency Gravitational wave observaton
・ Laser-interferometric detector
LIGO, VIRGO, GEO,
TAMA300, LCGT
Not sensitive below 10 Hz
ＬＩＧＯ
・ Space-borne mission
LISA（ESA, NASA）
DECIGO(JAPAN)
After 2018
Hopefully around 2027
No gravity→ mirror is free
No seismic noises
→Sensitive in low-frequency
But we need time, costs, risks…
ＬＩＳＡ
Evaluation of the magnetic coupling noise
Coil
Coil
B(f)
The horizontal magnetic moments are evaluated by
using magnetic sensors (Magnetoresistive element :
HMC1002, Honeywell) and coils
Testmass
m x = (3 ±1)×10-3 A・m2
m y = (6 ±2)×10-3 A・m2
( m z = 2 A・m2 )
GW strain-equivalent noise level
Spectra of the external magnetic field
Magnetic coupling noise
Principle of Torsion-bar Antenna
Tosion-bar Antennaz
GW
Resonant frequency : 5 mHz
→Sensitive for GW above 5 mHz
y
Equation of motion
x
: Moment of inertia
γ ：Damping constant
Detect the rotational
fluctuation of the testmass
Tidal force of GW
→ rotation
κ ：spring constant
：Dynamical quadrapole
moment
：Amplitude of GW
Above the resonant frequency,
Transfer coefficient : α ～1/2
Test mass
Results from one detector
8 hours observational run by two
detectors
on August 15, 2009
Upper limit on the GW
background
Ωgw(0.2 Hz) = 3.6 ×1017
Superconductor
Magnetic field
Magnet
by K Ishidoshiro
Test mass
How to improve the sensitivity?
・ Noise reduction
Magnetic shield, move to place with smaller seismic noise
・ Simultaneous observation
But if we use two detectors, we will enhance the
sensitivity according to the observation time
・ Bigger detector
Simultaneous observation
Tokyo
Timing: GPS module (Garmin GPS 15xL)
Distance : 510 km
Kyoto
Cross correlation analysis of 5 hours
Run will make a 102 times stricter limit
On stochastic gravitational wave
background
Coming soon
Optics
Vacuum chamber
Torsion bar
LASER
EOM
BS
Interferometor
Laser： wavelength1064nm YAG laser
４０ｍW
Rotation of the test mass → Output of PD changes
PD