Large-scale Cryogenic Gravitational-wave Telescope, LCGT
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Large-scale Cryogenic Gravitational-wave Telescope, LCGT. Keiko Kokeyama University of Birmingham. 23 rd July 2010 Friday Science. General introduction of LCGT project Introduction of gravitational waves (GWs) Introduction of LCGT Science goal and impact Technical features

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Large-scale Cryogenic Gravitational-wave Telescope, LCGT

Keiko Kokeyama

University of Birmingham

23rd July 2010

Friday Science

General introduction of LCGTproject

Introduction of gravitational waves (GWs)

Introduction of LCGT

Science goal and impact

Technical features

Underground, Seismic isolation system, Cryogenic, Optical configuration, Operation modes

Technical background

CLIO project as a LCGT prototype


Photos and plots are from “LCGT design document, “ CLIO/LCGT talks by Miyoki-san and Yamamoto-san, and “Study report on LCGT interferometer observation band”




Gravitational Waves

  • Ripples of spacetime propagating at the speed of light.

  • Changing the distances between free particles.

Coalescences of neutron star binaries, Supernova, BH coalescences, etc.

  • Einstein predicted its existence as a consequence of the general relativity in 1916.

  • Its existence is verified indirectly by the binary-neutron star observation, however, the direct detection has not been successful yet.

Significances of the direct detection

  • Experimental verification of the general relativity

  • The GW astronomy


Gravitational Wave Detectors

  • Laser interferometer (ifo) type

  • GW changes the mirror positions

  • The path length difference is detected as the phase difference between the two paths

  • GW has a very weak interaction to matters - very small path length change








Photo detector

Super accurate measurement to detect 10-20m change per 1 m


Large Scale ground-based GW detectors

The 1st Generation Detectors





Upgrading to the 2nd Generation Detectors

Advanced LIGO, Advanced VIRGO, GEO HF, LCGT


LCGT project

  • On 22nd June, the Japanese Next Generation Detector, LCGT was funded

  • 9.8 billion yen (£75M) for three years including 2010.

  • selected one of the projects for the forefront-research-development-strategic subsidy (40 billion yen in total) of Ministry of education, culture, sports, science and technology, Japan

  • The purpose of this subsidy is to develop the environment for the young or female scientists, and internationally high level researches

  • Further budget is being requested to run the project after the 3rd year. The result will be appear in August.


Scientific goals

Establish the GW astronomy

Main goal:

to detect gravitational waves from neutron star binaries (1.4 solar mass) at about 200 Mpc with > S/N 8

Expecting a few events

in a year from:

Coalescences of neutron star binaries

Goal sensitivity:

h=3 ×10-24 [m/rtHz] at 100Hz

Scientific Goals


GW detector network

  • LCGT plays a role of Asia-Oceania center among other detectors

Best sensitivity direction for


LIGO Hanford

LIGO Livingston


The good-sensitivity directions

are complementary for other detectors


Technical Features of LCGT

  • Underground Site

  • (2) Seismic isolation system

  • (3) Cryogenic Technique

    • Thermal noise design, Substrate of test mass

  • (4) Optical configuration

    • Four configurations and the observation plan


(1) Underground Site

More than 2 orders of magnitude better than TAMA site

The variance of 46 hours is about 0.1~0.2 degrees without temperature-controlling


(2) Seismic isolation system

  • Requirement: -190 dB at 3 Hzincluding the suspension part (*) and seismic isolation system

    • Seismic level in Kamioka is 10-9 m/rtHz at 3Hz

    • Sensitivity requirement is 3x10-18 m/rtHz at 3 Hz

  • The seismic isolation system (room temperature) is required

  • -130 dB isolation

(*) Test masses are suspended so that they act as free masses. Suspensions play a role of isolating the seismic motion, too.


(2) Seismic isolation system

  • Inverted pendulum

  • Three GAS (Geometric anti-spring) filters

  • This system achieves isolation ratios of:

  • -160dB for horizontal (w/ 4stages) at 3 Hz

  • -140 dB for vertical (w/ 3 stages) at 3 Hz

  • These satisfy the requirement

2-stage suspension (Low temperature)


(3) Cryogenic

  • We want to reduce the thermal noise

    • Thermal noise is…

  • To reduce the thermal noise, the main mirrors and suspension are cooled down to 20 K by refrigerators

  • sapphire f 250 ×150mm, 30kg


(3) Cryogenic

  • Heat links are used to release the heat occurred by the laser beam on the test-masses

SAS, 300 K

Heat link 1W,

7 ×f1mm, Al

Heat link 1W,

5 ×f3mm Al

Bolfur wire

40cm, f1.8mm

Recoil Masses

will be suspended by

Sapphire or Al wires


Sapphire wire, 860 mW

40cm, f1.8mm




  • Similar type to CLIO refrigerator (Sumitomo Heavy Industries Ltd, Pulse-tube refrigerator)


4 optical configuration1
(4) Optical configuration

Main IFO

  • Resonant-Sideband-Extraction (RSE)

  • In addition to the Fabry-Perot (FP) arm cavities, Power recycling and signal extraction cavities (PRC and SEC, respectively) are added to the interferometer

  • Advantages in capability of high laser power in arm cavities and flexibility in observation band

FP cavity

FP cavity




4 optical configuration2

Operation modes

(4) Optical configuration

  • BRSE: Broad band operation The carrier laser light is anti-resonant in SEC. Detector observation band is tuned to have a maximum sensitivity for neutron-star inspiral events.

  • DRSE: Detuned RSE. Detuning is a technique to increase detector sensitivity only in a slightly narrow frequency band. It is realized by controlling the SEC length between resonance and anti-resonance condition for the carrier laser beam.

  • V-BRSE: Broad band operation + slightly off resonance in the arm cavity

  • V-DRSE: Detuned operation+ slightly off resonance in the arm cavity






(4) Optical configuration

Operation modes

BRSE configuration has wider band. It can provides longer observation duration for an inspiral event. It is good for extracting information from observed waveforms, in accuracy of estimated binary parameters, the arrival time, and so on.

  • DRSE configuration has the best floor-level sensitivity at around 100 Hz, and the good observable distance for neutron-star inspiral events. Therefore the detuned configurations have advantages in the first detection and expected number of events.

V-BRSEand V-DRSEhave both advantages.


4 optical configuration3
(4) Optical configuration

Operation Strategy

Operate in the V-DRSEmode first for earlier detection of gravitational-wave signals

After the first few detections, they will switch to the V-BRSE mode.


Technical background
Technical Background

Suspended 4m RSE



  • In Kamioka mine

  • Prototype ifo for LCGT

  • To demonstrate the thermal noise reduction using cryogenic technique

  • 100m base-line unrecombined Fabry-Perot Michelson interferometer







Cryogenic in CLIO

  • 2008: 300K design sensitivity achieved.

  • 300K mirror thermal noise dominates the sensitivity around 150Hz.

  • 2009: Both near mirrors were cooled at about 20K.

  • 2010: Sensitivity around 150Hz were improved.

  • Total mirror thermal noise were reduced.

The suspended

sapphire mirror

(f100×60, 2kg)

6-stage vibration isolation

(3 stages in 300K,

3 stages in cryogenic)

Low vibration refrigerator


Displacement in November 2008

  • Almost the thermal noise limited at 300K (4/2008 to 12/2008)

  • Clio displacement touched the predicted thermal noise level

Sapphire mirror themal noise

Suspension thermal noise

(20-80 Hz)


Reduction of Mirror Thermo-Elastic Noise

  • It took 250 hours for cooling the mirror.

  • The near mirrors were cooled at 16.4k and inner shield was cooled at 11.5k.

  • The outer shield of the mirror tank and center of the cryogenic vacuum pipe were cooled at 69k and 49k, respectively.

  • Two near mirrors are cooled down to 20 K


Reduction of Mirror Thermo-Elastic Noise

  • CLIO has finally demonstrated the reduction of the thermal noise onsapphire mirrors around 200 Hz



  • LCGT

  • Just funded! The overview of LCGT project such as underground site, Seismic isolation system, cryogenic, optical configuration were reviewed.

  • CLIO

  • As the prototype for LCGT, CLIO successfully demonstrated the thermal noise reduction. Cryogenic, underground techniques are established for LCGT

  • Note:

  • Some parameters and materials are still under discussion toward the final design


Reduction of suspension thermal noise

Suppliment slide (3)

Reduction of Suspension Thermal Noise

Vacuum tanks
Vacuum tanks

  • Vacuum level

    • 2 x 10-7 Pa

  • Vacuum duct

    • 3km length

    • 1m diameter

    • steinless steel