lcgt technical review suspension point interferometer for parasitic noise reduction and an...

LCGT Technical Review

Suspension Point Interferometerfor

Parasitic Noise Reductionand

an Additional IFO

S.Miyoki(ICRR, Univ. of TOKYO)

LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

Contents

• Parasitic noise in LCGT• Heat link wires design as both a thermal

conductor and a seismic noise isolator• Designed thermal conduction and its seismic

noise isolation performance• SPI performance principle• SPI by-product• SPI experimental demonstrations• Technical issues toward LCGT


What is Parasitic Noise in LCGT ?

(i) Seismic noisethrough heat link wires(HLWs)

DampingMagnets

VibrationIsolationSystem(Room temp.)

CryogenicSuspension(4K-20K)Upper Mass(SPI)

Test Mass (Main IFO)

Recoil MassCoil-magnetActuators

Upper RecoilMass

Flex Joint

GAS filter

Displacement sensorCoil-magnet actuator

Acc. sensor

InvertedPendulum

SuspensionPlatform

77K shield4.2K shield

(ii) Magnet - CoilActuator Noise


Seismic Noise through the Heat Link Wires

To obtain enoughthermal Conduction...

To obtain enoughIsolation...

HLWDesign

- Diameter- Length- Purity- Number- CascadeNumber

LargeShortHighManyLeast(1)

SmallLongHighFewSeveral

- We should find a compromising design of HLWs -

Heat that is generated in the sapphire mirrors should be transferred onlyby the heat conduction of sapphire fibers and the HLWs to the innerRadiation shield at 8K. HLWs, however, introduce the seismic noise tothe mirror.

- How to decide and what consider about the Heat Link Isolation ? -

Trade Off Relation


How to design the Heat Link Wires

(1) The heat that should be transferred is 290 mW @ a near mirror. We set a safety factor over 3, i.e. 1W .(2) We select U- shape HLW because it provides the least spring constant for both horizontal and vertical motion, and the least length for thermal conduction.(3) From the requirement of seismic noise isolation, the least stage number is 6 for horizontal and 5 for vertical. Practically, the allowable stage number in the cryogenic area, however, is two (Mirror, Upper mass) or three (+ suspension platform).(4) In principal, the spring constant of HLW can be reduced by increasing the number of HLW that has less diameter, keeping the required heat transfer ability, because spring constant is proportional to fourth power of a wire diameter. (note : This U shape spring has an isolation plateau because the HLW weight is not negligible. So the excess reduction of the spring constant is nonsense.) (5) Survey the optimum pendulum stage numbers in the cryogenic area, and HLW diameter, length, number, and corres- ponding spring constant and isolation performance.


Present Heat Link Wire Design

Model- 3 stages in the Cryo- area.- 2 cascade of a HL path.- Number of HLW is less than 10.

SAS

Suspension Platform

GAS

UpperMass

Recoil Massfor Upper Mass

RecoilMass

MirrorMass

20K 15K

15K 14K

14K

300K

8K

HLWa HLWb

6N pureAluminum =3mmRadius of U =20cmNumber 5

6N pureAluminum =3mmRadius of U =25cmNumber 7

0.5Hz

1W

1W

RadiationShield

MGAS0.7Hz


Estimated Seismic Noise Isolationthrough HLWs (1)

The Vertical- to- horizintal coupling noise is dominant.


Estimated Seismic Noise Isolationthrough HLWs (2)

Expected SPI reduction : 1/100


What is SPI ?

SPI : proposed by Pro.Drever : forms an another interferometer using the upper masses.

Requirement : 1/100


How it works as an parasitic noise reducer ?

(FP cavity)

xuc

xud

HorizontalCommonMotion

HorizontalDifferentialMotion

xmc

xmd

Upper MassFP (SPI)

Main MirrorFP

Suspension Base

l1

T1 T2

Virtual Rigid Bar

HorizontalTransfer Function

f0h

≒l2 l1

l1 l2



x0c x0

dGround VerticalMotion x0

v

Expected displacement reduction

l2 - l1l1

Decided byxmd

1+Gxm

c CMRRxmd +

CMRR T2

T2-T1

G : SPI control loop gain, which can be large below 100Hz. f0h


How it works as an parasitic noise reducer ?

(Michelson Interferometer)

PDu

PDm

xuc xu

dUpper MassMichelson Interferometer(SPI)

xmc xm

dMirror MassMichelson Interferometer

f0Resonance Frequencyas a single Pendulumfor Horizontal motion

h



x0c x0

dGround

VerticalMotion x0

v





Expected displacement reduction

xmd

1+Gxm

c CMRRxmd +

In the case of FPMI or Locked FP control for SPI,almost same result is obtained.

G : SPI control loop gain, which can be large below 100Hz.


What dose the SPI contribute to ?(1) SPI reduces any kind of disturbance to the SPI masses including the seismic noise through the HLWs and actuator noise that originates from the electrical circuit noise. The reduction of the actuator noise allows stronger actuating force, which contributes to easier lock acquisition and wider dynamic range. Additionally, the RMS amount for the differential length control will be decreased.

(2) SPI could be an another Gravitational Wave detector that has a different observational frequency band using a higher laser Power and SPI mirror substrate.

Frequency [Hz]10 10k1k1001

10- 24

10- 23

10- 22

10- 21

10- 25


Experimental Demonstration (1)

Y. Aso’s WorkPLA 327 (2004) 1-8



Recoil massRecoil massMain massMain mass

SPI massSPI mass

MGAS filter 2MGAS filter 2Damping massDamping mass

Eddy current plateEddy current plate

MGAS filter 1MGAS filter 1

40cm40cm

1m1m


Experimental Demonstration (4)(Laser, Rigid cavity, Triple Pendulum)

Laser

Rigid cavity

Triple pendulum


Experimental Demonstration (5)(Each mass of the triple pendulum)

Upper massfor SPI

GAS at the top ofTriple pendulum

Mirror mass


Experimental Demonstration (6)Displacement of the mirror with and without SPI

SPI effect of 1/10~ 1/100 reduction is obtained below 2.5Hz. Above 2.5Hz,however, the vibration of other degree of freedom spoils the SPI effect.

with SPI

withoutSPI


Experimental Demonstration (7)Transfer function between the damping mass and the mirror

Frequency [Hz]

Tra

nsfe

r fu

ncti

on

[d

B]

Recently, SPI effect is obtainedbelow 10Hz.

Horizontal transfer function from a dampingmass(one mass is driven) to the main interferometer

2004 2005

Tra

nsfe

r fu

ncti

on

[d

B]

Frequency [Hz]1 100.1

10-1

10-2

10-3

10-4

10-6


Technical Issues toward LCGT

- SPI performance as an parasitic noise reduction device is now being evaluated, and displacement noise reduction of about 1/10 ~ 1/100 is obtained below 2.5Hz. Some improvements for the proto- type SPI suspension system to extend the frequency range, where the SPI effect is obtained, are being adopted.

- Individual alignment control system between the SPI mirror and the main mirror should be established.

- Practical design of the LCGT suspension using sapphire mirror and suspension fiber should be proposed.

lcgt technical review suspension point interferometer for parasitic noise reduction and an...

Documents

actuator noise

tokyocontentsparasitic

seismic noise tothe

parasitic noise reducer

electrical circuit noise

heat link isolation

spring constant of hlw

number of hlw