LCGT Technical Review

Suspension Point Interferometer for Parasitic Noise Reduction and an Additional IFO

S.Miyoki

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

What is Parasitic Noise in LCGT ?

Acc. sensor Displacement sensor Coil-magnet actuator Inverted Pendulum GAS filter Vibration Isolation System (Room temp.) (i) Seismic noise through heat link wires ( HLWs ) Flex Joint Upper Recoil Mass (ii) Magnet - Coil Actuator Noise Coil-magnet Actuators 4.2K shield Suspension Platform Damping Magnets Upper Mass(SPI) Cryogenic Suspension (4K-20K) Test Mass (Main IFO) Recoil Mass 77K shield LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

Seismic Noise through the Heat Link Wires

- How to decide and what consider about the Heat Link Isolation ? Heat that is generated in the sapphire mirrors should be transferred only by the heat conduction of sapphire fibers and the HLWs to the inner Radiation shield at 8K. HLWs, however, introduce the seismic noise to the mirror.

- We should find a compromising design of HLWs HLW Design - Diameter - Length - Purity - Number - Cascade Number To obtain enough thermal Conduction...

Large Short High Many Least(1) To obtain enough Isolation...

Small Long High Few Several

Trade Off Relation

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

How to design the Heat Link Wires

(1) The heat that should be transferred is We set a safety factor over 3, i.e. 1W (2) We select U- shape HLW 290 mW @ a near mirror.

.

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 cryo genic area, and HLW diameter, length, number, and corres ponding spring constant and isolation performance . LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

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 300K 8K GAS 0.5Hz

1W Suspension Platform 14K HLWa MGAS 0.7Hz

6N pure Aluminum  = 3mm Radius of U = 25cm Number 7 HLWb Upper Mass 15K 20K 15K 1W Mirror Mass Recoil Mass 14K 6N pure Aluminum  = 3mm Number 5 Radiation Shield Recoil Mass for Upper Mass Radius of U = 20cm LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

Estimated Seismic Noise Isolation through HLWs (1)

The Vertical- to- horizintal coupling noise is dominant.

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

Estimated Seismic Noise Isolation through HLWs (2)

Expected SPI reduction : 1/100 LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

What is SPI ?

SPI : proposed by Pro.Drever

: forms an another interferometer using the upper masses.

Requirement : 1/100 LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

How it works as an parasitic noise reducer ?

(FP cavity) Ground Horizontal Common Motion x 0 c Horizontal Differential Motion x 0 d Suspension Base Vertical Motion x v 0 Upper Mass FP (SPI) Main Mirror FP Horizontal Common Motion x u c Horizontal Differential Motion x u d x c m x d m l 1 f 0 h l 1 Virtual Rigid Bar l 2 l 2 ≒ l 1 T 1 Horizontal Transfer Function CMRR T 2 T 2 -T 1 T 2 Expected displacement reduction x d m x d m 1+G + c x m CMRR G : SPI control loop gain, which can be large below 100Hz.

f 0 h Decided by l 2 l 1 l 1 LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

How it works as an parasitic noise reducer ?

(Michelson Interferometer) Ground Horizontal Common Motion x 0 c Horizontal Differential Motion x 0 d Vertical Motion x 0 v f 0 h Resonance Frequency as a single Pendulum for Horizontal motion Upper Mass Michelson Interferometer (SPI) Mirror Mass Michelson Interferometer Horizontal Common Motion x u c Horizontal Common Motion x c m Horizontal Differential Motion x u d Horizontal Differential Motion x d m PD u PD m Expected displacement reduction d x m x d m + c x m CMRR 1+G G : SPI control loop gain, which can be large below 100Hz.

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

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

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.

10 - 21 10 - 22 10 - 23 10 - 24 10 - 25 1 10 100 1k Frequency [Hz] 10k LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

Experimental Demonstration (1)

Y. Aso’s Work PLA 327 (2004) 1-8 LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

Experimental Demonstration (2)

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

Experimental Demonstration (3)

MGAS filter 1 MGAS filter 2 Damping mass SPI mass Eddy current plate 1m 40cm Recoil mass Main mass LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

Experimental Demonstration (4)

(Laser, Rigid cavity, Triple Pendulum) Laser Rigid cavity Triple pendulum LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

Experimental Demonstration (5)

(Each mass of the triple pendulum) GAS at the top of Triple pendulum Upper mass for SPI Mirror mass LCGT Technical Review (2005/8/23) ICRR Univ. of Tokyo

Experimental Demonstration (6)

Displacement of the mirror with and without SPI with SPI 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.

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

Experimental Demonstration (7)

Transfer function between the damping mass and the mirror Horizontal transfer function from a damping mass(one mass is driven) to the main interferometer 2004 2005 10 -1 10 -2 10 -3 10 -4 10 -6 Frequency [Hz]

0.1

1 10

Frequency [Hz] Recently, SPI effect is obtained below 10Hz.

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

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 (2005/8/23) ICRR Univ. of Tokyo