Silica Research in Glasgow

Gianpietro Cagnoli – IGR - University of Glasgow

GEO CollaborationGinzton Lab, Stanford

Caltech

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 2

Summary

Silica suspensions in GEO600

Direct Measurement of Thermal

Noise

Coating Losses

Non-Linear Thermoelastic Effect

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 3

Silica suspensions in GEO600GEO600 Collaboration

Two monolithic silica suspensions have been installed in GEO600 (middle of June)

GEO600 is the first interferometer to use such suspension to reduce thermal noise

Some numbers:fibres length = 285 mmvertical freq. ~ 16 Hzmirror mass = 5.6 kg

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 4

Silica suspensions in GEO600

Features: 4 fibres welded on

“ears” 2 silicate bonded ears

each mass Selective damping of

violin modes with teflon coating on fibres

Masses are hold in a rigid frame during the welding and transportation inside the tank

Once the stage is suspended the holding frame becomes a catcher

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 5

Silica suspensions in GEO600

Coating Cutting

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 6

Silica suspensions in GEO600

Welding (test)

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 7

Direct measurement of thermal noiseGEO600 Collaboration

During the last year the 10m interferometer in Glasgow have been upgraded

A sensitivity limit lower than 10 –18 m/Hz ½ has been achieved above 500 Hz (blue curve at right)

Shot noise = 2·10 –19 m/Hz ½

Damping of the internal modes of one mass was increased by spreading 3 strips of grease on the barrel and noise was again measured (red curve)

1E-19

1E-18

1E-17

1E-16

1E-15

1E-14

100 600 1100 1600

Frequency [Hz]

Dis

pla

ce

me

nt

SD

[m

/sq

rt(H

z)]

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 8

1E-19

1E-18

1E-17

1E-16

1E-15

1E-14

100 600 1100 1600

Frequency [Hz]

Dis

pla

ce

me

nt

SD

[m

/sq

rt(H

z)]

Direct measurement of thermal noise

Q of several modes were measured Q ~ 1800 for greased mass Q ~ 1.2·10 5 for clean masses

Thermal noise estimation- semi-infinite mass - homogeneous structural damping (Levin Yu., Phys.Rev. D, 57 (2), 659-

663)

Results:Fluctuation-Dissipation Theorem has been verified experimentally

4 masses Q = 120000

3 masses Q = 1200001 mass Q = 1800

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 9

Coating lossesGlasgow and Stanford Collaboration

Identify and measure the coating loss from Q measurements on silica cylinders

Sample: - Corning 7980- 127 mm diameter, 100 mm length- IR coating on front faces and barrel

0.E+00

1.E-07

2.E-07

3.E-07

4.E-07

2.E+04 3.E+04 4.E+04

Frequency [Hz]

Measurements: - ring down method, 7 modes measured - 15 different sample suspensions tested

Finite elements analysis: - used for energy ratios (surface to bulk) - convergence with N. of points checked - agreement to a few % with measured frequencies

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 10

Coating losses

Two parameters model:

Cff is the front face to bulk energy

ratio

cffb C

then…another source of loss is significant

0.E+00

1.E-07

2.E-07

3.E-07

4.E-07

0.0E+00 5.0E-04 1.0E-03 1.5E-03

front face to bulk energy ratio

Cff

1st asym

1st sym

Results & comments: - all modes but 1st asymmetric are lined up - low loss measurement is not accidental - 1st symmetric mode loss too high? - but then the linear fitting is not good

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 11

Coating losses

Three parameters model:

Cb is the barrel surface to bulk energy

ratio

cbbcffb CC

0.E+00

1.E-07

2.E-07

3.E-07

4.E-07

0.E+00 1.E-07 2.E-07 3.E-07 4.E-07

theory

e

xp

eri

me

nt

1st sym

2nd asym

Results & comments: - apart for the 1st symmetric mode

(high loss) the other points are aligned- other sources of loss are currently

under investigation to justify the high value measured for the 1st symmetric mode

b = (5.8 ± 0.4)·10 –8 c = (3.8 ± 0.5)·10 –5 cb = (4.2 ± 0.3)·10 –5

From the best fit without 1st sym:

Coating losses could be a serious limit to the next generation interferometers

0.E+00

1.E-07

2.E-07

3.E-07

4.E-07

2.E+04 3.E+04 4.E+04

Frequency [Hz]

1st asym

1st sym

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 12

Asymmetrical thermal fluctuations are responsible of thermoelastic noise on silica fibres

Non-linear thermoelastic effectGlasgow and Caltech Collaboration

Loss angle:

22

1

C

TE

In linear thermoelastic effect thermal expansion coefficient transforms thermal fluctuations in strain fluctuations

= ·T

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 13

P

Non-linear thermoelastic effect

This second order effect becomes relevant due to the large longitudinal static stress o in the suspension fibre

In non-linear thermoelastic effect thermal fluctuations are transformed in strain fluctuations through the thermoelastic coefficient ·dd

Loss angle:

2

2

1

C

TE

E

T

P

SE

LPL

P

TLLL )(

11th July 20014th Edoardo Amaldi Conference on

GW, Perth, 8th – 13th July 2001 14

Non-linear thermoelastic effect

With a static stress o = 200 Mpathermal expansion is compensated and

thermoelastic noise is cancelled

An article will be submitted soon to Phys. Rev. D by Cagnoli G. and Willems P.

Some numbers for silica:

= 5.5·10 –7 K –1

= 2·10 –4 K –1

E = 72 GPa

POSITIVE !

= 0·T