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Low thermal noise suspensions for Virgo

The Virgo Perugia Group

•Luca Gammaitoni

•Fabio Marchesoni

•Michele Punturo

•Ciro Cattuto

•Helios Vocca

•Paolo Amico

•Flavio Travasso

•Leone Bosi

Perugia University and INFN

(2)

H.Vocca – GWADV Isola d’Elba, Italy, May 19-26 2002

The expected Virgo Sensitivity

10

0

10

1

10

2

10

3

10

4

10

-25

10

-24

10

-23

10

-22

10

-21

10

-20

10

-19

10

-18

10

-17

10

-16

10

-15

Total sensitivity Total thermal noise Thermal noise: Mirror Thermal noise: Pendulum Shot noise

Radiation Pressure Seismic

Creep Magnetic Laser Power Acoustic Quantum Limit Newtonian (Cella) Newtonian (Thorne)

h(f) [1/sqrt(Hz)]

Frequency [Hz]

(3)

Pendulum Thermal Noise

S b

wire w

g w

B m L T C

E m

n g E L

T g k

X θ φ

π

ω ω

 

 +

5 2 2

2

4 4 1

) (

Wire loss angle:

• Excess loss angle

– Frictional processes in the marionetta-wire clamps

– Frictional losses in the wire- mirror contact point

T

b

: Breaking strength

+ +

= e

wire φ φ

φ 0 φ th

: Material loss angle

φ th

φ 0

φ e

: Thermoelastic loss angle : Excess loss angle

C

S

: Confidence factor

• Thermoelastic loss angle

( )

2

1 ωτ φ ωτ

∆ +

th

=

c T E α

2

=

D d

w

2 2

τ = π

(4)

H.Vocca – GWADV Isola d’Elba, Italy, May 19-26 2002

Present solution

The Virgo present solution for the suspension last stage uses four 200 µ m diameter C85 steel wires.

In these conditions:

- φ 0 ˜ 10 – 4

- φ th (max) ˜ 10 – 3 (at 680 Hz) - C S = 0.6

- T b ˜ 2.9 GPa

(5)

The next step will be

the monolithic solution

(6)

H.Vocca – GWADV Isola d’Elba, Italy, May 19-26 2002

1 10 100 1000

10

-7

10

-6

10

-5

10

-4

10

-3

C85 steel Sapphire Fused Silica New F.S. with bob Y a g + A l2O3+ZrO2

φ

w

(f)

frequency (Hz)

Measured losses of materials

φ≈ 10 -4

φ≈ 10 -6

A factor

10 on

“h”

sensi- tivity

Synthetic FS with bob

(7)

FS Production facility

• Pure O

2

-H

2

flame

• No fiber-support contact

• Direct

measurement of the fiber diameter

• Directly tested

the fiber breaking

strength using a

new automatic

device

(8)

H.Vocca – GWADV Isola d’Elba, Italy, May 19-26 2002

FS Breaking strength (1)

1000 2000 3000 4000 5000 6000

7000 0

1 2 3

4 5

6 7

Acid

Alcohol Dirty Breaking Strength [MPa]

The breaking strength of FS fibers depends on material purity, production and handling procedure.

GPa T

GPa T

C b

SiO b

02 . 0 90 . 2

55 . 0 05 . 4

85

2

±

=

±

=

φ

w

/T

b

is a factor 200 lower for FS fibers, but C

s

is lowered by the large T

b

fluctuation.

H.Vocca et al., Proc. of the3th Lisa symposium (2000, sub. to Class. Quant. Grav) M.Punturo et al, talk to the GREX meeting (Grasse, 2000)

P.Amico et al, Nuclear Inst. and Methods in Physics Research A, pp 297-299 Apr.2001

(9)

• The Breaking strength is dramatically reduced by:

– Cracks on the fiber surface

• contact with metals or other hard surfaces

– Humidity

• some worry for ageing caused by humidity

– no statistical evidence for this effect in our labs, but some “alarms”

– this effect is reported by optical fiber producers (closest to the theoretical FS breaking strength)

– good news from Glasgow for fibers under vacuum – more tests needed

FS Breaking strength (2)

• Very strong under elongation (and compression)

Si O

O O O

Si O

O O

H2O

H+ OH-

Si O

OH O O

Si O

O O

OH

• Very fragile for shocks

(10)

H.Vocca – GWADV Isola d’Elba, Italy, May 19-26 2002

100 1000

10

-6

10

-5

10

-4 F.S. with C coating F.S. with C plus N=30%

F.S. with C plus N=60%

F.S. without coating

φ

w

(f)

frequency (Hz)

Coating of the wire surface

•Surface coating could prevent FS ageing

•Effect of a Carbon coating (with Nitrogen contaminations) on loss angle has been tested

•An evident increase of the wire loss angle has been measured

•More tests are needed with different coatings and better thickness control.

•It could be useful to

damp the violin modes

of the wires.

(11)

Potassium silicate bonding

Silicate bonding is a Hydroxide-Catalysis chemical process.

Test are made in a class 10000 clean room, under a class 100 laminar flux.

For λ/10 samples we are dominated by the clamping

•Mean λ/4 =2.5±0.6 MPa

•Mean λ /7 =3.3±0.8 MPa

It is important a:

–control of the purity of H

2

O and KOH

–accurate cleaning of the SiO

2

samples

In this conditions the breaking strength vs time of samples with different flatness quality has been investigated ( λ /4, λ /7 and λ /10).

0 7 1 4 2 1 2 8 3 5 4 2 4 9 5 6 6 3 7 0

2 4 6

0 1 2 3 4 5 6 7 8 9 1 0

1 5 . 6 3 1 . 3 4 6 . 9

1 0 1 0

1 0

1 0

1 0

1 0

7 4 7

4

Breaking load [kg]

T i m e [ w e e k ]

Breaking Stress [Mpa]

T i m e [ D a y ]

λ=633 nm

(12)

H.Vocca – GWADV Isola d’Elba, Italy, May 19-26 2002

Silicate bonding as current technology

• Silicate bonding technology is used in the current Virgo assembling:

– Spacers will be attached to the lateral surface of the mirror (polished @ λ /10 in a 40mm height strip) to improve the pendulum Q

– Camera targets will be attached to the mirror – Magnets will be bonded through an intermediate

fused silica disk to the face of the mirror directly on

the reflective coating.

(13)

Future Implementation

•2 flat strips

•4 40×40mm λ/10 polished region

Marionetta clamp

•Because of the complexity of the Virgo SA the hanging procedure will be very crucial (and difficult)

FS λ /10 polished ear

(14)

H.Vocca – GWADV Isola d’Elba, Italy, May 19-26 2002

Monolithic solution

• With FS fibers we can gain in

– φ

0

– thermoelastic damping because of

– clamping losses – breaking strength

• On the other hand we lose in C S due to the dispersion of the breaking strength

85 2

85 2

1 6 1

6

17 10

10 5 .

0

C SiO C

SiO

≈ ⋅

K

< α ≈ ⋅

K

⇒ ∆ < ∆

α

(15)

Virgo sensitivity with FS suspension

1 10 100 1000

10

-23

10

-22

10

-21

10

-20

10

-19

10

-18

h(f) [1/sqrt(Hz)]

Frequency [Hz]

C85

Pendulum thermal noise (φ=4*10- 7) Mirror Thermal noise (Q=106) Mirror Thermal noise (Q=107)

(16)

H.Vocca – GWADV Isola d’Elba, Italy, May 19-26 2002

All Fused Silica Suspension

November 1998, Perugia, quartz suspension system, in collaboration with GEO Glasgow

• Mass 2.8 kg

• Length 30 cm

• Frequency 0.93 Hz

• 2 fused silica wires 400 µ m diameter

Very high Q measurements on a fused silica monolithic pendulum for use in enhanced gravitational wave detector.

G. Cagnoli , L. Gammaitoni, J. Hough, J. Kovalik , S.

McIntosh, M. Punturo, S. Rowan. Phys. Rev.Phys. Rev. Lett Lett. 85, 2442-2445,. 85, 2442-2445, 2000

2000

(17)

0 5 10 15 20 25 30 35 40 45

1E-4 1E-3 0.01

Q=2.1X10

7

Q=2.4X10

7

Q=2.5X10

7

amplitude in m

time in days

Pendulum Q for Fused Silica Suspension

(18)

H.Vocca – GWADV Isola d’Elba, Italy, May 19-26 2002

What can we

do best?

(19)

1 1 0 100 1000 10000 10-8

10-7 10-6 10-5 10-4 10-3

10-8 10-7 10-6 10-5 10-4 10-3 C85

SiO2 Si GSGG SiO

2 cris.

YAG

φ ( f )

Frequency f (Hz)

Thermoelastic loss angle (T=300°K)

(20)

H.Vocca – GWADV Isola d’Elba, Italy, May 19-26 2002

1 10 100

10

-25

10

-24

10

-23

10

-22

10

-21

10

-20

10

-19

10

-18

φ

SiO2

=4*10

-7

φ

Si

=2.8*10

-8

h(f) [1/sqrt(Hz)]

Frequency [Hz]

h

th

(f) SiO

2

T=300K h

th

(f) Si T=300K h

th

(f) Si T= 70K

Cryogenic approach

(21)

Localized cooling

• Mono-crystal fibers are more robust with respect to ageing and OH contamination then amorphous ones

• With the addition of impurities of Yb

3+

, it is possible to cool the fiber optically with an anti-Stokes mechanism

• In the case of fibers, the optical path of the pumping radiation can be increased by using Total Internal Reflection to raise the intensity and absorption of the radiation.

(In collaboration with M. Tonelli, Dipartimento di Fisica of Pisa)

I.R.laser

• With this technique, it is possible to decrease the temperature by a few tens of

(22)

H.Vocca – GWADV Isola d’Elba, Italy, May 19-26 2002

Conclusions

• Thermal noise is and probably will be the main limit to the GW interferometers

– Radiation pressure will limit the sensitivity if the laser power will increase dramatically

• Fused Silica suspension is the right solution if the ITF operates at room temperature

• Si fibers are, from the thermoelastic point of view, a possible alternative at RT.

• Cryogenic ITFs are the most promising evolution for the GW detectors

– Mono-crystal fibers/rods (Si, or…) must be used to suspend and to extract the heat from the mirror

– Localized cooling could be a useful technology to further reduce the temperature in the bending points of the suspension fibers

• We have enough work for the next years!!!

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