1. Michele Punturo WP3 meeting, Cascina 9-July-2004
Low frequency noises in GW ITF: R&D activities in VIRGO on Thermal noise reduction
Michele Punturo
WP3 meeting, Cascina 9-July-2004

2. Virgo Expected Sensitivity Curve

3. Virgo under commissioning
C4 sensitivity (no PR) dominated by technical noises

4. Low frequency fundamental Noises
Seismic Noise
Newtonian Noise
Thermal Noise
Suspension
Mirror
Radiation pressure noise

5. Virgo SuperAttenuator
Multi-pendulum (6) seismic filter + IP
Cantilever blades for vertical seism attenuation
Active controls to damp the resonant modes
Seismic noise lower than expected thermal

6. Thermal noise in Virgo
In Virgo the suspension thermal noise is expected to be dominant in the 4-40Hz range
The mirror thermal noise “could be” dominant in the Hz
In an advanced version of the detector, with at least 100W power laser, the thermal noise will be the largest enemy up to 500Hz

7. Current design
Currently the Virgo mirrors are suspended by a system of two steel wires:
C85 Steel selected to minimize the loss angle ant the creep effect
Special clamps to reduce the clamping losses
Lateral spacers to reduce the pendulum and mirror Q degradation

8. Silicate Bonding
In each mirror are attached camera targets, frontal and lateral magnets and lateral spacers
Silicate bonding technique has bee adopted to reduce the mirror Q degradation

9. Current performances A pendulum Q of ~106 is expected for Virgo
We cannot measure it directly
Pendulum thermal noise limit is still far from our sensitivity
Read-Out mirror Q between 3×105 and 1.6×106
Effective loss angle computed through FEM of the dissipations in the mirror
Perfect coincidence with the expected sensitivity curve

10. Mirror vibration in C4 data
Input Mirrors
Mirror vibration in C4 data
End mirrors

11. R&D on Thermal noises
“Historical” R&D activity on fused silica suspension
Scientific targets reached few years ago
Production of SiO2 fibers through H2-O2 flames
Silicate bonding process under control
21 kg mirror suspended
Engineering procedures to be defined:
Complex SA: safety of the suspension
Heavy mirror
Transport
Cleanliness issues
R&D financed by EGO to Glasgow on fiber production with a CO2 laser

12. Intermediate term R&D Suspension fibers in mono-crystalline material
Silicon is a good candidate both for a room temperature ITF and for a cryogenic 3rd generation detector
Low intrinsic loss angle (10-8 – 10-10)
High thermal conductivity at low temperature
Zero thermal expansion coefficient at some “magic” temperatures
Contribution at room temperature
Thermo-elastic

13. Si thermal properties vs Temperature
a(T): C.A. Swenson, J. Phys. Chem. Ref. Data, vo. 12(2),
k(T) Thermophysical Properties of Matter, v1, Y.S. Touloukian, R.W. Powell, C.Y. Ho & P.G. Klemens, 1970, IFI/Plenum, NY

14. Production of Mono-Crystalline fibers
A small furnace is available in the Pisa Labs (M.Tonelli)
A larger one soon available (mPull down technology)

15. Expected sensitivity

16. Local Cooling Pcool(l)= Pabs(l)(l-lF)/lF
We can used a traditional cold finger or a more advanced (and difficult) anti-stokes fluorescence mechanism
It needs the doping of the crystalline fiber with rare earth impurities
Possibility to cool down a localized region (flexural point in suspension fibers)
R. I. Epstein et al. NATURE 377 (1995)
Pcool(l)= Pabs(l)(l-lF)/lF

17. Optical cooling Low efficiency energy extraction TIR
Possibility to increase the efficiency through a multi-pass technique using Total Internal Reflection in a fiber
YAG:Er fiber
f  800 mm

18. Cryogenic ITF Cryogenic Facility at the VIRGO-EGO site financed by EGO
Cryostat (pulsed tubes) R&D financed by INFN at Rome
Cryogenic payload R&D supported by INFN, STREGA and EGO

19. Cryogenic Payload Executive design of the prototype ready
The request of offer to the company will be submitted next week
To be mounted in the cryogenic facility in Cascina to test thermal and control issues

20. Radiation Pressure R&D
In the 3rd generation ITF a power of 1-10MW could be stored in the FP cavities to reduce the shot noise
Thermal noise could be reduced through a cryogenic approach
Radiation pressure becomes a dominant noise at low frequency

21. Radiation pressure noise

22. New Idea Courty et al., [Phys. Rev. Lett. 90, 083601 (2003)]
It is possible to reduce the radiation pressure noise in the frequency range of interest coupling the ITF cavity with an auxiliary control cavity

23. Coupling of the cavities
Experimental and theoretical studies in progress:
Effect of the others noise sources on the quantum locking
Locking schemes and total number of cavities
Experimental tests supported by INFN (Perugia) and EGO (LKB)
Optical read-out noise cavity 1 free
Optical read-out noise
Cavity # 2
Optical read-out noise caviity 1 with feedback

24. Possible reduction of R.P.