Low frequency noises in GW ITF: R&D activities in VIRGO on Thermal noise reduction

1. 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 already @4Hz

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 40-300Hz • 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. 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 Thermo-elastic Contribution at room temperature

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 larger one soon available (mPull down technology) • A small furnace is available in the Pisa Labs (M.Tonelli)

15. Expected sensitivity

16. Local Cooling • It needs the doping of the crystalline fiber with rare earth impurities • Possibility to cool down a localized region (flexural point in suspension fibers) • We can used a traditional cold finger or a more advanced (and difficult) anti-stokes fluorescence mechanism 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800mm

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 Optical read-out noise cavity 1 free • 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 # 2 Optical read-out noise caviity 1 with feedback

24. Possible reduction of R.P.