Research and Development for Gravitational Wave Detectors

1. Research and Developmentfor Gravitational Wave Detectors Raffaele Flaminio CNRS/LMA Lyon

2. Ground-based GW detectors • Focus on ground based laser interferometers • Most sensitive detectors in operation • LIGO, Virgo, GEO, TAMA, • Some of the following applies to other kind of detectors (e.g. resonant detectors)

3. Present reach • Gravitational collapses in the galaxy (or nearby ones) • Test upper limits of known galactic pulsars (and look for unknown ones) • Search for coalescing neutron stars up to a max distance of ~ 30 Mpc • Search for merger of binary black holes to a max distance of ~150 Mpc

4. Toward GW astronomy • Present detectors will test upper limits • Even in the optimistic case rate too low to start GW astronomy • Need to improve thesensitivity • Increase the sensitivity by 10  increase the probed volume by 1000 • Plans to improve thepresent detectors

5. GW roadmap: time scale • ET: Einstein Telescope • Design study selected by the EU within FP7

6. Virgo+ 2009 LIGO 2005 AURIGA 2005 Virgo Design GEO-HF 2009 DUAL Mo (Quantum Limit) Ad LIGO/Virgo NB Advanced LIGO/Virgo (2014) Credit: M.Punturo Einstein GW Telescope GW roadmap: sensitivity scale

7. Present limitations …. • Shot noise • - Depends on quantum nature of light • - Decreases when more photons are used • - Depends on the optical configuration adopted

8. …. and possible improvements • Increase power stored in the interferometer • - increase laser power • - decrease optical losses • But pay attention to: • 1) Mirrors heating and thermal lensing • - better thermal compensation • - decrease light absorption • 2) Radiation pressure noise • - increase mirror mass • - optimize optical configuration • signal recycling • - use non classical light • light squeezing/quantum optics • 3) Non-linear coupling between the light fields and the mirror suspensions

9. Present limitations … • Mirror thermal noise • - brownian motion • - due to temperature …. • - …plus any source of friction in the mirror

10. … and possible improvements • Reduce friction in the mirrors • Friction in the coating • - Main source of friction today • - Multi-layers SiO2/Ta2O5 used today • - Ta2O5 is the lossy material • look for new materials • materials science • - SiO2 layer lossier than raw material • improve deposition process • Friction in the substrate • - Best material so far: silica • - Avoid attaching anything to preserve • mechanical quality • - Move to electrostatic actuators avoiding magnets attached to the mirror

11. Present limitations … • Pendulum thermal noise • - same kind of brownian motion • due to temperature …- … plus friction in the suspension wires • - or friction between the wires and the mirror

12. … and possible improvements • Decrease pendulum friction • better suspensions wires (new materials)- better wire clamping • monolithic suspensions • fused silica fibers • silicate bonding

13. Further reduction of thermal noise • Thermal noise decrease as √T • - go to low temperatures • - friction vs temperature? • - depends on materials (materials science) • - look for optical materials with good • mechanical properties at low temperature • (silica not a good choice) • Thermal lensing • - due to laser power deposited in the mirror • - higher mirror thermal conductivity • lower thermal lensing • higher wires thermal conductivity • heat extraction more efficient • Silicon • - silicon a good candidate • - silicate bonding behavior at low T? • - thermal conductivity across bonding? • - on-going R&D Si

14. COLD FINGER Cryogenics for GW detectors • Need to cool large masses • Vibration free cryogenics • Soft thermal links • Points of contact with underground detectors for rare events search

15. ILIAS: the STREGA joint research activity • Strong component within the ILIAS project • Goal: thermal noise reduction for GW detectors • All the european groups working in thermal noise reduction involved • INFN (Ge, Fi, Fr, Le, Pd, Pi, Pg, Rm1, Rm2), CNRS (LKB, ESPCI,LMA), Univ Glasgow, CNR (Trento), Leiden, Jena, … • All collaborations: Virgo, GEO, ROG, Auriga, MiniGRAIL • Ingredients: • Cryogenics suspensions • Cryogenics mirrors • Materials • Thermo-elastic studies • A key role for starting the ET design study • A lot more to do • But ILIAS ends in 2009 and support available within ILIAS-NEXT very much reduced

16. Present limitations: seismic noise • Seismic noise • residual transmission of seismic motion through the suspensions system • 'relatively large' motion at very low frequency → need for a control system → control system noise - sensitivity to weather conditions

17. Seismic noise: possible improvements • Better active isolation • More sensitive accelerometers • Very low-frequency tilt-meters • Gryo-lasers • Softer springs ?

18. Forthcoming limitations • Gravity gradient noise • Limitation to existing infrastructure • Will limit advanced detectors Figure: M.Lorenzini

19. Improvement: go underground ! • LISM: 20 m Fabry-Perot interferometer, R&D for LCGT, moved from Mitaka (ground based) to Kamioka (underground) • Seismic noise much lower • Operation easier • 102 overall gain • 103at 4 Hz

20. Further improvements: spherical cavern 102 less seismic noise x 104 geometrical reduction 106 overall reduction (far from surface) Spherical Cavern G.Cella Reduction factor (Compression waves not included) NN reduction of 104 @5 Hz with a 20 m radius cave 5 Hz 10 Hz 20 Hz 40 Hz

21. Combination of improvements Upper experimental hall 50-100 m tower to accommodate long suspension for low frequency goal Ellipsoidal/spherical cave for newtonian noise reduction Credit: R.De Salvo 10 km tunnel

22. Rüdiger, ‘85 The ET concept • Need to improve sensitivity at low frequencies • More physics is there • Present facilities limited by environmental disturbances • Seismic noise • Gravity gradients • ET Einstein Telescope • Concepts • Underground • Less seismic noise • No wind • Temperature stability • Cryogenic • 30 km beam tube • 100 m suspensions • Different geometry • Triangle?

23. Conclusion • Present detectors are testing upper limits of GW predictions • A few upgrades ready to be implemented (Virgo+, Enhanced LIGO) • Advanced detectors should see several events/month • Sensitivity will profit from on-going R&D (e.g. coating thermal noise) • Engineering needed (e.g. monolithic suspensions) • ET Einstein Telescope • Design study should start soon • R&D activity started within FP6 (STREGA) • Should continue within FP7 • More investment needed • Points of contact with other fields of astroparticle physics • Cryogenics • Vibration isolation • Underground operation • GW will participate to ILIAS-NEXT • GW networking • Networking with underground labs • A few small R&D activities • But more investment will be needed