Lisbon, 8 January Research and Development for Gravitational Wave Detectors Raffaele Flaminio CNRS/LMA Lyon

Lisbon, 8 January 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)

Lisbon, 8 January 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

Lisbon, 8 January Toward GW astronomy Present detectors will test upper limits Even in the optimistic case rate too low to start GW astronomy Need to improve the sensitivity Increase the sensitivity by 10  increase the probed volume by 1000 Plans to improve the present detectors

Lisbon, 8 January GW roadmap: time scale ET: Einstein Telescope  Design study selected by the EU within FP7

Lisbon, 8 January GW roadmap: sensitivity scale Ad LIGO/Virgo NB Credit: M.Punturo LIGO 2005 AURIGA 2005 Advanced LIGO/Virgo (2014) Virgo Design GEO-HF 2009 Virgo Einstein GW Telescope DUAL Mo (Quantum Limit)

Lisbon, 8 January Present limitations …. Shot noise - Depends on quantum nature of light - Decreases when more photons are used - Depends on the optical configuration adopted

Lisbon, 8 January …. 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

Lisbon, 8 January Present limitations … Mirror thermal noise - brownian motion - due to temperature …. - …plus any source of friction in the mirror

Lisbon, 8 January … 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

Lisbon, 8 January 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

Lisbon, 8 January … and possible improvements Decrease pendulum friction - better suspensions wires (new materials) - better wire clamping - monolithic suspensions - fused silica fibers - silicate bonding

Lisbon, 8 January 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

Lisbon, 8 January 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 COLD FINGER

Lisbon, 8 January 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

Lisbon, 8 January 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

Lisbon, 8 January Seismic noise: possible improvements Better active isolation  More sensitive accelerometers  Very low-frequency tilt-meters  Gryo-lasers Softer springs ?

Lisbon, 8 January Forthcoming limitations Gravity gradient noise  Limitation to existing infrastructure  Will limit advanced detectors Figure: M.Lorenzini

Lisbon, 8 January 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 10 2 overall gain 10 3 at 4 Hz

Lisbon, 8 January Further improvements: spherical cavern Reduction factor Spherical Cavern G.Cella 5 Hz 10 Hz 20 Hz 40 Hz NN reduction of 10 Hz with a 20 m radius cave 10 6 overall reduction (far from surface) (Compression waves not included) 10 2 less seismic noise x 10 4 geometrical reduction

Lisbon, 8 January Combination of improvements Upper experimental hall Credit: R.De Salvo m tower to accommodate long suspension for low frequency goal Ellipsoidal/spherical cave for newtonian noise reduction 10 km tunnel

Lisbon, 8 January 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? Rüdiger, ‘85

Lisbon, 8 January 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