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Cavalier Fabien LAL Orsay IIHE Bruxelles 30 Avril 2004 The Quest for Gravitational Waves Slide 2 Virgo and the quest for Gravitational Waves I.The interferometric detection of gravitational waves 1.Gravitational Waves: nature and effects 2.Sources 3.Principle of interferometric detection and improvements II.The Virgo Challenge 1.The infrastructures 2.The seismic noise and the super-attenuator 3.The thermal noise 4.The control system 5.The Virgo sensitivity III.The first experimental results 1.CITF 2.Fabry-Perot cavities 3.Recombined Interferometer IV.Interferometric search of GW in the world Slide 3 Gravitational Waves A little bit of General Relativity In Special Relativity, the space-time interval is given by: ds 2 = dx 2 + dy 2 + dz 2 c 2 dt 2 = dx dx where is the Minkowski metric tensor In General Relativity, we have: ds 2 = g dx dx with g metric tensor which follows Einsteins equation Weak Field Approximation g = + h with || h || Seismic Noise Measurement: h seismic ( ) 10 -10 -2 Hz -1/2 Isolation Principle: chain of pendulums with internal dissipation each pendulum behaves as a low pass filter: H( ) = ( 0 / ) 2 for > 0 Slide 18 Performances motion of the mirrors about one micron speed about few microns per second The Super-Attenuator Slide 19 The thermal Noise Each suspension wire and each mirror behave as an oscillator excited by thermal agitation Characterized by 0 and Q quality factor Q Measurements: silica : 10 6 steel wire : 10 4 10 5 Limiting factor between 3 and 500 Hz Mirror weight: 30 kg (noise when M ) Test of new materials (sapphire, silicon) Monolithic suspensions Slide 20 The mirrors Reflectivity defined better than 0,01 % Reflectivity of end mirrors > 0.9998 Losses (absorption, diffusion) about few ppm High Radius of curvature (3400 m) and defined with 3 % precision Surface defined with /40 precision over 30 cm of diameter Coating realized by SMA at IPN Lyon Metrology made at ESPCI Solution : silica mirrors (SiO 2 ) = 35 cm and h = 10 or 20 cm Slide 21 Position Control Fabry-Perot resonant: L < 5 10 -10 m Recycling Cavity resonant: l R < 2.5 10 -10 m Dark Fringe (coupling with laser power noise): l DF < 10 -10 m Alignment End Mirrors: 3 10 -9 rad Alignment Entry Mirrors: 2 10 -8 rad Alignment Recycling Mirror: 10 -7 rad Fully Digital System running at 10 kHz for Locking and 500 Hz for Alignment Slide 22 The errors signals Pound-Drever technique for Fabry-Perot cavity phase modulation of laser frequency side-bands anti-resonant use reflected beam Generalization for Virgo Use all signals coming out of the ITF Slide 23 The Virgo Sensitivity If all technical noises are under control Slide 24 Virgo and the CITF Slide 25 The CITF (Central area InTerFerometer) Central Part (no kilometric arm) used from June 2001 to July 2002. Tests and validation : super attenuators electronic and software data acquisition output mode cleaner injection optics Main Output: Learn how to control a suspended interferometer with digital systems Slide 26 CITF: Michelson Control Slide 27 CITF Engineering runs : results RunDateConfiguration E009/2001Simple Michelson E112/2001+ control SA E204/2002+ recycling E305/2002+ automatic alignment E407/2002+ injection system RunLosses of LockDuty CycleMax Duration E0198%51h E1185%27h E2398%41h E3498%40h E4473%14h 5 runs , 3 days long from September 2001 to July 2002 Slide 28 CITF Engineering runs : results Alignment Noise Frequency Noise Slide 29 The Virgo Commissioning Started in September 2003 after the upgrade to full Virgo Strategy: North arm Lock acquisition Frequency stabilization Auto Alignment Hierarchical control (top stage, marionette, reference mass) West arm (same activities) Recombined ITF (no recycling mirror) (same activities) full ITF (same activities) Slide 30 Locking at the first trial first lock ~ 1 hour frequency noise and alignment noise Transmitted power Frequency noise reduction North Arm Slide 31 North Arm First Sensitivity Shot noise Electronic noise (ADC) Frequency noise Control noise MC length control noise Slide 32 North Arm with Automatic Alignment Linear alignment OFF Linear alignment ON Slide 33 Arms Sensitivity during C1 and C2 runs Slide 34 Frequency Stabilization on North Arm laser 50 Hz 300 kHz Ref Cav 50 Hz A free running laser has a too noisy frequency You need to stabilize the frequency on a more stable reference The Virgo arms are the best available reference at high frequency Slide 35 Frequency Stabilization on North Arm laser 50 Hz 20 kHz. 300 kHz Ref Cav 1 Hz Loop needed to stabilize the frequency in DC Slide 36 Hierarchical control: 3 points Slide 37 Fast corrections (f > 70 mHz) Slow corrections (f < 70 mHz) 3.5 mN Force applied to mirror No feedback to top stage with feedback to top stage Hierarchical control: top stage Done during the CITF commissioning Slide 38 Recombined Interferometer B7_demod B8_demod north arm west arm B5B5 B1B1B2B2 3 steps strategy Slide 39 Recombined Interferometer North lockedWest locked DC Error signal Correction Michelson locked Slide 40 Near Past and Near Future 4 days C3 run last week North Cavity with Automatic Alignment and Frequency stabilization West Cavity with Automatic Alignment only Recombined Interferometer Analysis under progress New Hierarchical control under test Improvement of robustness for Recombined Interferometer Transition to full Virgo beginning of this summer Slide 41 GEO TAMA AIGO VIRGO The other ITF detectors for GW 3 kilometric antennas : VIRGO (3 km) LIGO (2 antennas, 4 km) Coincidences and position reconstruction GW Astronomy needs at least 3 detectors LIGO Slide 42 LIGO Sensitivities Slide 43 LIGO Detection Range Slide 44 Conclusions Lot of debugging and experience acquired on the CITF Commissioning of full VIRGO started in September 2003 After 8 months: North cavity locked, aligned and frequency stabilization OK West cavity locked, alignment robustness to be improved Recombined Interferometer locked, alignment under progress, frequency stabilization to be done. Robustness has to be improved Full Virgo locked in simulation, preliminary measurements under progress. Experimental work to be done First run with full Virgo at the end of the summer ? First science run in 2005 R&D effort started for next generation Slide 45 GW: a never ending story The future of gravitational astronomy looks bright. 1972 That the quest ultimately will succeed seems almost assured. The only question is when, and with how much further effort. 1983 [I]nterferometers should detect the first waves in 2001 or several years thereafter () 1995 Kip S. Thorne Km-scale laser interferometers are now coming on-line, and it seems very likely that they will detect mergers of compact binaries within the next 7 years, and possibly much sooner. 2002