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Environmental noise studies at VIRGO

The 9th annual Gravitational Wave Data Analysis Workshop – December 15-18, 2004 Annecy, FRANCE. Environmental noise studies at VIRGO. Irene Fiori – University and INFN Pisa, Italy (the Virgo Collaboration). Summary:. Environmental contributions to Virgo readout noise (C-runs)

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Environmental noise studies at VIRGO

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  1. The 9th annual Gravitational Wave Data Analysis Workshop – December 15-18, 2004 Annecy, FRANCE Environmental noise studies at VIRGO Irene Fiori – University and INFN Pisa, Italy (the Virgo Collaboration) Summary: • Environmental contributions to Virgo readout noise (C-runs) • many sources identified through • coherency analyses with seismic and acoustic sensors • and dedicated tests • Understanding the noise path through detector • preliminary results

  2. resonances SA Coherency Analysis: Low Frequencies (< 1 Hz) Dark Fringe noise below 1Hz is all seismic: • Residual seismic motion of mirror suspensions (Super Attenuators) • excited by the site microseismic activity (mainly oceanic microseism) • Multi-coherence analysis (NAP library, see poster session) : • - tri-axial seismometers in Central bld., North and West terminal blds vs. Dark Fringe • - disentagled contributions of seismicity at different locations along ITF • - correlation terms subtracted I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  3. Coherency Analysis: Higher Frequencies  10 Hz Dark Fringe coherent with acoustic/seismic sensors on some peaks/regions Watts / sqrt(Hz) Major sources identified through dedicated tests Frequency (Hz) 10 100 1000 Coherence (DF, microphone and seismometer) Noisy devices: air conditioning, pumps, racks coherence NE VIRGO C1 (single arm) 10 100 1000 Frequency (Hz) Microphone on laser optics table MC Seismometer on laser optics table WE LASER LAB Dark Fringe Central building I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  4. Air Conditioning low/high cycle RMS acoustic noise in laser lab. microphone C1 • AC switches to “high power regime” from Monday thr Friday 8:00 – 18:00 • Broadband acoustic noise in laser lab. • Dark fringe “breaths” at 11. and 14. Hz I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  5. Amplitude [Watts/sqrt(Hz)] Hz 600.8 1201.5 1802.5 2403.2 3004.3 3604.5 4806.7 5407.5 6008.0 2 x 10^-7 1 x 10^-6 4 x 10^-8 8 x 10^-8 3 x 10^-9 2 x 10^-9 6 x 10^-9 2 x 10^-9 8 x 10^-10 Turbo-molecular vacuum pumps: sweep test • 1 pump per SA tower (UHV < 10-9mbar in tower lower section) • magnetically levitated, rotation speed 400 Hz or 600 Hz VIRGO – C2 IB tower pump: sweep 600Hz 400Hz • fundamental and harmonics sweep coherently in dark fringe and seismometer Seismometer near IB tower Dark fringe photodiode I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  6. From single-arm to Virgo recombined • Single-arm (C1, C2): coupling to common noise (i.e. frequency noise) is maximum • Recombined (C3, C4): common noise suppressed by CMRR factor  0.004 • C4 recombined : laser frequency locked to arms common mode Single arm C1 C1 C2 C3 C4 Recombined I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  7. From single-arm to Virgo recombined • Single-arm (C1, C2): coupling to common noise (i.e. frequency noise) is maximum • Recombined (C3, C4): common noise suppressed by CMRR factor  0.004 • C4 recombined : laser frequency locked to arms common mode 150 Hz (mirror mount) 231 Hz (water chiller laser) 219 Hz (laser ele. rack) C4 C4 421 Hz (laser ele. rack) 2402 Hz (turbo pump) Recombined Recombined I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  8. Which path for seismic/acoustic noise to dark fringe ?

  9. Acoustic test during C4 run • Broadband white signal sent to a loudspeaker in laser laboratory, • with 5 levels of increasing intensity • acoustic noise increase in laser lab. • up to 50 times the • standard noise floor • at [30, 4000] Hz • noise increase in dark fringe • up to 10 times • at [150, 1500] Hz I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  10. Effects of acoustic noise: signals layout VIRGO C4: IMC North arm RC LASER LAB. microphone West arm loudspeaker Acoustic noise Dark Fringe • Injection SYS: • - Laser clean room: laser, beam forming optics, photodiodes&piezos on non suspended benches, in air • Input Mode Cleaner: plane concave triangular FP, 144m, reference cavity, suspended, under vacuum • - Alignement: laser on RC (<1Hz), IMC optical axis (<10Hz) I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  11. Effects of acoustic noise: signals layout • Misalignements of IMC (a,  ) IMC translation (a) IMC rotation ( ) • Power fluctuations of MC transmitted beam RC LASER LAB. microphone loudspeaker ITF trans. power Acoustic noise IMC trans. power Dark Fringe I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  12. Which path to dark fringe ? A look at coherences: Dark Fringe vs. microphone is low  non linear path Microphone vs. Dark Fringe Fluctuations of IMC trasmitted Power IMC a, Microphone Dark Fringe Jitter of laser beam is non compensated by IMC alignement control Misaligned MC gives power fluctuations of transmitted beam Power fluctuations converts into ITF readout noise Microphone vs. IMC(a, ) IMC(a, ) vs. IMC out Power IMC Out Power vs. Dark Fringe I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  13. Non Linear effects a(t) a  Effect of misalignements (a,) of IMC optical cavity : Opt. axis translation: W0  (t) Etra E00 (1-½(a2+  2) + i2a )  PtraP(a2, 2)   Opt. axis rotation:   Coherence: MC trans. Power vs. a, Coherence: Dark Fringe vs. a, a, a2 coherence ,2 coherence frequency (Hz) frequency (Hz) • Linear components may indicate a static (or low freq.) misalignement of the cavity: a(t)+ aS  (t)+S ~ ~ ~  aS a + S P I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  14. Power noise propagation model P Spn(t) = S Power noise contribution to Sensitivity : P S(t) = sensitivity [m] 1) Naïve model:S  SRMS S = displacement from the dark fringe P 2) More accurate model S low freq. part (<50Hz) of S = relative power fluctuations P • C4 sensitivity (S)during acoustic noise injection • Power noise estimate(S)(Naïve model) • Power noise estimate (S)(more accurate model) I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

  15. Conclusions • We have identified and characterized seismic/acoustic noise • sources affecting detector sensitivity during Virgo commissioning • through coherency analyses and dedicated tests • Effects of these sources on Virgo dark fringe reduced, and almost • disappeared (C4), as laser frequency noise reduced when ITF was • operated in the recombined configuration • A test was performed (C4) to verify the robustness of our injection • system against acoustic noise, by injecting noise 50 times larger • than std. level • This noise produced a jitter of the beam at the Mode Cleaner input, • which caused disalignemnets of the MC cavity, • and at least partially converted into dark fringe power noise. • A power stabilization of the MC output beam is currently being • commissioned I Fiori - GWDAW 9 - Annecy - Dec 16, 2004

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