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

2. I Fiori - GWDAW 9 - Annecy - Dec 16, 2004 2 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) resonances SA 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

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

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

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

6. 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 C1 C2 Single arm C1 C3 C4 Recombined C4 recombined : laser frequency locked to arms common mode

7. I Fiori - GWDAW 9 - Annecy - Dec 16, 2004 7 C4 Recombined 421 Hz (laser ele. rack) 2402 Hz (turbo pump) 231 Hz (water chiller laser) 150 Hz (mirror mount) C4 Recombined From single-arm to Virgo recombined 219 Hz (laser ele. rack) 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

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

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

10. I Fiori - GWDAW 9 - Annecy - Dec 16, 2004 10 LASER LAB. loudspeaker microphone Dark Fringe Effects of acoustic noise: signals layout VIRGO C4: IMC North arm West arm Acoustic noise RC 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)

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

12. I Fiori - GWDAW 9 - Annecy - Dec 16, 2004 12 Which path to dark fringe ? A look at coherences: Microphone vs. IMC (a,  ) IMC (a,  ) vs. IMC out Power IMC Out Power vs. Dark Fringe Microphone vs. Dark Fringe Microphone IMC a,  Fluctuations of IMC trasmitted Power Dark Fringe Dark Fringe vs. microphone is low  non linear path 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

13. I Fiori - GWDAW 9 - Annecy - Dec 16, 2004 13 a, a 2 , 2, 2 Non Linear effects Effect of misalignements ( a,  ) of IMC optical cavity : E tra  E 00 ( 1- ½(a 2 +  2 ) + i2a  ) coherence  P tra  P( a 2,  2 ) coherence Coherence: MC trans. Power vs. a,  Coherence: Dark Fringe vs. a,  frequency (Hz) Linear components may indicate a static (or low freq.) misalignement of the cavity: ~  a S a +  S  ~ ~ P  a  Opt. axis translation: a(t) W0W0    Opt. axis rotation:  (t)    a(t)+ a S  (t)+  S

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

15. 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