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

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

I Fiori - GWDAW 9 - Annecy - Dec 16, 2004 3

Coherency Analysis: Higher Frequencies

LASER LAB Dark FringeCentral building

10 Hz

Seismometer on laser optics tableMicrophone 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

10 100 1000 Frequency (Hz)

Wat

ts /

sqrt

(Hz)

cohe

renc

e Noisy devices:air conditioning, pumps, racks

VIRGO C1(single arm)

Coherence (DF, microphone and seismometer)

Frequency (Hz)

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

I Fiori - GWDAW 9 - Annecy - Dec 16, 2004 5

Turbo-molecular vacuum pumps: sweep test

IB tower pump: sweep 600Hz 400HzVIRGO – C2

600.81201.51802.52403.23004.33604.54806.75407.56008.0

Hz Amplitude[Watts/sqrt(Hz)]

2 x 10^-71 x 10^-64 x 10^-88 x 10^-83 x 10^-92 x 10^-96 x 10^-92 x 10^-98 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 < 10-9mbar in tower lower section)

• magnetically levitated, rotation speed 400 Hz or 600 Hz

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

C1C2Single armC1

C3

C4Recombined

• C4 recombined : laser frequency locked to arms common mode

I Fiori - GWDAW 9 - Annecy - Dec 16, 2004 7

C4Recombined

421 Hz (laser ele. rack)2402 Hz (turbo pump)

231 Hz (water chiller laser)

150 Hz (mirror mount)

C4Recombined

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

Which path for seismic/acoustic noise to dark fringe ?

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

I Fiori - GWDAW 9 - Annecy - Dec 16, 2004 10

LASER LAB.

loudspeakermicrophone

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)

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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

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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 ofIMC trasmitted

PowerDark Fringe

Dark Fringe vs. microphone is low non linear path

Jitter of laser beamis non compensated by IMC alignement control

Misaligned MC gives power fluctuations of transmitted beam

Power fluctuationsconverts into ITF readout noise

I Fiori - GWDAW 9 - Annecy - Dec 16, 2004 13

a, a2

, 2

Non Linear effects

Effect of misalignements (a, ) of IMC optical cavity :

Etra E00 (1- ½(a2+ 2) + i2a )

cohe

renc

e

Ptra P(a2, 2)

cohe

renc

e

Coherence: MC trans. Power vs. a, Coherence: Dark Fringe vs. a,

frequency (Hz) frequency (Hz)

• Linear components may indicate a static (or low freq.) misalignement of the cavity:

~ aS a + S ~ ~P

a Opt. axis translation:a(t)

W0

Opt. axis rotation: (t)

a(t)+ aS

(t)+S

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Power noise contribution to Sensitivity :

Power noise propagation model

S = displacement from the dark fringe

Spn(t) = S PP

1) Naïve model: S SRMS

• C4 sensitivity (S) during acoustic noise injection

• Power noise estimate (S) (Naïve model)

• Power noise estimate (S) (more accurate model)

PP = relative power fluctuations

S(t) = sensitivity [m]

2) More accurate model S low freq. part (<50Hz) of S

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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