environmental noise studies at virgo
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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)
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
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 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
I Fiori - GWDAW 9 - Annecy - Dec 16, 2004 14
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
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
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