Commissioning Data on Vibration Isolation & Suspensions
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LIGO-G020486-00-W
Commissioning Data on Vibration Isolation & Suspensions
Fred Raab
24 October 02
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LIGO Laboratory 2LIGO-G020486-00-W
Vibration Isolation Systems
» Reduce in-band seismic motion by 4.5 – 6.0 orders of magnitude» Little or no attenuation below 20Hz» Large range actuation for initial alignment and drift compensation» Quiet actuation to correct for Earth tides, microseism at 0.15 Hz, and active
stack damping during observation
HAM Chamber BSC Chamber
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LIGO Laboratory 3LIGO-G020486-00-W
Seismic Isolation – Springs and Masses
damped springcross section
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LIGO Laboratory 4LIGO-G020486-00-W
Seismic System Performance
102
100
10-2
10-4
10-6
10-8
10-10
Horizontal
Vertical
10-6
HAM stackin air
BSC stackin vacuum
HAM chamber view
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LIGO Laboratory 5LIGO-G020486-00-W
Earth-Tide CompensationUsing Fine Actuation
Feedforward
Calibration
Feedback
Earth tide magnitude is hundreds of fringes
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LIGO Laboratory 6LIGO-G020486-00-W
Tidal Compensation Data
Tidal evaluation on 21-hour locked section of S1 data
Residual signal on voice coils
Predicted tides
Residual signal on laser
Feedforward
Feedback
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LIGO Laboratory 7LIGO-G020486-00-W
Microseism
Trended data (courtesy of Gladstone High School) shows large variability of microseism, on several-day- and annual- cycles
Reduction by feed-forward derived from seismometers
Microseism at 0.12 Hz dominates ground velocity
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LIGO Laboratory 8LIGO-G020486-00-W
Core Optics Suspension and Control
Local sensors/actuators provide damping and control forces
Mirror is balanced on 1/100th inchdiameter wire to 1/100th degree of arc
Optics suspended as simple pendulums
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LIGO Laboratory 9LIGO-G020486-00-W
Background Forces in GW Band = Thermal Noise ~ kBT/mode
Strategy: Compress energy into narrow resonance outside band of interest require high mechanical Q, low friction
xrms 10-11 mf < 1 Hz
xrms 210-17 mf ~ 350 Hz
xrms 510-16 mf 10 kHz
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LIGO Laboratory 10LIGO-G020486-00-W
Actuation/Isolation Transfer Function Should Be 1/f2
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LIGO Laboratory 11LIGO-G020486-00-W
Thermal Noise Observed in 1st Violins on H2, L1 During S1
Almost good enough for tracking calibration.