MSC - 18 Oct 071 LOW FREQUENCY SEISMIC NOISE: LOCKING AND SENSITIVITY ISSUE Paolo Ruggi noise meeting

MSC - 18 Oct 072 VSR1 IN BAD WEATHER CONDITIONS analysis of ITF behavior over one day on Sep 29 – high level of sea activity at the beginning; low level at the end  SEISMIC NOISE REJECTION how the low frequency seismic noise is attenuated by the suspensions PSEUDO  SEISMIC NOISE INJECTION trying to reproduce the ITF behavior by injecting noise at the top stage, for each suspensions and each d.o.f.

MSC - 18 Oct 073 VSR1 IN BAD WEATHER CONDITIONS

MSC - 18 Oct 074 Sensitivity and stability Vs  Seism level  Seism level > 1.5 effect on sensitivity and stability  Seism level < 1 no effect on sensitivity and stability 1 <  Seism level < 1.5 small effect on stability questionable effect on sensitivity

MSC - 18 Oct 075 Stability: B5_DC Vs alignment The power fluctuation of the carrier in the recycling cavity is due to IB oscillation in theta z

MSC - 18 Oct 076 Stability: B5_2f_ACq Vs alignment The power fluctuation of the sidebands in the recycling cavity is due to IB oscillation in theta z

MSC - 18 Oct 077 Stability: B1_DC Vs alignment The power fluctuation of B1 is due to IB oscillation in theta z

MSC - 18 Oct 078 Stability: B1p_DC Vs alignment The power fluctuation of B1p is due to IB oscillation in theta z and differential end oscillation in theta y

MSC - 18 Oct 079 CONCLUSION #1 The stability of the power resonating in the ITF should improve a lot if we stabilize the IB control in theta-z

MSC - 18 Oct 0710 Sensitivity Vs  Seism level  Seism level: 3  Seism level: 2.5  Seism level: 1.5  Seism level: 2  Seism level: 1  Seism level: 0.5

MSC - 18 Oct 0711 Sensitivity Vs  Seism level: Hz  Seism level: 3  Seism level: 2.5  Seism level: 1.5  Seism level: 2  Seism level: 1  Seism level: 0.5

MSC - 18 Oct 0712 Sensitivity Vs  Seism level: Hz  Seism level: 3  Seism level: 2.5  Seism level: 1.5  Seism level: 2  Seism level: 1  Seism level: 0.5

MSC - 18 Oct 0713 Sensitivity Vs  Seism level: Hz  Seism level: 3  Seism level: 2.5  Seism level: 1.5  Seism level: 2  Seism level: 1  Seism level: 0.5

MSC - 18 Oct 0714 Sensitivity Vs stability: Hz The noise below 100 Hz is strongly non stationary: we observe glitches from time to time. The glitches are not coincident to the power fluctuations

MSC - 18 Oct 0715 Sensitivity Vs alignment: Hz The noise between 100 Hz and 500 Hz (mystery noise) uses to grow as the inaccuracy of alignment. The effect of q81 mis- centering (BS AA) is known since the beginning of VSR1. The differential end theta-x has been observed to be responsible as well: on Sep 27 an instability of locking at Hz, affecting the alignment of the dark fringe, was the cause of a large increase of mystery noise. In case of  Seism, all the alignment d.o.f. are disturbed. The incidence of each d.o.f. upon the mystery noise modulation is an open matter.

MSC - 18 Oct 0716 Sensitivity Vs stability: Hz The black curve is the inverse of the noise level at 5 kHz. This is a measurement of the noise floor at high frequency. A correlation with the level of power on B1 is visible. At this level, we cannot say if the increase of the floor is simply due to an increase of the optical gain (no effect on the sensitivity), or if there is something else.

MSC - 18 Oct 0717 CONCLUSION #2 From this rough analysis of VSR1 data, we learn almost nothing about the path from the excess of ground motion at low frequency to the worsening of ITF sensitivity

MSC - 18 Oct 0718  SEISMIC NOISE REJECTION

MSC - 18 Oct 0719  Seism at the ground We call ‘  Seism’ the ground motion in the region 0.2 – 1 Hz. Usually it is due to the sea activity. The top stage Lvdt measures the relative motion between the ground and the suspension point. At the frequency of  Seism, this is the best measurement of the ground motion for each suspension, provided that the suspension point moves much less.

MSC - 18 Oct 0720  Seism at the suspension point

MSC - 18 Oct 0721 As already said, in the region of  Seism the suspension point moves much less than the ground: the attenuation is a little less then two orders of magnitude.

MSC - 18 Oct 0722 By using the control signal of the ITF – the locking correction applied to the marionette - one can compute the equivalent free motion of the payload, as it would be without the applied force. The attenuation with respect to the ground motion is about one order of magnitude in rms (above 100 mHz).

MSC - 18 Oct 0723 Impact on the ITF longitudinal error signals

MSC - 18 Oct 0724 Impact on the ITF angular error signals, in loop full bandwidth

MSC - 18 Oct 0725 Impact on the ITF angular error signals, DC controlled in loop full bandwidth

MSC - 18 Oct 0726 Impact on the IMC angular error signals, in loop longitudinal

MSC - 18 Oct 0727 The attenuation of  Seismic noise performed by VIRGO suspension is not so bad, even if it is centered in the region of the superattenuator resonances. An increase of noise by one order of magnitude, with respect to quiet conditions, produces a worsening of the ITF error signals accuracy by a factor of two. CONCLUSION #3

MSC - 18 Oct 0728 PSEUDO  SEISMIC NOISE INJECTION – ONE BY ONE

MSC - 18 Oct 0729 Noise injection at end suspension top-stages We tried to reproduce a longitudinal motion of the top stage similar to the actual one, seismically excited, by injecting noise on the ID actuators. We obtain a reliable angular motion in tx, while ty is not perturbed. The longitudinal accuracy is much worse, due to an excess of noise at 0.2 Hz.

MSC - 18 Oct 0730 The mystery noise grows to the level of Sep 29. An input port of  Seismic noise Vs mystery noise is located at the end towers, passing through the diff end tx perturbation Actual  seism OBO Noise injection at end suspension top-stages

MSC - 18 Oct 0731 Comparing to a reference curve, taken when no noise was injected, the injection of noise has an effect only at the level of mystery noise. Nothing happens in the other frequency regions. Noise injection at end suspension top-stages

MSC - 18 Oct 0732 Noise injection at BS suspension top-stage With a similar excitation of the top stage, the longitudinal accuracy is much worse. The angular motion is larger in tx, smaller in ty.

MSC - 18 Oct 0733 Some small change with respect to the reference curve, below the region of mystery noise. Not enough to explain the noise in actual condition, considering that the applied disturbance was much larger in OBO test. Noise injection at BS suspension top-stage

MSC - 18 Oct 0734 Noise injection at IMC suspension top-stages Very difficult to excite the IB in a reliable way simply injecting pseudo  Seismic noise at the top stage. We obtained a sufficient excursion of IBtz only applying a strong line tuned at the frequency of the main resonance. In this case, we were close to reproduce the instability of the power observed in actual conditions.

MSC - 18 Oct 0735 Noise injection at IMC suspension top-stages About the sensitivity, no effect has been observed.

MSC - 18 Oct 0736 Many other OBO injections have been performed on the suspensions. In some cases we observe small effects, but no major channel of disturbance for the sensitivity has been found, even considering that we were exceeding with the input noise (BS is just one example). However, it is clear that the worsening of the ITF AA error signals accuracy in actual conditions is centered in the  Seism region, and comes from the ground through the suspensions. The effect of diff end tx on the mistery noise is proved, and we cannot exclude an overall effect of the other degrees of freedom. But it is important to say that the mystery noise should not be there. Removing it, the requirement for  Seismic noise rejection could change. CONCLUSION #4