Again there was a sudden change in MICH noise. This time a lot of structures disappeared for about one hour, and than they came back.

The exact moment of the noise turning off is visible in the data: 17:17:39 UTC.

The event of MICH noise reduction, reported in the previous comment, is an effect of SSFS boost engagement, as we can see in Fig. 1. But in Fig. 2 we see the engagement of SSFS boost one hour later, and this time no reduction of MICH noise is visible.

In the other figure we compare SSFS and MICH error signal in thne first lock (blu lines) with respect to the second lock (red lines). Fig. 3 and Fig. 4 show the signals before the boost; Fig. 5 and Fig. 6 show the signals after the boost. No relevant difference in SSFS noise between the two locks, and a small difference in MICH noise before the boost. So it is not clear the huge difference in MICH noise after the boost.

We should check if this is an effect of an occasional ultra-fine tuning of B4 demodulation phase.

In the middle of the good period the phase was adjusted by 0.15 rad, because the phase monitor was measuring a mistuning (Fig. 1). On MICH error signal the effect was a further small improvement (Fig. 2). A larger improvement was visible in SPRB_B4_56MHz_I, out-of-loop monitor of frequency noise (Fig. 3). No change at all in SSFS in-loop signal (Fig. 4). The phase was still well tuned later, when MICH noise was largely higher; the possibility that a further phase tuning would have reduced the noise seems not realistic.

By tha way, the different behavior of SSFS_Err and B4_56MHz_I seems interesting to investigate.

In Fig. 5 SSFS error signal is superposed to SPRB_B4_56MHz_I in the two different conditions of noise on MICH. In both cases the out of loop signal is higher than SSFS signal starting from a few hundred Hertz. This happens not only in the floor, but also for the frequency noise peaks, and the level of the peaks changes when 'MICH noise' changes.

The fact that the peaks are 'frequency noise' is shown in Fig. 6, where instead of SSFS error signal the RFC error signal is plotted (input mode cleaner stand alone, MC locked on RFC). The peaks which go up and down in B4 are already present in IMC stand alone.

Finally, in Fig. 7 B4_56MHz I and Q are shown again in the two different conditions of noise. The common noise which goes up and down seems to be frequency noise.

We extracted from ENV channels some indications about the nature of the structures in  the frequency noise which is coupling to DARM.

Figure 1 indicates the peaks in DARM (MICH) we have investigated.

Figure 2 shows the ENV coherence projection of RFC channel. Indicated are the structures which are common to MICH and DARM noise, as reported in 37537.

• 20Hz, large cohence with seismometers on the External Inj Bench, it looks like a bench resonace, Figure 3
• 24.6Hz, looks like a fan or scroll pump. It is present more or less in all accelerometers in CEB plus the microphone in DET lab. Antonio says there are only two active Scroll pumps at present: one is in the MC tunnel close to MC end (but the absence of coherence with MC environemnet would exclude it) and the other one is the CEB remote scroll. Figure 4.
• 29.8Hz, this is NOT present in RFC signal and frequency noise. It was indicated as an SDB2 resonance, but seems it is not, since it did not reduce significantly after SDB LC improvements, and moreover  it is observed to move in frequency: see spectro in Figure 5
• 30.9Hz, this seems stable in frequency and it is a good candidate for being a mechanical resonance of something suspended. There is some coherence with accelerometers placed around the IB tower. In particular the peak at 30.5Hz in the accelerometers on IB cryoTrap seems to match the left "shoulder" of the 30.9Hz peak. Figure 6.
• 1Hz comb between 40 and 70Hz: it is coherent with CEB magnetometers and UPS, good candidates are the CT heaters. Indeed the NI CT heater power modulation seems to matche the comb on/off: Figure 7.
• 62.5 Hz, this peak is stable, another candidate for being a resonance of some suspended bench, no coherence at all with ENV channels.
• 101.3Hz, this peak is stable. As for the 30.9Hz, there is coherence with accelerometers around IB, the most coherent is the accelerometer on the IB flange facing SIB2, yet there is no correspondent peak in accelerometers. Figure 8.
• 107 Hz, no coherent ENV channels. Yet the peak frequency moves. We notice one long and one short periodicity,The short periodicity  is of about 1 minute: Figure 9.  It resambles something driven by temperature.

SUMMARY

Some switch off can help sources identification (CT heaters for the 1Hz comb, remote scoll for the 24.6Hz line and possibly coherent seism noise in the 30Hz region). It could be worth a deeper search of the IB area (SIB2, SIB1, cryo-trap,...) for example with shakings and tappings.

We extracted from ENV channels some indications about the nature of the structures in  the frequency noise which is coupling to DARM.

Figure 1 indicates the peaks in DARM (MICH) we have investigated.

Figure 2 shows the ENV coherence projection of RFC channel. Indicated are the structures which are common to MICH and DARM noise, as reported in 37537.

• 20Hz, large cohence with seismometers on the External Inj Bench, it looks like a bench resonace, Figure 3
• 24.6Hz, looks like a fan or scroll pump. It is present more or less in all accelerometers in CEB plus the microphone in DET lab. Antonio says there are only two active Scroll pumps at present: one is in the MC tunnel close to MC end (but the absence of coherence with MC environemnet would exclude it) and the other one is the CEB remote scroll. Figure 4.
• 29.8Hz, this is NOT present in RFC signal and frequency noise. It seems it is not a SDB2 resonance (as I erroneosly said in 37512) since it did not reduce significantly after SDB LC improvements, and moreover  it is observed to move in frequency: see spectro in Figure 5
• 30.9Hz, this seems stable in frequency and it is a good candidate for being a mechanical resonance of something suspended. There is some coherence with accelerometers placed around the IB tower. In particular the peak at 30.5Hz in the accelerometers on IB cryoTrap seems to match the left "shoulder" of the 30.9Hz peak. Figure 6.
• 1Hz comb between 40 and 70Hz: it is coherent with CEB magnetometers and UPS, good candidates are the CT heaters. Indeed the NI CT heater power modulation seems to matche the comb on/off: Figure 7.
• 62.5 Hz, this peak is stable, another candidate for being a resonance of some suspended bench, no coherence at all with ENV channels.
• 101.3Hz, this peak is stable. As for the 30.9Hz, there is coherence with accelerometers around IB, the most coherent is the accelerometer on the IB flange facing SIB2, yet there is no correspondent peak in accelerometers. Figure 8.
• 107 Hz, no coherent ENV channels. Yet the peak frequency moves. We notice one long and one short periodicity,The short periodicity  is of about 1 minute: Figure 9.  It resambles something driven by temperature.

SUMMARY

Some switch off can help sources identification (CT heaters for the 1Hz comb, remote scoll for the 24.6Hz line and possibly coherent seism noise in the 30Hz region). It could be worth a deeper search of the IB area (SIB2, SIB1, cryo-trap,...) for example with shakings and tappings.