Summary

Referring PEM's tapping activies around the OMC chamber in the past (For example 27442, 26011, 27258, 25919), I manually (by my hand/fingers) tapped or rather touched around the OMC chamber. Everywhere was somehow sensitive, but particulary I confirned that some sticking out parts (or mechanically "weak" parts) of the vacuum pump unit attached on the tube in-between OMM-OMC was quite sensitive to the in-vac geophone in the OMC chamber. Moreover, the geophone's behavior itself is strange.

Background/Motivation

In O4a we found that still some acoustic coupling at the OMC chamber to the interferometer sensitivity was large. This coupling should be the first prioritized enemy for us to overcome for the better sensitivity. So far some PEM acitivies revealed that the invac geophone itself was largely responding to tapping the chamber. This would mean the vibration isolation situation in the OMC chamber would be, say, out of question, to the level that it would be even too early to discuss about stray-light noise. This coupling mechanism has to be fully revealed and to be taken care well enough at first.

Details

Firstly, I looked at the filter bank of the relevant invac geophone channel K1:PEM-ACC_OMC_VACTABLE_OMC_Y_OUT. (The relevant MEDM can be accessible by sitemap -> OMC -> OMC Dock -> GEOPHONE OMC; see 25919)In short or in conclusion, I am still not confident on what are activated  in this filter bank (see note below). I compared the filters activated in the filter bank with those for the other geophones, but could not become convinced fullily. This point should be clarified.

I firstly tried to rely on OUT channel after the filters, but I felt the behavior was strange and unuseful. Maybe due to the filters, the filtered signal showed large low-frequency surge when shook was applied, and the surge always started with going "positive". Whether this behavior would be real DC moves or not, the DC trend was not useful for my today's work. So I mainly looked at the singal just before the filter chain. As written in the note below, still there is a strange thing, though.

Anyway, I strated tapping. What surprised me was that my light stomping on the floor outside of the clean booth was clearly observed with the invac geophone. Also, I just tapped the top surface of the desk outside the clean booth, and it was also observable. Too sensitive. I maybe like to do the similar thing for the other place for comparison.

Next, I entered in the clean booth. I checked that the vacuum gauge attached to the OMC-OMM chambers showed 1.2e-5 Pa, so the inside of these chambers must be vacuum. Tapping around the vacuum chambers, I found everywhere was somehow responsible, but particulalry the vacuum pump unit was the worst (best response, in some sense, but worst for detecting GW). Particularly, some sticking-out parts such as a slender tube of a mass spectrometer, and manual handles for gate valves (there are two gate valves). The structure of the gate valve is thin, so the structure might be easily fluctuating. The white-ish support structure of the vacuum pump unit was also sensitive.

Although still the coupling path is not clear, one solution would be that these mechanically weak structures should be put away from the most sensitive part of the interferometer, the OMC chamber.

Note

• As shown in Fig. 1, the input stage itself of this geophone signal is strange. For some reason the lower was bounded by something. Maybe as a result of this lower bound, the filtered signal always start with positive surge. Moreover, the low-frequency surging itself would indicate that the filter setting might be not good. I think that this should need to be checked at first from the bottom.