[Washimi, Takahashi]
We installed the geophone pod into the OMC chamber. The in-vacuum cable and outside cable were connected. The signals were confirmed on the MEDM screen.
I just compared the geophone spectrum (red) to that before replacement of the OMC stack (blue) around April 2nd (klog:29043).
I cannot see a significant improvement at high frequencies, but this might be due to the in-air measurement.
An in-vacuum measurement is necessary.
unfortunately, there are no klog posts about the OMC vacuum opening, but in the schedule spreadsheet, it should be performed on April 8.
So the April 2 data was in the vacuum.
A comparison of the in-air ACC spectrum before/after the stack replacement was reported in https://klog.icrr.u-tokyo.ac.jp/osl/?r=30109.
I checked the current geophone signal (red) at 9:30:00 UTC on July 5th, 2024 and compared it to the previous one (blue).
We can see significant improvement above 100 Hz!
Green : ADC noise (1st Jul. 13:00(JST))
Blue : Before starting pumping down (but all chamber closed) (5th Jul. 12:30(JST))
Rlue : After starting pumping down (a few e-3 Pa) (6th Jul. 8:30(JST))
Fig.1. spectrum for the geophone
Above 140 Hz, the difference of the spectrum appeared.
I didn't evaluate the sensor noise
Fig.2. spectrum and coherence for the microphone
limited coherence above 60 Hz.
Position of the microphone is not good or other reason for the discrepancy of the geophone spectrum?
Fig.3. spectrum and coherence for the OMC seismometer Y direction
I measured OMC geophone spectrum with TP for checking the spectrum at high frequency above 1 kHz.
I also measured the coherence between the geophone and the other channels(fig1).
Above 1 kHz, there are so many lines on the geophone signals.
These lines have a coherence with the AS TABLE GND and OMC CHAMBER GND as shown in the top right graph in fig1.
Since the AS TABLE GND, which is probably connected to OMC rack through RF PDs and QPDs, has higher coherence, it would somehow come from the GND fluctuation of the OMC rack even though the geophone signals are differential.
In addition, some frequencies like 1.2, 2.5, and 5 kHz, small bump of the spectrum can be seen in geophone signals.
Although I'm not so sure that these are real vibrations, a careful check is necessary, especially around 5 kHz.
Since the vibration around 5 kHz would be downconverted due to OMC dithering (5kHz), it could contaminate the DARM sensitivity.
Another important note is that, interestingly, the Z-direction signals of the seismometer at OMC booth have a greater coherence compared to that between the geophone and the Y-direction seismometer signals.
This implies that the current vibration on the OMC stack along the Y-axis is limited by the coupling from the vertical vibration even though the Z-direction seismometer signals are slightly lower than the Y-direction signals.
Since the dominant source of vibration on the OMC stack around 80 Hz, which is close to the OMC blade resonances, appears to be the vertical ground vibration, it might be problematic, so further investigation with OMC LSC is necessary.
In addition if we would like to reduce the vibration on the OMC stack, vertical vibration isolation seems more important than the other direction.
Measured data is stored at /users/Commissioning/data/PEM/OMC/2024/0706/OMC_vibration_20240706.xml.
>unfortunately, there are no klog posts about the OMC vacuum opening, but in the schedule spreadsheet, it should be performed on April 8.
Since our interest is the situation on April 2, the situation doesn't change (OMC should be vacuum on April 2) but according to the vacuum guage signals, vacuum venting was started on April 11 10:20 JST and finished on April 11 15:40 JST , not April 8 (fig1).
