We followed the prescriptions described in the git task.
At first we observed that during quiet time the 63Hz bump was not present both in Hrec and B4_DC signals. A quick look at VIM spectrograms showed that the bump was present at least in B4_DC signal up to before yesterday maintenance. This has to be further investigated.
We proceeded with injecting vibration noise with the shaker (which is positioned on EPRB since yesterday). As a first attempt we injected a line at 63 Hz (Figure 1). However the volume was set a bit too high and we lost the B4 beam signal after 13s of shaking, the ITF unlocked right away.
Once the ITF relocked we injected a colored noise shaped between 50 Hz and 70 Hz at what we judged to be the maximum volume allowed. This injection lasted 300s. This injection produced a corresponding bump in B4_DC but not in hrec (purple lines in Figures 5 and 6).
Camilla added an high absorption optical density filter (transmission 1/1000) in front of the razor stack beam dump which blocks the beam from the scanning Fabry Perot, and closed a bit two irises along the path of the phase camera beam. She also had to move the shaker in another positon on EPRB because of space constraints. Figure 2 and Figure 3 are picture taken respectively before and after these actions. The ITF did not unlock.
We repeated the injection of the same colored noise (same shape and same volume). The bump excited on B4 changed slightly shape and size, but this seems to follow a similar slight reduction in the excitation sensed by the EPRB accelerometer (orange curves in Figures 5 and 6).
Camilla performed the second part of the planned intervention. She added one optical density with 30% transmission of the phase camera beam path. Figure 4 is a picture taken after this actions.
Again we repeated the injection of the same colored noise. We did not observe any reduction of the bump we excited in B4: see the blue curves in Figures 5 and 6.
We moved the shaker on the PR tower base (West side) and repeated the injection of a few lines around 60 Hz in the attempt to investigate further what obseved in https://logbook.virgo-gw.eu/virgo/?r=64685
Precisely we injected lines at 70Hz, 60Hz, 65 Hz, and 64 Hz. When injecting at 60 Hz we see clearly a line in Hrec (and also in B4): see Figure 7. Lines at 70, 65 and 64 produced someting in B4 Hz. Finally the injection of the line at 64 unlocked the ITF.