Commissioning task ENV02. PURPOSE: investigate the bunch of Hrec peaks between 41 and 43Hz (more precisely 41.15, 41.9, 42.4, 42.9 Hz). See first attached plot. Previous investigations in 46037 (peaks have some small coherence with WEB magnetometers and UPS).

MAGNETIC SNIFFING
We sniffed with portable magnetometer (tmp channel ENV_WEB_MAG) various locations (PCAL rack, PCAL boxes placed on tower base at East side, LC SE corner, camera NE corner) but found no evidence of magnetic emissions associated to the peaks.

TAPPINGS
We tapped various components, see attached log file with times.

• We saw no effect in DARM when tapping: SWEB link pipe, SWEB vacuum chamber, HWFS bench pipe, Cryotrap bellows, Cryotrap tank (we hammered on the joints of lateral rods with the help of Antonio Pasqualetti).
• We saw an effect when tapping on "costolone": some bumps apeared in Hrec in correspondence with loud vibration peaks seen by the vacuum chamber ENV accelerometer, see Figure #2.  But nothing at 41-43Hz. The frequency pattern resembles scattered light. The coupling seems not critical, but worth to ty to extract estimated coupling from the data.

TOWER VACUUM CHAMBER EXCITATIONS, using Kagra portable shaker
We attached one accelerometer to WE vacuum chamber at the mid stage (second ladder): ENV_WE_TOP_ACC_X (same used for 46534). Then:

• *Without ITF* we knocked on the virola and clearly excited resonances at 41.35 and 70.3Hz: Figure #3 and Figure #4. We repeated using the Kagra bluetooth shaker attached to the tower mid stage virola: Figure #5
• *With interferometer* we injected some noise using kagra's shaker this time attached at the bottom flange (W):
  • 41.3Hz line (to match the chamber mode): Hrec excites easily
  • recursive sweep between 35Hz and 45Hz: we see Hrec excites each time the sweep frequency crosses 41.3-41.4Hz but not elsewere: Figure #6
  • 70.3 hz line (to match the other chamber mode): Hrec excites easily
  • recursive sweep between 65Hz and 75Hz: we see Hrec excites each time the sweep freq. matches exactly 70.3Hz: Figure #7

PCAL

Since we noticed the 41-43Hz peaks have some (small) coherence with PCAL_WEB_PSD2 signals (but no peaks in this channels), we want to try to have it off. (note the WEB PCAL was disabled since Sept.5 because of a failure). We contacted Dimitri Esteves (PCAL on call) who first set to zero the voltage sent to the Pcal PSD and then advised to disconnect the the 4 DAC cables of the WE PCAL PSD1 and PSD2 driving, which we did. We lesft the cables disconnected. Yet the 41-43 Hz peaks today (Oct 5) are still there.

Conclusions:

• the source of the 41-43 Hz is still not found
• we understood the origin of the peaks that we excited when injecting acoustic noise at WEB (see 46708 ): they are vibration modes of the vacuum chamber. at least the 70.30Hz peak is present in the quiet sensitivity (see https://logbook.virgo-gw.eu/virgo/uploads/46708_1566222859_FFT_highRes_Hrec.PNG) so it is worth to further investigate the coupling. Couls it be scattered light? However the chamber mode at 41.35 Hz seems not to be one of the 41-43Hz family, but it has to be better evaluated.
• we find some scattered light (tipical bumps pattern) when knocking on the costolone, which seems not critical, but should be better quantified.

TO DO: the magnetic sniffing of 41-43Hz source can continue also without ITF, i.e. next TUESDAY morning.