The goal of the shift was to look for the noise source of the 63 Hz bump and the coupling location. We performed the following actions.

• We installed a beam dump at the input of EPRB, Fig 1. After the relock of ITF, the bump on Hrec was no longer visible, Fig 6 and Fig.7 (compare purple vs orange). It is worth noticing at that time, the noise level of the 63 Hz vibration was smaller on all accelerometers. To validate the effect of the beam dump, we placed the shaker on EPRB and generated different kinds of vibration:
  • sweep in the free. range (60-65) Hz
  • lines at 60Hz and 62 Hz
• Although the vibration level on the bench increased by at least 10 times, we did not generate any bump in Hrec, blue curves in Fig.6,  blue and green curves in Fig.7.
   
• We reconnected the vertical accelerometer ENV_EPRB_ACC_Y which was already on the bench but not acquired, Fig 1. Comparing the ASD, we found that this accelerometer measures the 63 Hz vibration 10 times larger than the SPR accelerometer.
   
• We used the accelerometer of the INJ cryotrap ENV_IB_CT_ACC_X as a sensor to perform our sniffing investigation.
   
• We found the NN geophone #209 in contact with the metal H-beam. We moved it to the floor nearby and in this position, the geophone did not see any more vibration at 63 Hz,  Fig 2. We deduced that the vibration is transmitted through the H-beam.
   
• We disconnected the cables of the piezo of EPRB without effect on EPRB/SPR accelerometers. We can exclude this source of vibration. The cables have been reconnected.
   
• We tapped the EPRB and we observed the excitation of the bench resonances at 60 Hz and 110 Hz, Fig 3. This explains why the 63 Hz vibration is amplified on the bench. 
   
• For the sniffing, we installed the ENV_IB_CT_ACC_X on the base of the WI SAT rack and on the Sa suspension crate, Fig. 4. To understand if the noise is associated with the SAT cooling fans,  we asked Alberto G. to increase the speed. No effect is observed on 63 Hz vibration on EPRB, therefore we can exclude this noise source, Fig. 5.
   
• As a last action, we moved the small shaker on the PR tower base. The injected line at 60 Hz is well visible in Hrec (Fig. 8) as a narrow line. This seems indicating a different coupling path (linear), which is not the non-linear path due to EPRB scattering. It is worth further investigation.

We concluded our investigations around 19:30 LT because the ITF was repetitively unlocked, not because of our actions.

At the end of the shift, we removed the beam dump and shaker from EPRB. It would be interesting to repeat the bench shaking without the beam dump.