Virgo Logbook

I had a look at some trends of the bubble noise during an entire refilling cycle of the crytrap. Fig 1 shows the spectra of B1, the new seismic sensor attached to the cryotrap and their coherence. The legend indicates the number of hours since the last refill. The noise measured by the sensor reduces with time over a large band. Note that this does not necessarily mean the noise by the boiling LN reduces, it could be that the evaporation of 100 kg(?) of LN has a significant influence on the mechanical coupling between fluid and sensor. It might also simply be the increasing distance between top of the fluid and the sensor. Only the coherent bump at 65 Hz seems clearly related to the cryotrap. The bump seen earlier around 120 Hz is going down too fast to be positively identified.

Fig 2 shows some trends over time. The refilling cycle is clearly seen in the RMS of B1 in the band 60 to 70 Hz, it is not seen in any other bands.

Accoring to Julien, since the removal of the baffles, the wall of the cryotrap can now be seen from the detection bench. Any light scattering e.g. from L1 could thus hit the wall of the tank. At the moment we install a new AR-coated window of the detection tower, we might also have to install a 'non-suspended, non-vibrating' baffle to cover up the naked cryotrap.