Virgo Logbook

The (unconfirmed!) hypothesis is that the magnetic noise seen at the NE building is somehow caused by the UPS of the 2400N pumping station. What is still missing is an explanation how this could be seen as magnetic noise in the NE building. It seems unlikely that the noise is generated along the tube, and then measured at the terminal building 600 meter further. It is more likely that the noise is present as a current on the IPS cable between the pumping station and the technical building at the end. This cable is traveling via a cable-tray along the west-wall of the tunnel. In the NE-building, this cable tray is mounted about 3 meter high, again along the west wall and passes just a few meters from the permanent magnetometers.

Measuring with the portable magnetic probe and the Onosokki, we could indeed see the bumps near the power cables that exit the tunnel (no plots available). A few meter away, these bumps cannot be seen by the probe (which is not as sensitive as the big magnetometers).

Since the big magnetometers in the end-buildings have some problems with saturation, we brought one of the old ones from the central building, which are 'desensitized' by a factor 5. It was connected in the place of Em_MABDNE02, without adjusting the calibration value.

Events (all local time on 08/03/2012):
12:20 connected desensitized magnetometer from CB in place of MABDNE02
12:25 huddle test with MABDNE01 (east-west direction, close to west wall)
12:34 magnetometer placed east-west in middle of hall in front of entry doors
12:38 magnetometer placed east-west close to east wall, in mirror position of MABDNE01 with respect to tower
12:44 back to west side of the tower, in line with MABDNE01, but closer to tower. We left it like this.

Fig 1 shows the huddle test, showing the factor of 5.
Fig 2 compares the spectra for the huddle test (red) middle of hall (blue), east wall (green) and close to tower on the west side (black).
Fig 3 compares the spectra during the night with the 3 original magnetometers. The bumps at 14, 36, 64, ... Hz are well visible along the east-west and up-down direction, but are less clear along the north-south direction. This is consistent with a magnetic field generated by a cable in the north-south direction.

The evidence is not conclusive, but the fields seem a bit stronger on the west-side. This is consistent with a field generated by the IPS cable along the west-wall. The bumps seem to be the strongest close to the tower, which might be caused by distortion due the large amount of metal of the tower. This does not say anything about what is happening to the field inside the tower, which is what counts.