On Nov 3rd, 2025 at 09:20 UTC, we observed a decrease of the magnetic field inside the West end building (ENV_WE*_MAG_*) and this decrease was visible only in the current monitor of T-phase of the IPS line (ENV_WEB_IPS_CURR_T).
Figure 1 shows the spectrograms of the magnetometer installed in the experimental hall and of the IPS current monitor.
Figure 2 shows a comparison of the magnetic field measured by the WEB magnetometers installed in the experimental hall (including one located on the tower base), before (blue curve) and after (purple curve) the device switch-off.
A possible explanation is that a device powered by the IPS T phase was switched off around that time. Supporting this interpretation, during the WEB UPS replacement on July 1st, 2025, we observed that the magnetic field inside the hall did not change (see attached plot of elog 67815).
By comparing the magnetic spectra of the two terminal buildings, we notice that they are now more similar (Figure 3). Regarding the residual structures in Figure 3, our current understanding is as follows:
- the NEB magnetometers show sidebands around the 50 Hz line at ±13 Hz, which are associated with the vacuum UPS modules installed at 2400 m along the North arm.
- the WEB magnetometers show sidebands around the 50 Hz main line at ±6 Hz. During a past investigation at WEB (elog 65201), we placed a magnetometer close to the cable tray, and we already observed the presence of these ±6 Hz sidebands in the magnetic signal. Subsequently, we identified the source of this noise as the vacuum UPS module installed at 2100 m along the West arm. This module is connected to the T phase of the IPS line (elog 65205).
A follow-up investigation is ongoing to identify the device responsible for the magnetic field decrease.
