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Environmental Monitoring (Environmental Monitoring)
fiori, derossi, paoletti - 18:47 Tuesday 12 November 2019 (47617) Print this report
hunting for 5hz comb and the higher magnetic fields close to WE chamber

Today during maintenance we started investigating about the observed apparently larger magnetic fields close to WE tower.

Figure 1: shows the spectra of our ENV magnetometers at NEB and WEB: the WEB ones are larger, also in the region 40Hz to 50Hz where we are measuring magnetic coherence with Hrec.

Note that the comparison is not completely fair since, while all three WEB magnetometers are close to WE vacuum chamber North side  (since end of May, 45983), at NEB just one magnetometer (ENV_NEB_MAG_V) is close to NE vacuum chamber East side, since June (46079).

Another common feature of these magnetometers is a 5Hz spaced comb: both at NEB and WEB, which is coherent among the two: Figure 2. The peaks are not exact multiples of 1Hz, but slightly displaced and seem not coherent with Virgo clocks.

 We did some sniffing with a magnetic probe (ENV_WE_MAG) trying to locate the 5Hz comb source at WEB. Which we could not find. However we observed that:

  • we compared the three sides of the WE tower, and found that the noise looks stronger by a factor 2 to 10, depending on direction on the NORTH side (the one that faces the racks): see Figure 3
  • the 5Hz comb rises together with a broadband white noise and also another (?) comb located below 10Hz and spaced by 0.5Hz
  • we sniffed the racks, cables, cable trays and found nothing stronger than when close to the tower. We sniffed some VAC devices located both on the upper floor and at tower base, north side, bout found no evident emitter.
  • we left the probe in the location where we measured the most intense noise, about 5 times larger than elsewhere: see ENV_WE_MAG signal in Figure 4. This location is on top of a piece of metal of VAC equipment, but apparently passive... see picture in Figure 5. It will be interesting to check for coherence with Hrec.

In order to do a fair comparison, a similar investigation should be done at NEB.

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fiori, paoletti, francescon, pasqualetti, tringali - 14:59 Thursday 19 December 2019 (47985) Print this report

With the help and advice of vacuum team today we switched off some devices at WEB. We found that the the ITR gauge pressure devices were contributing to the 0.5Hz and 5Hz magnetic combs. One such device is located at the tower base level, N side (close to where magnetometers are presently located) and a second device is located at the platform first floor, East side.

Figures 1 and 2: once switched off the devices (power plug disconnected) the magnetic noise measured by the magnetometers reduced in a broadband range by up to 1 order of magnitude.The 0.5Hz magnetic comb at low frequency seems completely removed. The 5Hz comb, affecting mainly above 100Hz is greately reduced but some residual is still present.

Figure 3: we do not see a sizeable effect in Hrec. Also coherence with magnetometers and UPS probes in the region from 40 to 60Hz seems more or less unchanged.

The devices were powered on but, according to VAC team, not actively queried for data. Now both devices have been left powered OFF with the power plug disconnected.

We could not investigate, because of limited time, if the stray mag fields were radiated by the source and/or by the cables.

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