Summary: Magnetic fields from aLIGO electronics were measured in the LVEA test stand racks. Fields from the switching power supplies in the black IO boxes dominated, and were larger than fields from VME crate power supplies, that had produced lines in DARM during iLIGO. I measured magnetic coupling to some of the channels that were working and estimated that the switcher fields were within a factor of 6 of producing features in those particular channels. We may want more of a safety margin than this, so I suggest investigating different power supplies.


I had a look at magnetic fields inside of the aLIGO electronics racks that were set up for the test stands holding the ITMY ISI and SUS, and compared the fields to those in iLIGO racks. I checked several sites but discuss 2 of the most important iLIGO magnetic field coupling sites below. The magnetometer was calibrated in-situ using a coil calibrator. 

Figure 1 compares the magnetic fields in the electronics rack near a test mass controller for aLIGO (ITMY coil driver in test stand racks in the +Y arm of the LVEA) and one for iLIGO (4k ITM controller measured during S6). The 60 Hz fields are significantly greater in the aLIGO setup than in iLIGO. However, this was not due to components in the racks: the fields outside of the test stand racks were about the same as inside and were more than an order of magnitude greater than at typical LVEA sites during science runs because the test stand racks are located near LVEA electrical panels and we are using a lot more current for clean rooms etc. than normal. Otherwise, the spectrum is dominated by broad features from wandering lines produced by the switching power supplies in the black IO expansion chassis discussed below.

Figure 2 shows that the magnetic fields are greater around an aLIGO black box IO expansion chassis than they were near an iLIGO VME crate and are dominated by wandering lines. I opened up h2susb478 and used the magnetometer to search for the source. The fields were much greater right at the Synocean 24V input switching power supply. I confirmed that the fields and source were typical by also checking the TCS black IO box. 

I tried to determine if these fields were deleterious by estimating the coupling of the fields to a few of the channels that were working (OSEM channels). I set up a small coil on the Synocean power supply and generated large 55 Hz and harmonics fields with the coil. I checked that the fields from my dipole coil dropped off with distance about the same way the power supply fields did (ratio of switcher and diagnostic fields varied by less than 3 over a factor of about 5 in distance). 

Figure 3 shows that 2e-6 T/sqrt(Hz) from my injection coil produced about 1 count/sqrt(Hz) on the channels, and so the 2e-8 T/sqrt(Hz) signal from the power supply would be expected to produce 0.01 counts in the channel. While this value is too low to show up on this channel, it is only low by a factor of 6 or so. Considering that 1) other channels may be more susceptible to magnetic coupling, 2) other Synocean power supplies may be noisier, 3) this power supply was not drawing maximum current, and 4) we had line features in iLIGO DARM that were attributed to the quieter switchers in VME crates (here), we may want to consider other power supplies for the IO boxes. 

In a closer look at stationary lines, I saw 10 Hz and 0.5 Hz combs. In the analogous locations during iLIGO, 10 Hz and 1 Hz lines were present, not 0.5 Hz. Possibly something is happening every 2 seconds that happened every second in iLIGO electronics.

Robert S., Richard M.