Summary: no coupling hot spots (e.g. coil drivers, satellite amps, or feedthroughs) were found in the OSEM systems; L4Cs were the coupling hot spots in HPI and ISI systems. Magnetic coupling to sensors was not high enough to be a problem in normal operation but is a problem for our large magnetic injections.  Cable coupling is worse for AOSEMs than BOSEMs.

Our magnetic coupling experiments suffer from our inability to rely on L4C, T240, BOSEM, and AOSEM channels to measure motion because of direct magnetic coupling to these sensor channels. I studied coupling to these channels, particularly whether there were local sites (e.g. to magnets in the sensors, or to feed-throughs) or diffuse (e.g. to cables) in order to help us interpret coupling results. 

The technique I used was to generate a global field (roughly the same over the entire system) with a distant coil and a local field at a slightly different frequency with a local coil. A magnetometer was set up at the tested location (e.g. a feedthrough) and the fields from both coils were adjusted to give equal amplitudes at the magnetometer (Figure 1). If the feedthrough was the dominant coupling site, then the peak from the local field would be comparable to the peak from the global field. If the local field made a much smaller peak in the OSEM channel than the global field did (as for the Figure 1 location) then the feedthrough could not be the dominant coupling site.

In addition to balanced magnetic fields, I also sometimes increased the local field to see how much leeway there was at the local coupling site. Figure 2 shows that coupling at the coil driver is lower than elsewhere even when the local field is much larger (good design!).

Results (good between 10 and 40 Hz)

SUS: I tested for local coupling at the chamber, the feedthroughs, the satellite amp, and the coil drivers. Coupling at each of these locations was small compared to global coupling, suggesting that the cable runs were the most important coupling sites. Unfortunately, the worst coupling was to the AOSEMs on the PUM, the closest OSEMs to the test mass. To see if there was a specific problem with the AOSEM cabling etc., I switched PUM AOSEM and UIM BOSEM cables at the BSC feedthrough, and the coupling was still high, so it is not because of a bad cable, etc. Tests of an isolated AOSEM and its in-chamber cabling indicated very low coupling, so it seems that the EMF generated in cables influences the AOSEM signal more than the BOSEM signal. For ITMY we are getting a coupling to the AOSEMs of as high as 0.05 m/T for a15 Hz field that is roughly uniform in the LVEA and ebay. The BOSEMs are lower by at least an order of magnitude.

ISI: Coupling to the L4Cs is greater than to the T240s or the GS-13s. It is good that they are only on the first stage. Coupling to the L4Cs dominates over cable coupling and ebay coupling. I am not sure what dominates in the T240 and GS-13 sensor channels because the coupling was so low.

HPI: the dominant coupling is at the L4Cs. 

Robert S., Richard McCarthy.