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Reports until 14:20, Thursday 08 November 2012
robert.schofield@LIGO.ORG - posted 14:20, Thursday 08 November 2012 (4640)
Magnetic coupling at quad levels 1 and 2 and potential mitigation

Summary: We have removed eddy current damper magnets from M0 of the quad in hopes of reducing magnetic coupling. However, investigations show that there is also excess coupling at levels L1 and L2. I suggest removing the blade ECD magnets from L1 and cutting slits to reduce eddy currents in certain L2 parts around the AOSEMs.


We have now removed the eddy current damping magnets from stage M0 of the quad suspensions (Figure 1). This should reduce magnetic coupling, but my investigations at ETMY suggest that M0 is not the only level that has large magnetic coupling. Figure 2 demonstrates that there is also coupling at L1: there are two peaks evident on the BOSEM signals from M0 (top plot), the one on the left is produced by a BOSEM injection, only at M0. The one on the right is produced by a magnetic field generator outside of the ETMY vacuum chamber. The bottom plot shows that the relative amplitude of the two peaks is very different on the level just below, L1: the BOSEM injection peak on the left is relatively more attenuated. If the external magnetic field coupled only at M0, then the two peaks would keep their relative sizes because both would be equally attenuated between M0 and L1 (actually, the slightly higher frequency magnetic peak would be slightly more attenuated). Instead, the magnetic peak is much bigger than would be expected if it coupled mainly to the eddy current dampers on M0. A similar experiment shows that we also have excess coupling at L2, the PUM.

Since it is clear that we will have excess magnetic coupling even after removing the M0 ECDs, I had a look at the suspension to pick out what I thought would be the worst coupling sites. Here are my two favorite sites for easy improvement, that might be possible to accomplish before reinserting ITMY.

L1:  The blade spring eddy current dampers (Figure 3). I have checked with Mark Barton to see if there are any problems removing these eddy current dampers before we reinstall ITMY.

L2: The AOSEM mount base plate should probably have a slit in it so that eddy currents can not make a loop around the AOSEM so near the actuation permanent magnet (Figure 4 and drawings: ETM PUM can, AOSEM base plate. Two secondary candidates for slits would be the AOSEM translation plate and the control ring (Figure 4 and drawings  AOSEM translation plate, control ring). The control ring would simply need a non-conducting screw to tighten it.

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