Summary: magnetic field injections suggest that the coupling to test mass motion is at least 3 orders of magnitude too high.

We measured magnetic coupling by injecting known magnetic fields with two coils set up in pseudo-Helmholtz configuration on opposite sides of ITMY (Figure 1). We measured the resulting test mass motion using the calibrated one arm test length channel and the ITMY optical lever channels. The optical lever calibrations were run past Jeff K. to make sure we had the right ones. The magnetometer was placed right under the floor of BSC8, within a meter of the test mass, and was calibrated in-situ using a calibration coil. To estimate the motion produced by ambient fields, the motion we observed was reduced by the ratio of our injected fields to ambient magnetic fields measured during S6 science mode (about 20 pT/sqrt(Hz) between 2 and 15 Hz, a few nanotesla at 60 Hz).

Figure 2 is an example of our injections, showing the signal produced by a  63 Hz magnetic injection on the magnetometer and the ITMY yaw channels.  Figure 3 is a summary of the predicted motion from ambient fields. The point at 63 Hz is calculated using the ambient 60 Hz background instead of the 63 Hz background so that it serves as an estimate of the 60 Hz peak in the gravitational wave channel produced by the ambient 60 Hz field. The predicted motion at 11.5 Hz is about 1e-16 m or radians/sqrt(Hz), and at 60 Hz is almost 1e-14. These values are at least 3 orders of magnitude higher then desired.

Our preliminary guess is that the excess coupling is to HEPI or the ISI because motion sensors on the HEPI and ISI suggest that our injections cause HEPI and ISI motions that are orders of magnitude over background (Figure 4). Arguing against this is the observation that predicted motion only goes as roughly 1/f^3 (see Figure 3); if the coupling were only to HEPI and ISI, we would expect that the 4 passive pendulums would result in a greater frequency dependence. Our first planned step in trying to sort this out is to compare coupling with HEPI and ISI turned off.

We investigated the possibility that the magnetic fields were coupling at HEPI and ISI cable feedthroughs using a small coil. We saw a variation in coupling for different feedthroughs, but we have not reached definite conclusions. 

Robert Schofield, Maggie Tse