Tyler, Fil, Richard, Robert

I have been looking for the source of the several peaks at harmonics of 13.1 Hz in DARM. These started or got worse on a past Tuesday. So I tried backing out the ITMX oplev and the CER grounding activities that took place that day, but to no avail. I had noticed that there was a large acoustic peak in the CER that might be the source (its hard to tell because the peaks are broadened in DARM by bilinear coupling). I tracked this acoustic peak to AC-1 in the CER. Today we shut it down, and it turns out that this AC was the problem and the appearance of the peaks on a Tuesday was probably coincidental.

The figure shows that the DARM peaks go away when AC-1 is off, and we have left this AC off to improve DARM. We also shut down all six ACs a couple of times for a test, and DARM spectra for the tests are also shown in the figure. The AC that we shut off makes the biggest difference, but the rest of the ACs also appear to increase the noise slightly in the 50-80 Hz region.

I will revisit this some tomorrow, now that Camilla, Jenne, et al. have significantly improved the feed-forward and the range (this afternoon after the figure plots were made). I will also be looking more into the coupling mechanism.

The second figure compares summary page DARM spectra showing that the improvements held overnight.