Evan, Sheila
Since the power reduction from 50W to 35W at about 2:25 UTC Sept 11, we have been sitting with the IFO mostly undistured other than changes to PI damping. We have not been in low noise (no ISS second loop, we are still on the high noise ESD driver), but we have seen some hugh noise in DARM that looks like a fringe wrapping shelf. We reduced the power again at 3:46 UTC, and the noise got better and went away after 10s of mintes.
We don't see anything in our LSC loops or main ASC loops that had a similar shape, do the detchar fringe wrapping/scattering tools offer any clues? Especially at the beinging of this time window, there is a lot of noise in DARM (a high frequency comb) which is due to a rung up PI mode.
Andy, Josh This doesn't seem like fringe wrapping. We're looking mostly at the time from 2:50 to 3:30 UTC (actually Sep 12 in UTC day), after the IFO has settled a little from the transition from 50 down to 35 Watts, and before the power changes again. We find no optics moving enough longitudinally to make a scattering shelf, though it could be something moving in angle. But also, this doesn't really seem to have the time structure of a scattering shelf. The first attachment has spectra starting at 2:50 and stepping every 10 minutes. It's very confusing, and things are moving in different directions. But focusing on the feature between 20 and 40 Hz, it's hard to get a sense of what it actually is. The next two attachments are 15-minute spectrograms that show a wide band of noise slowly walking down in frequency. But an Omega scan shows bursts of noise - they don't look like scattering. They also don't look big enough to account for all of this noise, so maybe there's a glitchy bit and a background of Gaussian noise. The next two plots are coherences with MICH/PRC/SRC at the beginning and end of the period. The coherence is significant with all, and it goes up at the low frequencies toward the end (when the excess noise has moved down in frequency). So maybe it's just because of changing coupling to some auxiliary degrees of freedom. I'll quickly mention that there's a lot of other things going on. The last attachment is a spectrogram showing a huge moving comb around 300 Hz. We also saw clear RF beatnote whistles. We'll look into all of these more.