Summary: Both injection based noise projections of HAM1 HPI Z to DARM indicate that vertical motion was a limiting noise source for O2 from 10- 17 Hz. These projections also suggest that there is a another coupling of HAM1Z to DARM in addition to the coupling through the refl wfs.
The first attachment is a plot from our noise budget showing a projection of CHARD noise to DARM (based on injections made June 14ths 2017) and HAM1Z motion to DARM (based on injections made in March 2017). The projection of HAM1 Z to DARM roughly matches the projection of CHARD to DARM up to about 13 Hz, so the coupling HAM1Z-> REFL WFS -> DARM can probably explain this noise. However, above 15 Hz, the projection for HAM1Z is larger than the projection for CHARD. I've also attached screenshots of the CHARD injection data, you can see that the CHARD motion was increased by 2 orders of magnitude up to 100 Hz, so the upper limit that we get from the CHARD projection is below the noise that is predicted by the L4C projection above 15 Hz.
I also looked at coherence of CHARD and the HAM1 Z with DARM for 1000 averages on July 3 2017. This also indicates that there is coherence with the HAM1 Z L4Cs above the frequencies where the CHARD loops can explain it. The final attachment is a projection based on these coherence of the noise in DARM. This kind of measurement isn't as convincing as the injection based projection because there could be other explanations for the coherence, and I might be having a problem with numerical precision in DTT with the CHARD drives.
I took the injection of HAM1 referenced above, March 17, and I made estimates for DARM contributions both based on HAM1 L4C witness and CHARD P as witness. Comparing this to LLO (also March 17 injection), we see that the projection at LLO match. I think this means that CHARD at LHO is not dominated by HAM1 Z, while that is kind of true at LLO.
After some discussion with Jeff, Sheila, Hang and Arnaud, we agreed that one of the ways to decrease the HAM1 to DARM coherence is to push up the gain of the Z feedback loop to be more similar to LLO's configuration. I think our installed loops have a ugf of ~10hz, LLO are around 30 hz. I found data to compare the LLO and LHO plants is seismic SVN, they are pretty similar but not identical, see attached plot. Other dofs show similar levels of similarity. I have some higher ugf loops pretty much ready to install, probably do that Tuesday morning.
