Because we are missing the out of vac crs damping driver, we have to use the ISI to damp the CRS down. This is temporary, we will eventually get a driver for the CRS. This works pretty well for the CRS, but imposes CRS motion on the table. This is enabled by setting the HAM3 ISI RY blend to blend 7, and turning up the gain on the INERT_LO path. The minimum effective gain is somewhere around 10-15, 30 works pretty good, we've gone as high as 100.
We turned the damping on this morning, while Jeff was doing some measurements on one of the MC suspensions, we have kept it running at low gain (3) while the IMC started locking, and so far I don't think this gain interferes with the IMC. There is still some crs signal visible on the RY CPS signal. The CRS is still rung up, and I don't think this gain is sufficient to damp the CRS but I don't see any evidence we are causing problems. I do think a gain of 10 would still allow us to lock (if the CRS is not too rung up) and slowly damp the CRS.
Attached trend shows ISI and IMC trends during the IMC locking this morning. Top trace is (blue) the CRS input into the blends in nrad, second trace (yellow) is the damping signal, third trace (green) is the table motion seen by the RY CPS, fourth trace (red) is the gain applied to the damping signal, last trace is the IMC F. The IMC was finally locked at about -15minutes on the X-axis, you can see we left the CRS damping at gain at 3 when they finally locked, the the CPS is still seeing some of 25mhz CRS motion imposed by the damping, but it doesn't really show up in the IMC signal. I think the bigger issue for the IMC at this time was people climbing on HAM2, for some reason this also seems to be a bigger impact on the HAM3 CPS as well, the extra "hair" on the CPS at -5minutes is not due to the CRS.
Currently with a gain of 3 on the CRS damping, the signal is barely visible on the CPS, not visible in the IMC. CRS is moving about 1 urad peak to peak, CPS motion is ~40nrad peak to peak from the CRS damping. The IMC seems to be losing lock from exterior disturbances (people walking nearby or climbing on the chamber) when the CPS peak to peak is over 200nrad.
With a gain of 10, the CRS is able to get quiet enough (eventually) that even with the damping engaged, the crs signal is no longer visible on the CPS. I think we should try running in this state for now.
