This has been kind of hard to pick out, but it definitely seems like the IFO is less robust now against earthquakes that it was before increasing power over 60W in the IMC. Basically since then, we haven't been able to stay in NLN for any earthquakes over ~.5um/s, as measured by the peakmon eq witness channel.
Attached plot shows peakmon vs lock state trends since Feb 10 this year. The blue trace is the peakmon ground velocity minute trend, each green point indicates where the IFO was still locked 2 minutes after a local maximum ground velocity (found using the matlab peakfinder routine with some prominence and time separation requirements). The two marked points indicate where IMC power was increased above 60W (X=79730 minutes after Feb 10th) and the start of the run (X=148255 minutes after Feb10). Before power up we had multiple locks survive multipe 500nm/s eqs, a few around 1micron/s and one at almost 3micron/s. After the power up, the IFO doesn't ride out any eqs over 500nm/s and we basically not survived any notable eqs since the start of the run.
This is further reinforced by TJs alog from earlier today, when SEI_CS transitioning knocked us out of OBSERVE. Unless SEI_CS got dropped off the exclude recently, I suspect we never noticed because we were losing lock before the seismic system switched.
There have been no changes to the SEI eq controls. I looked at the small eq Camilla noted this morning and didn't see anything suspicious in the seismic systems, but I'll keep digging.
I will also add that this ~.5micron/s eq-band velocities are a level that the primary microseism can hit for a week or two at a time during the winter. Conditions that were already challenging for us in prior runs.
Another possible culprit from around the time of April 7: removing 12 dB of low-frequency (< 1 Hz) gain from the Michelson loop to try to reduce the amount of sensor noise reinjection in the GW band (LHO:68432).
It would be a relatively simple test to (1) increase the EPICS gain of the loop by 3 dB to place the UGF back around 10 Hz, and then (2) re-engage the 4:1 Hz boost (LSC-MICH2 FM3). If the noise increase in DARM is acceptably low, perhaps you could run like this for a week to see if the duty cycle improves.
If this extra gain helps during EQs, it could also be made part of the EQ guardian response.
Brina, Sheila,
We ran some excitations while locked, need to review plots (will come back to alog to update info)
