Peter, Sheila, Evan
We have been having trouble keeping the IMC locked when powering up (without the interferometer).
We found that the UGF of the loop was something like 110 kHz. During O1, we ran with a UGF that was more like 50 kHz. Recall also that the IMC loop at one point had some questionable resonance features above 100 kHz, so it is probably in our interest to keep the UGF on the low side (though we should confirm this with a high-frequency OLTF measurement).
We turned down the loop gain by 4 dB, giving a UGF that is more like 60 kHz. We were able to power up from 2 W to 23 W without lockloss.
Attached is an OLTF measurement after turning down the gain. As usual, the last point is garbage.
To keep the IMC length crossover stable in full lock, Stefan and I increased the gain by 7 dB. This brought the crossover form 16 Hz (nearly unstable) to 40 Hz (stable).
In fact the right way to compensate for the decreased electronic IMC gain is to also decrease the AO gain (not the MCL gain). Now both are decreased by 4 dB.