Sorry no plots, the SR785 ate all of them... (for some reason it didn't like my USB stick, it pretended to save successfully all the traces, but there are no files on the device)

Following Craig's measurements reported in 46836, we wanted to increase the ISS gain to 12 dB, and compensate the slow servos. So with the IFO unlocked, we increased the ISS second loop gain to 12 dB and decreased the matrix elements for the slow servo accordingly: H1:PSL-ISS_SECONDLOOP_REFERENCE_IN_MTRX_1_2 from 10 to 2.5 and H1:PSL-ISS_SECONDLOOP_REFERENCE_IN_MTRX_1_4 from 0.5 to 0.125. We could close the ISS second loop at 2 W, then increase power to 10 W. We lost it when increasing power to 15 W. It doesn't work this way.

Even in the nominal gain configuration, there are glitches in the second loop output, that gets larger with increasing input power. With low gain they are not a problem, but with increased gain they seem to get too large and kick the ISS out of lock.

However, we were able to start with 2 W, close the ISS second loop AC coupled, increase power to 30W, then DC couple the ISS second loop. The boosts squeeze down the glitches. At this point we could change the ISS second loop gain by as many dBs as we want, without causing problems, and without compensating elsewhere. So maybe this is a viable strategy.

We went out to the floor to take some ISS second loop transfer functions, and understand if the gain of the ISS first loop was marginal (as it happened in the past, see for example 44841). The ISS second loop TF looks ok for gains of the first loop between 6 and 12 dB. A gain of 10 dB seems to be the best to get a smoother TF at high frequency. The gain was originally 8 dB, now we left it at 10 dB.

Increasing the gain of the second loop more than 12 dB does not do what one would expect. From 8 dB to 12 dB the ISS second loop transfer function scales roughly as expected. By gains of 15 dB, 18 dB or 20 dB results all in a smaller amplitude of the transfer function, and a steeper roll off at high frequency (>50 kHz). I wish I had some plots, but see the note at the beginning). So the suspicion is that gains larger than ~10-12 dB cause a saturation somewhere in the ISS loops.