Sheila, Georgia
We took a transfer function of the ISS second loop, and increased the gain by 12 dB. OLTF is shown in the first figure: blue trace is at the original slider value (20dB), red is at the new slider value (32 dB). The UGF has been increased from 7kHz to to 28 kHz. We were originally going to increase the gain by 10 dB only, but that put the ~19 kHz feature right at the UGF which seemed unwise.
The guardian does not set the gain of the loop, only turns it on and off. I've updated the SDF with the new value. If tomorrow morning's commissioners have intensity noise problems turning the second loop gain might be a good start. We won't get to relock tonight after the earthquake in Tonga.
Sheila ran an intensity noise excitation and added the new intensity noise to the noise budget, shown in the second attachment, dark blue dots. This can be compared to the previous noise budget reported in 45169.
To take the OLTF I hooked the SR785 near the PSL racks up to the ISS board (note the second loop excitation point is labelled as the Outer Loop transfer function, and is part of the ISS [first loop] board. There is a board labelled second loop, but the "servo out" lemo cable is disconnected? Obviously the system works, but I'm not sure if there should be so many cables dangling from the boards.
In fact the morning (ok, noontime) commissioners found an intensity noise problem: the power was oscillating by 10%, with a period of 20 seconds. We reduced the second loop gain slider back to 20 dB and it stabilized.
This sounds like an instability of the AC coupling loop.
I think the best strategy is to increase the gain when the second loop is DC coupled
The 7kHz UGF with 20dB gain was measured at 21.3W input power.