Sheila, Matt, Lisa The ISS has been oscillating a couple of times when reaching 50W in the last two locks. Sheila fixed that by turning off the AC coupling. We leave the IFO locked at 50W with the ISS AC coupling off, at Sep 15, 9.45UTC. It has been locked for about 20 minutes, powers are stable, no PIs. Matt updated SDF with the latest PI settings. We just noted that the PSL NOISE EATER is oscillating -- so please fix that in the morning.
The 50 W lock lasted for about 3 hours. It is unclear what broke it. There were two PI modes (modes 10 and 26) which rung and weren't successfully damped. See the attached.
It's possible MODE 26 broke the lock, although lockloss occured when the amplitude of the mode was much lower than the usual breaking point. We're still re-finding settings after the ETMY ring heater change that happened yesterday afternoon and probably the gain sign was wrong on this mode.
More interesting is the unusual broad noise between 14kHz and 16kHz seen the entire lock. In spectra below, orange is OMC trans showing high frequency behavior during last nights lock and a reference 'normal' lock from a few hours prior. Despite the suspicious frequency range, we don't think this noise is associated with PI (we've checked to make sure we weren't injecting, had wrong settings, etc.); even very high amplitude PI create a symmetric peak with much higher Q. Perhaps laser noise?
Below left is just after locking last night (note the cursor is sitting ~15410 Hz, an area where there's no known mechanical modes). Below right is ~5 seconds before the lockloss 3 hours later. PIs are ringing up in the latter, but amplitudes are below those that have broken locks in the past.
Below is a 'normal' spectrum from a lock earlier in the day while two PIs were ringing up to similar amplitdue.