I've stepped all four ring heaters consecutively tonight by 4 W each to identify which test mass belongs to the new 17783 Hz/47.7 kHz PI mode (alogs 29820 and 29838). The shift in frequencies of the test masses is dominated by a change in the Young's modulus which we can alter via the ring heater power; by stepping each ring heater and finding which causes a larger relative frequency shift of the mode in question, we can identify which test mass the mode is from. See T1600080 and LLO alog 18002 for more info. All power changes below were applied to both top and bottom ring heater. All times UTC.
ETMX: 0.75 --> 2.75 @ 9:29 --> 0.75 @ 9:48
ETMY: 0 --> 2 @ 9:49 --> 0 @ 10:08
ITMX: 0.25 --> 2.25 @ 10:08 --> 0.25 @ 10:29
ITMY: 1.25 --> 3.25 @ 10:31 --> 1.25 @ 10:41
Note we just lost lock around 10:42, probably due to these ITM steps.
Ring heater tests indicate that the new 17783 Hz (aliased from 47753 Hz) belongs to ITMY, in agreement with Slawek's prediction.
Below shows the relative frequency shifting of one mode per test mass and mystery 17783 Hz mode ('o') during the ITM ring heater steps. ITMX was stepped up first then stepped down at the same time ITMY was stepped up. As expected, the 15518 Hz mode on ITMY rises slightly with the ITMY ring heater increase. At the same time, we see a drop in 17783 Hz; since 17783 Hz is the aliased-down frequency, we see it decrease as the 47.7 kHz mode shifts upwards.
I've set up damping settings for 17783 Hz on MODE 7. I tried many times to drive it up but got little response; as seen in Slawek's mechanical mode shape simulations, there is poor overlap with ESD actuation. Recently we were unable to damp 18041 Hz (aliased down from 47.5 kHz); ultimately we had to change ring heater power to avoid.