Summary: ETMY PI 18041 Hz - aliased down from 47495 Hz, Mode27 - is (more readily) dampable if the OMC DCPD is used as error signal.  ESDs provide adequade force if actuating on mode while it is < 4 orders of magnitude above OMC-PI_DCPD noise floor (~10^4 magnitude in OMC-PI_DCPD channel), ~2.5 orders of magnitude above QPD noise floor.  

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Previously we had been unable to damp Mode27 (alog 29685) and turned off ETMY ring heater (alog 29702) to have less optical mode overlap with this 47495 Hz mechanical mode. Wednesday night I turned the ETMY ring heater back on to revert back to the bad zone and more thoroughly test our ESD damping capabilities. This mode has the highest gain during the transient time.

Over the past 20 hours and about 10 locks, I've found we have the actuation force to damp this mode if it is caught early ( < 4.5 orders of magnitude above noise floor in OMC signal ) which is much more likely when using the OMC DCPDs. All PI damping nominally uses error signal from TransMon QPDs. The OMC DCPDs also see mechanical modes and with an SNR ~15x the QPDs (see here for example from tonight of Mode27 seen in both error signal paths). We have avoided using DCPDs as error signal becaues the coupling path is not well understood and we see gain sign flips and much less steady phase changes. However, the high SNR seems necessary for PI with high gain that rings up quickly so that the damping loop can actuate against it more immediately while the mechanical mode amplitudes are still lower. This and other PI in the 47kHz ring up especially quickly (and twice) during the first hour transient sweep. Mode is very responsive and easily damped with a gain sign or phase change under 4 orders of mag above noise floor (1k magnitude in H1:OMC-PI_DCPD_64KHZ_AHF_DQ); above 4 is gets less responsive and is essentially non responsive by 5 (10k mag). So while we do have an error signal from QPDs within the effective damping range, the smaller window combined with high PI gain/rapid ring ups make the QPD signal inadequate. 

Operators: I've left Mode27 in the new OMC DCPD scheme. Patrick and I found a pattern of needing to change the gain sign twice during the beginning of the lock and adjust phase after that during the next hour or so. This mode responds very quickly, so be ready to immediately revert gain signs if it turns out to be the wrong direction. 

Note: Rang up with single quadrants (LL or UR) a few times and saw slightly less effective actuation than with LL+UR, though still need to process actual ring up time constants to compare effect. I switched once to LR, UL drive but many modes went crazy and broke lock so didn't try again tonight. Will repeat at lower power to get relative quadrant coupling for a given mode.