Kiwamu, Matt E, Terra

We broke our overnight lock this morning from Mode2 (ITMX 15520.7 Hz) ringing up; usually this is easily damped back down with a slight phase change but there was no operator in the chair at the time. This mode has been especially prone to ring up with very minor (a few degrees) phase drifts as the frequency drifts over ~3 hours lock across a static bandpass. 

I measured this mode's open loop transfer function last night (with the same settings it later rang up with). Width of loop is 24 mHz; cursors at UGFs shown on transfer function plots. Measured Q of mode is ~20 M, giving a half-width of 0.776 so the gain on resonance of our loop is about 31. Phase differential across this fairly wide frequency range is ~160 deg, so we're close to instability. 

To this end, I've halved the gain of the damping drive. I also shifted the bandpass filter: previously the resonant frequency range was sitting slightly off center picking up ~30 deg and now it shifts around within ~2 deg phase window (for a five hour lock at least). We had trouble locking tonight and are now working at low power, so I'll have to remeasure another time.