We have been having continuing problems with a 4.18 Hz oscillation saturating ETMX L2 within 10 mins of reaching NLN. Yesterday afternoon Sheila and Jenne removed the ITMs from DHARD P to help with a similar oscillation (but at 3.2 Hz) that was also saturating EX L2. Sheila put the ITMs back in to DHARD P this evening (we've accepted this in SDF). The 4.18 Hz caused another 4 locklosses since that change. The ETMX L2 P2L gain was 4.0 for all of these locks.
We suspected the oscillation was related to the pro-spring detuning of DARM, and tried some SRCL offsets, but this did not work. In the last lock we lowered the ETMX L2 P2L gain to 1, this seems to have fixed the problem. To keep the dither loop maintaining the same old spot position, we added a 0.25 offset in the PIT4 loop, just for this lock. This will compensate for the offset added by the P2L gain change. This might need to be reverted by hand tomorrow.
We also lost lock several times this evening around CARM_TO_ANALOG. When we tweaked up alignment of PRM, SRM, PR2, and SR2 at PREP_AS_SHUTTERS we made it through ok. Before we pass CARM_TO_ANALOG the 60 Hz, 120 Hz and harmonics visible in the DARM FOM seemed much bigger than last week, unsure why.
Our lockloss tool isn't working at the moment (Jamie and others are working on fixing this), so I went through and found the approximate lockloss times that I think Georgia is talking about:
The 60 and 120 Hz noise was very high durring the final stages of the CARM offset reduction, as Georgia says, this has been causing us saturations and locklosses since Tuesday. We found that the aliglent was plugged into the IMC board excitation points, although the excitation switch was off disconnecting the cables seems to have fixed at least the 60-120 Hz problems in the CARM offset reduction.
Investigations of CHARD P and the 4 Hz problem last night are ongoing.