[Evan, Jenne, JimW]
We turned the ITM pitch oplev damping back on, set some QPD offsets for POP and transmons, and were able to go to 20W without trouble. We have finally made it to nominal low noise for the first time in weeks! Hooray! Sadly, a 2.84Hz oscillation killed us after about 10 seconds. Work continues, but I was excited enough to write an alog.
We lost lock several times very suddenly within 20 minutes of powering up. Also, POP90 seemed to be escaping upward. Looking at the 36 MHz wavefront sensors, one can see several quadratures of AS A are escaping. (AS A has not been used in any alignment loops for the past six weeks or so). In particular, AS A 36Q seems to correspond to the motion of SRM yaw as seen by its OSEM (see attachment).
We switched control of the SRM yaw loop from AS B 36I to AS A 36Q immediately after powering up to 20 W. This stops POP90 from escaping and seems to hold SRM yaw in place. So far we have been at >20 W for more than 90 minutes. The new setting (−0.5 ct/ct for AS A 36Q to SRC1 yaw) is not in the guardian.
Overall, the carrier power in the PRC seems less stable than before. It seems to have ~0.2 Hz fluctuations that sometimes approach 5% RIN, and correspond to fluctuations in cHard pitch, PRM pitch, and beamsplitter pitch. This frequency seems too low to correspond to an optomechanical resonance, so we suspect some alignment control problem. We tried turning down the PRM pitch loop gain by a factor of 2, but this didn't seem to change anything.
As Sheila anticipated, angular noise in DARM is quite bad right now. Some of it can probably be improved with a careful A2L retuning. The rest may require loop reshaping.
We also chose new QPD offsets for the soft loops and the PRM loops, in order to maximize recycling gain at 2 W.