The unlock reported by the operator is of the same kind as the ones from yesterday, an oscillation at 9.5 Hz that causes saturation in the B1 photodiode.

It is an oscillation of WE payload roll resonance. There is a damper which is supposed to suppress it, and in fact most of the time it seems to work, but sometimes for unknown reasons the oscillation grows; it looks like a loop close to the instability. It can stay excited for a while, then go down again and stay quiet for a long time. At the beginning of the run the WE roll damper was the only one not used; the 9.7 Hz peak was well visible in DARM, with some extra noise above 10 Hz. Then it was engaged after a small tuning, obtaining a certain improvement.

If now the occasional malfunctioning determines systematic unlocks, maybe it is better to go back and disengage the damper. Or maybe spend half an hour of commissioning time and try to tune better the loop.

The attached plot shows the bad behavior at 9.7 Hz, happening from time to time. It was not so frequent, some time ago.

Considering that the latest locklosses belong to the 9.7 Hz cathegoty, the WE roll damper has been disengaged during the lock acquisition. Hopefully we will see the benefit soon.

Fig 1 - the spectrogram shows the behavior of DARM at 9.7 Hz before and after the disengagement of the damper: now it is stably yellow, no more very unstable from green to red.

Fig 2 - the rms at 9.7 Hz of DARM and DIFFpTX is computed ad gives the same information.

Fig 3 - Hrec now is worsen at 10 Hz. No big effect on the quality of the data: it is much better to avoid locklosses. But we could try to tune the damper and have the good performance without the undesirable side effect.