I have looked at three unlocks that happened this afternoon during the lock acquisition between CARM NULL 1F and LN3.
Figure 1 and 2. Two of them look very similar, with a ~1s time scale transient on ASC_BS_TX and ASC_PR_TX, which is followed by a quick rise in B1s power which eventually crosses the 45mW threshold and triggers the safety fast shutter.
Figure 3. One unlock is different, the power on B1s increases over ~30s before crossing the safety threshold. There is also ASC_BS_TX going far away from zero on the same time scale, but it is much less clear that it is related. There are deviation in BS TX in the previous 100s that are comparable, there are also B1s power increases at the same time, but the correlation is not very clear.
The following lock attempted worked without any intervention.
The first unlock, which was caused by a drift from zero of BS TX, was preceeded by a little 'glitch' (?) on the local control of the BS TX, see fig.1. In that stage we are not in full bandwidth and this is a problem that sometimes occurs when we are still in drift control with the loop (https://logbook.virgo-gw.eu/virgo/?r=65234).
