After the long lock of the weekend, on Monday 23/12 the interferometer started to struggle, not in the lock acquisition, but in keeping LOW_NOISE_3, where we consistently unlocked upon arrival or shortly after, with a ~2.3-2.5 Hz oscillation.

We tried to figure out what, at the end of the lock acquisition, could go wrong or leave the ITF in a unstable state:

• we disabled the BS full bandwidth, to no gain;
• we disabled the new PR/BS reallocation (which is still off), as we could also see some glitches in the top-stage corrections;
• we checked the MICH/SRCL demodulation phase: although the one used in CARM Null was quite good, the second one we put just before opening the slow shutter at the end of the transient was not, so that one was changed;
• in general all DRMI UGF servos were struggling as, maybe because of the bad weather, they had low coherence so the servo was not effective all the time; the lock which survived up to this morning was characterized by setting by hand a SRCL gain quite higher to the one that the servo put (before it lost coherence) (0.26 vs 0.2), which makes sense considering that the last thing we do in the lock acquisition is to weaken the SRC.

I put higher (~50-80 %) lines for MICH and PRCL in ACQUIRE_LOW_NOISE_3, and more than +100% for the SRCL one; today after the Maintenance the ITF locked at first attempt, with the servos behaving better (but also the weather is better today) (see Figure); I will adjust offline a couple of amplitudes for the next locks (PRCL looks overkill now), and consider whether to raise the SRCL UGF setpoint a bit from 0.2 (arbitrary units) as the current configuration doesn't have that much margin. I already put a very minor increase to 0.21, to be monitored.