This afternoon we resumed the recovery of the ITF, starting again from pre-aligning the arms themselves (and the PRM position). Outcome:
- the demodulation phases of RFC, B7 and B8 have been retuned, the arms' ones in particular with good precision;
- we locked both arms, after some work on the WI driving balancing (see Fig.1);
- the arms are in much better condition w.r.t. yesterday, due to the decreased frequency noise and work on the INJ side (see Fig.2);
- the corrections on the ITMs are therefore much less saturating, allowing a more stable lock and, in principle, a sensitive error signal for the drift control to work as intended;
- indeed we restored the drift control, but quite big offsets are needed in order to correctly zero the error signal;
- we restored the normal functionality of Metatron, now we can start using ITF_LOCK again.
At this point we looked into MICH, but our fringe signal (B1p/(B1p+x B4), a.k.a. LSC_B1p_SUM_NORM) is much smaller than usual, spanning from 0 to just 0.2; to be checked if the intercalibration between B1p_DC and B4_DC has changed, or something needs to be realigned. Looking at the signals, the issue seems related more to B1p than B4.
We leave the arms locked, so ITF_LOCK is in LOCKED_ARMS. Note that the automatic FmodErr tuning is still disabled (via ITF_LOCK.ini), it could be worth tomorrow to check also the automated tuning, but after a first manual one (which can be done much faster).
