At the end of the afternoon shifts, we had to recover the ITF locking in Dark Fringe again; this was completely impossible throughout the evening; the issues we faced are the following:
- already from the start of the lock acquisition, we saw a much increased noise which resembles the issue we had before the tuning of the Faraday isolator some time ago; this became consistently worse during the acquisition, preventing us completely from reaching dark fringe, in fact causing several unlocks at MICH_SET = 0.1, or even before at 0.3;
- after each unlock, the North Arm only was quite misaligned, needing us to tweak the mirrors' position by hand and/or temporarily enable the dampers on the mirrors;
- the PR alignment did not seem as robust as before during the MICH offset reduction, but this may be a consequence of the other issues, as they were present also before the PRM alignment;
- for safety we reduced the SSFS gains down to 80/10, but this didn't cause any evident change in the ITF behaviour; this was saved in Metatron;
- we tried using higher MICH gains, since it seemed the most problematic loop, but this had little effect on the stability; this was done on the fly.
We are not sure at all about the possible causes of this behaviour; as far as we could tell at the time, the activities done in the the afternoon shifts were the following:
- TCS work on the NI CO2 laser
- DAQ installation/update activities
- DET works on SDB2 and EDB
- INJ work on several picomotors near the Farady isolator
- SUS works on the retuning of the PR OPLEVs
We left the ITF in LOCKED_SSFS, since it is the last stable state we reached, and also in order to check if the delay issue of the SSFS DSP is still present and with which time constant, but this is hardly related to the issues of this evening.