J. Kissel
FRS Ticket 10805

At the beginnings of IFO alignment recovery in corner station, H1SUSSRM's top mass would saturate, cause local damping loop instability, and eventually lock-loss when the requested/required alignment drive (either from sliders or ASC DC centering loops) for cavity resonance was large (see e.g. LHO aLOG 42198).

Now that we're regularly locking the full IFO, with all ASC loops -- including that which drive SRM -- running, and we have a robust initial alignment system we've not seen this problem. As quantitative evidence, I attach screenshots of the M1 DAC output request during a recent lock stretch that had the conditions described above (2018-08-31 17:50 - 18:16 UTC), which shows the request, at its largest is in the 55k DAC count mean range, with a few 100 ct max and min deviation from that mean. Recall the SRM has 18-bit DACs in which each channel theoretically has +/- 2^17 = 131k counts (practically ~120k counts before non-linearity and intermittent saturations sets in), so we're only using up ~45-50% of the DAC range.

Attached are screenshots of the SRM M1 stage and ASC overview during the above mentioned lock stretch (1st and 2nd attachment, respectively), as well as 40 minutes of trend data that shows the DAC output for each M1 coil, and the requested output from the alignment sliders and (slow) global control (3rd and 4th attachment, respectively).

Anecdotally, the commissioning vangaurd has confirmed that they don't recall any recent occurrence of this issue.

I deem this evidence enough to close out the FRS Ticket.