[Alexa and Kiwamu]

 We became unable to lock the reference cavity this morning. The autolocker wasn't capturing a fringe. After some investigation and fiddling some parameters it started locking.

There was an oscillation which seemed preventing the FSS from a stable lock. In the end the oscillation somehow went away.

• What we did:
  • We changed the temperature scanning range and its sweep speed
    • Neither narrowing nor expanding the range helped
  • We decreased the transmitted light threshold from 1 to 0.5 with a hope that this gives an earlier trigger for a fringe
    • No improvements found.
  • We increased the oscillation threshold from 0.5 to 2.0 because we saw the fast feedback hits this threshold.
    • No improvements
  • We switched the system to a manual mode by toggling the button for the autolocker in the FSS medm screen
    • We manually set the temperature to be 0.1359 [K]. This is where we found the zero-zero mode.
    • We diabled the active temperature feedback by disabling the input of H1:PSL-FSS_TEMP_LOOP
    • Once we reached the point close to the resonance we toggled on the fast loop.
  • The FSS was found to be oscillating.
    • We were still in the manual mode at this point
    • In H1:PSL-FSS_FAST_MON_OUTPUT display we could see that the voltage fully swung +/- 10 V. We didn't check the oscillation frequency.
  • We toggled the noise eater (see Rick's alog).
    • No difference was observed.
  • We decreased the common gain by 19 dB to see if the oscillation is due to a control issue.
    • The oscillation disappeared.
    • H1:PSL-FSS_COMMON_GAIN was 8 dB
  • We set the common gain back to the nominal
    • Strangely we didn't see the oscillation any more
    • H1:PSL-FSS_COMMON_GAIN was 27 dB which is nominal
    • So the oscillation was fixed somehow
  • We switched it to the autolocker mode and restored the parameters which we changed.
    • It has been locking and fine so far.