[Kenta, Yuzurihara]
For the lockloss (2025-04-06 20:26:15 UTC), the lock duration was longer than other recent locks (50335 s = 13.98 hours)!! So, we checked the stability of the lock.
There are two isses: (1) the BPC for EY pitch was drifting away from 0. (2) the unknown excess in 500~2000 Hz started in the middle of the lock
Details
Figure 1 shows the BPC error and feedback signals during the lock. We found that the BPC for EY pitch had drifted so that the error signal (K1:BPC-PIT_ETMY_OUTF_INMON) was away from 0. See the y-corsor located at y=0. (In my understanding, there was no BPC control for IY pitch.)
Figure 2 shows the normalized the spectrogram of the strain signal. From -5 hours before the lockloss (~15:00 UTC), the excess in 500~2000Hz had appear. The BPC frequency for EY pitch and yaw was 860 Hz and 855 Hz, respectively . So, this excess could disturbed the BPC control.
Note that, during this lock. the laser power and AS17 were stable (Figure 3).
Figure 4 shows the non-Gaussianity monitor (Gauch) of the corresponding time. I can't find any coincident change (also in other time and channels).It's possible that the excess was not far from Gaussian noise or that the time resolution should be much more large to catch this excess.
Figure 5 and Figure 6 show the behavior around 860 Hz and 855 Hz of the strain channel. I picked up them from line monitor (developed by Takahiro S Yamamoto-san). The amplitude of the injected line for BPC was changing in time, as seen Figure 1.
In addition,the magnitude of the spectrum was different before and after 15:00 UTC. This is corresponding with the finding in Figure 2.
Figure 7 shows the channels related to the PMC. Once the PMC temperature control was saturated, it recovered from the saturation.
