[Yokozawa, Yuzurihara]
We performed the lockloss investigation for the recent lockloss of 2025-07-10 10:53:19.187500 UTC. The previous lockloss investigation was posted in klog34259. This is the longest lock in the O4c by now.
Quick summary
Although I check all the lockloss phenomena which were reported in the past klog, those phenomena except for the OMC DCPD saturation did not occur just before this lock loss.
The OMC saturation occurred just before the lockloss. But, the quickness of the saturation seems to be different from the past saturations. I and Yokozawa-san checked the timeseires and listed up the possible cause of the saturation.
Details
- There was no excess on seismic motion of 1~10 Hz. (Figure 1)
- About the oscillation, we can see the oscillation on the PR2 pitch with 0.83 Hz. But, the amplitude is not large. (Figure2)
- We can see the coincident oscillation (0.83 Hz) on the ASC PRC2 pitch and ASC MICH pitch. (Figure3)
- There was small excess on PRM feedback signal. But, it will be small to cause the lockloss. (Figure4)
- About the drift, there was no drift on Xarm and Yarm mirrors over 30 minutes. (Xarm, Yarm)
- About the BPC control, there was no strange behavior just before the lockloss
- About the saturation of control, there was no saturation on BS and ETMX.
- There was no glitch on ETMY MN oplev.
- There was no earthquake around the lockloss time.
- We can see the large seismic motion in 3~10 Hz and 10~30 Hz. This excess ended 20 minutes later. But, this amplitude is not enough to cause the lockloss. (Figure)
Quick OMC saturation
At this lockloss, we can see the OMC saturation, which we reported many times in the past klog. This will be the direct cause of the lockloss. I and Yokozawa-san checked the original cause of the OMC saturation.
One important difference between this saturation and past saturation is the quickness to reach the saturation. As seen in Figure9 and Figure10, the DCPD signal reached the saturation within 1 ms, which is so quick!
We are thinking it's difficult to make this quick saturation by the suspension. So, we guess it might be associated with the electrical signal or glitch.
In the past saturation, we could see the several oscillation just before reaching the saturation as shown in Figure 11 or this. This might be a hint to investigate the cause more.
Anyway, this was new phenomenon for us (or I missed this phenomenon...).
Other findings
- The volage monitor channel
- While K1:PEM-VOLD_OMC_RACK_DC_M18_OUT_DQ is stable, K1:PEM-VOLD_OMC_RACK_DC_P18_OUT_DQ shows the drift behavior and it reset somehow. (Figure 12)
- K1:PEM-VOLT_AS_TABLE_GND_OUT_DQ is almost stable but shows the unstable behavior at -5 hour and just before the lockloss.
- It's highly important to check the ground condition around AS port and OMC rack.
- PMC control
- K1:PSL-PMC_LO_POWER_MON_OUT16 shows some jumps before the lockloss. It would be very helpful if the PMC expert could comment on this phenomenon.