[Yamazaki (Aogaku), Shuta Tanaka (Aogaku), Kenta, Yuzurihara]
We started the lockloss investigation on 2025/02/19(19:17 UTC) -23, with powerful supporters from Aogaku. Last lockloss investigations were posted in klog32744 and klog32686.
During this period, there were 43 lockloss. The 39 lockloss was the lockloss with the IMC power 10 W. In this report, we focused the lockloss with the 10 W operation. The plots are summarized in wiki, wiki and wiki.
We observed majorly two kinds of phenomena just before the lockloss.
- The first phenomena is the oscillation of the OMC PD with around 40 Hz. This happened 20 times in 39 lockloss. 11 of 20 lockloss seems to be caused by the reduction of the optical gain.
- The other phenomena is the excess of 1Hz seismic motion and the failure of the IMC length control. This happened 12 times in 39 lockloss.
- Minor phenomena
- The oscillation of MICH error signal with ~10 Hz happened twice.
- The cause of four lockloss were still unclear. It's necessary to investigate more.
- One lockloss happened when the excitation was done. We will leave it.
Compared with the previous lockloss investigation, thanks to the tuning of filters by the experts, the number of lockloss related the oscillation of the suspension seems to have reduced significantly!
Oscillation of the OMC PD with around 40 Hz
Just before the lockloss (typically shorter than 1 s), we observed the oscillation of the OMC PD (K1:OMC-TRANS_DC_A_IN1_DQ) with around 40 Hz. Mainly the frequencies of OMC PD were distributed in 35 ~ 45 Hz (see Fig). Note that the plots are summarized in wiki.
We considered that two hypothesis for the oscillation and the lockloss and tried to test the hypothesis:
- The resonance of Type-A suspensions was excited due to some reason and the oscillation appeared in OMC PD.
- Due to the reduction of the optical gain, the UGF frequency shifted from ~80 Hz to ~40 Hz. Accordingly, the DARM control start to oscillate around the 40 Hz.
To test the first hypothesis about the resonance of suspension, the frequencies of Type-A resonance are summarized in Ushiba-san's slide (JGWdoc). We checked the channels of K1:VIS-ETMX_MN_DAMP_V_IN1_DQ (38.9 ~ 39.8Hz), K1:VIS-ETMX_MN_WIT_P_DQ (43.26 ~ 44.48 Hz) and K1:VIS-ETMX_MN_WIT_Y_DQ. We also checked the channels for ITMX, ITMY and ETMY.
We compaed the spectrum just before the lockloss and several minutes before the lockloss. We checked two lockloss, but we didn't find the significant enhancement on the peak around 40 Hz or other frequency. We plan to check the other lockloss.
To test the second hypothesis, we checked the change of K1:LSC-AS_PDA1_RF17_Q_ERR_DQ, just before the lockloss. The typical value of this channels is around 0.02. When K1:LSC-AS_PDA1_RF17_Q_ERR_DQ is less than 0.02, and is approching to 0 or is touching 0, it's possible that the optical gain is reducing. We found that for 11 of 20 lockloss, this phenomena was happening. The plots can be seen in wiki.
For the other lockloss with the oscillation of the OMC PD, the duration of how long 40 Hz oscillation is different case by case. In most cases, the duration is less than 1 second.
In some cases, the duration is a few seconds (wiki) (wiki) (wiki), ~12 seconds (wiki), or 30 seconds (wiki).
For the rest of lockloss, we need the other characterization from other perspective.
Failure of IMC length control due to 1Hz seismic motion
The other phenomena is the excess of 1Hz seismic motion. This happened 12 times in 39 lockloss. The time is summarlized in Fig.
Figure shows the typical phenomena: the seismic motion started at 1 Hz or 3 Hz band --> IMC length feedback signal increased --> The condition of IMC_LOCKED state was not satisfied. --> lockloss
Compared with the previous lockloss investigation, the number of the IMC lockloss due to 1 Hz seismic motion increased. I guess that the lock duration increased and other lockloss causes were solved. As a result, this kinds of lockloss appeared frequently. This lockloss might be the bottleneck in future.
The other scenario related 1 Hz seismic motion was reported in klog32352: the seismic motion started at 1 Hz or 3 Hz band --> IMC length feedback signal increased --> The ISS error and feedback signals increased --> ISS lockloss --> BPC control moved a lot --> the alignment of interferometer was moved a lot --> lockloss
This phenomena was labeled as ISS_AOM by SYS_LOCKLOSS guardian. In this period, this phenomena happened once with 1.2 W operation, not 10 W operation.
Others
The analysis is underway. I will summarize and post it later.
