This morning, Yokozawa-san and Yuzurihara-san asked me to investigate the cause of the IMC lock loss because whole IFO lock loss seemed to be caused by IMC lock loss when seismic motion around 1Hz became larger during O4a. So I started the investigation.

As the first step, from the summry page of lock losses during O4a, I selected one event labeled "1 Hz" at 22:32:42 UTC on June 12 2023 and saw some channels related to IMC (K1:IMC-CAV_TRANS_OUT_DQ (IMC trans. power), K1:LAS-POW_PMC_OUT_DQ (PMC trans. power), K1:IMC-SERVO_SLOW_DAQ_OUT_DQ (PZT feedback signal), K1:LSC-MCL_OUT_DQ(MCE feedback signal)) and the states of the LSC_LOCK and IO guardians with ndscope.

I found that IMC and PMC trans. power oscillated at high frequency several seconds before the LSC_LOCK guardain went to the DOWN state (fig.1). This indicates that ISS lock lost at that moment. After the ISS lock loss, the BPCs seemed to be broken due to becasue the BPC error signals varied in the intensity noise and ITMs moved a lot. Then, the whole IFO fell down.

I chosed other 3 events (07:32:21 UTC June 15 2023 (fig.2), 05:58:22 UTC May 30 2023(fig.3), and 11:46:07 UTC June 09 2023(fig.4)) randamly and observed the same trend. Therefore, it is likely that ISS lock loss is the cause of whole IFO lock loss when 1 Hz seismic motion became larger.

However, I'm not sure of the reason why ISS lock was lost when 1 Hz seismic motion became larger for now. We need more investigation.