Lucia, Kiwamu,

Last night, the X arm cavity seemed moving a lot at frequencies around 0.1 Hz. The first attachment shows the arm cavity motion in terms of IR frequency noise in Hz/Hz^1/2.

Comparing this spectrum against those we took in the past (for example see 6709), the spectrum at around 0.1 Hz last night is higher by a factor of 4 or 5. Also, the coherence suggests that it mainly from ITMX. We are not sure why it has to do with ITMX at this point rather than the case where both ETMX and ITMX equally affected.

As a consequence the VCO needed to follow the motion by sending a voltage signal of something like 2 Vp-p which is close to the linear range (6544). See the second attachment showing the typical feedback signal to the VCO last night.

However, looking into a handful of the individual lock loss events last night, we haven't yet found a lock loss triggered by the VCO saturation, which is encouraging. We will further look into the lock losses.