Matt H. at LLO noticed that their PMC transmission changes slightly with IFO lock state (alog here) and asked me if we see the same thing. Never having noticed this before, I took a look. Sure enough, we do, see attached; when the IFO is locked PMC transmission drops slightly. The change is slight (~0.3 W), so I don't think this is an issue, just noting it here so folks are aware.
I should note that we are using the new, all-bolted PMC while LLO is still using the original aLIGO PMC. Also, the movement in PMC transmission immediately followed by it's loss, return, and resulting ramp-up seen on the right half of the plot were caused by yesterday's PSL enclosure incursion and small water leak repair.
I had a closer look at exactly when the PMC transmission drops - as Adam noted at LLO, it occurs when we turn on the ISS second loop. The second loop is first engaged (with AC the coupling servo on) quite early in our lock acquisition sequence - in the DRMI_LOCK_CHECK_ASC state of the ISC_LOCK guardian, this causes the first step in PMC power in the attached screenshot. We get a second kick when we DC couple the ISS second loop, at the end of the lock acquisition sequence.
