J. Kissel, J. Warner, S. Dwyer, O. Patane, E. Capote, T. Sanchez

After finally making progress past Beam Splitter optical lever glitching (LHO:84631), a poorly phase REFLAIR sensor set for PRMI (LHO:84630), and adding loop gain to the REFL DC centering loops (LHO:84623), we were just about to see some success and made it ISC_LOCK's RESONANCE state.

Alas, Jim started commissioning/debugging the HAM1 ISI loops (in a different room), trying to carve out some time to deal with a ~70 Hz feature in some DOFs of the plant. In doing so he made sure to turn off the DOFs he thought were problematic (RX / RY). LHO:84638

We (including Jim) now know that even only having the X, Y, Z, and RZ loops closed, the ISI HAM1 X loop slowing goes into oscillation slowly, on the order of 5 to 10 minutes after the ISI is isolated.

This manifested in IFO commissioning in very confusing ways mostly because of the timing of events (and the "front row" didn't know that Jim had started commissioning the ISI). The first attachment "shows" this story.
    - The first re-acquisition after an unrelated lock loss started with the ALS COMM's out-of-loop sensor (H1:LSC-TR_X_NORM_INMON) oscillating at 70 Hz, i.e. "looking fuzzy" on the wall "DRMI buildups" screen.
        (See bottom, 5th panel.)
    - It started to dissipate in the middle of this acquisition (we now know because Jim was independently cycling the ISI Isolation loops)
        (See 4th panel)
    - But by the time that DRMI was locked, the "fuzziness" oscillations started to reappear.
        (again, see 5th panel)
    - All the while we notice that MC2's M2 stage -- feedback for the IMC -- is saturating. This in itself is confusing because our impression is that the IMC F / IMC L cross-over is at ~15 Hz, so "there shouldn't be any drive from the IMC common mode board to MC2 up there, especially not offloaded to the M2 stage."
        (see 2nd and 3rd panels)
    - But also, "why is the IMC seeing any HAM1 motion?"

I also attach an ASD of IMC F, the middle stage of MC2's DAC drive, and the HAM1 and HAM2 ISI motion. These clearly show the 70 Hz feature, and show nothing in HAM2.
I call out the "nothing in HAM2" to dispel the suspicions that vacuum equipment that's still vibrating and moving the HAM1 / HAM2 chambers is causing this noise. It's not. It's the oscillation in the ISI HAM1 loops.

Here's why "the IMC is seeing HAM1 motion:" at this point in the lock acquisition, ALS COMM is inserted as an additive offset to the IMC's Common Mode board, which drives MC2 length and the PSL frequency (see e.g. the difference between pg 12 and 13 of G1400519). This is why the MC2 M2 DAC request is peanuts when the IMC is locked alone, but then is gigantic once the ALS COMM signal is ramped in to push the laser frequency around to find the X arm resonance in IR. The ALS beam is "caught" in HAM1 with a periscope and steered around the table before being sent on to ISCT1. So the unstable 70 Hz motion in the new HAM1 ISI is imprinted there on the beam, which is then used to inform ALS COMM, which is then fed to the IMC.

For now, we're going to leave the HAM1 ISI loops off and carry on with ISI commissioning, and give Jim some dedicated time to figure out the issues with the ISI.