J. Kissel, O. Patane, I. Abouelfettouh, B. Weaver

Betsy asked Ibrahim to gather some health check transfer functions in air prior to their CP work yesterday. He'd struggled for a bit to drive standard templates on the reaction chain, and called in re-inforcements. We reminded each other that "R0 tracking," or "L2 to R0 damping", or "the L2DAMP path" exists that uses the PUM (L2) OSEMs in feed back to the reaction chain (R0) to keep the chains moving together (ECR E2000011, built into all QUAD front-end models Jan 10 2020 LHO:54416, installed the next Tuesday LHO:54506, quickly and obviously became useful on the ETMs LHO:54556, but only turned on for the ITMs for the first time on Nov 10 2022 LHO:65728). 

It is *not* a DC coupled loop, it's an AC-coupled "velocity damping" controller, but it does have a giant amount of gain with a resgain filter right at 3.3 Hz.

This tracking needs to be turned off during standard M0 or R0 "health check" transfer functions for *all* quads. They go unstable with huge (but standard health check) drive going to the main chain. We've run into having accidentally left these loops on for the ETMs before (LHO:60482), but this is the first time it's popped up for the ITMs, since it's the first time in a while we've characterized the ITMs.

Anyways, we found it, turned the ITMY L2 to R0 damping/tracking OFF (ITMX remains ON), and the main and reaction chain TFs that Ibrahim gathered went without a hitch after turning it off.
These TFs are 
    /ligo/svncommon/SusSVN/sus/trunk/QUAD/H1/ITMY/SAGM0/Data/
        2025-04-09_1330_H1SUSITMY_M0_Mono_WhiteNoise_V_0p02to50Hz.xml
        2025-04-09_1330_H1SUSITMY_M0_Mono_WhiteNoise_P_0p02to50Hz.xml
        2025-04-09_1330_H1SUSITMY_M0_Mono_WhiteNoise_Y_0p02to50Hz.xml

    /ligo/svncommon/SusSVN/sus/trunk/QUAD/H1/ITMY/SAGR0/Data/
        2025-04-09_1800_H1SUSITMY_R0_WhiteNoise_V_0p02to50Hz.xml
        2025-04-09_1800_H1SUSITMY_R0_WhiteNoise_P_0p02to50Hz.xml
        2025-04-09_1800_H1SUSITMY_R0_WhiteNoise_Y_0p02to50Hz.xml
Pressed for time, we did NOT take all DOFs.
Just wanted to get these TFs mentioned in the aLOG.