Summary - Jim fixed it with a little shake.
We cycled through the various ISI state controls. The 1Hz line would start appearing when stage 1 is isolated. In any states below this point, we can clearly see the difference in the 'sensor response' of the local H1 L4C wrt T240 or CPS. While Jim was taking an olg measurement, the ISI tripped, which seem to have changed the response of the H1 L4C back to the nominal response, see the first pdf attached, showing the L4C/T240 local TF, before (p1) vs after (p2) the ISI trip. The 2nd pdf attached shows the ISI spectra in the isolated state before (p1) vs after (p2) the shake, no other changes.
We've had sticky L4Cs in the past and solved it in a similar way (see alog 38939), but the symptoms were much more obvious than a slight change in resonant frequency as we are seeing here.
Timeline of tests :
16:04 - 16:12 UTC ETMX ISI/HEPI OFFLINE
16:15 - 16:30 UTC HEPI ON / ETMX ISI OFFLINE
16:31 - 16:37 UTC HEPI ON /ETMX DAMPED
16:38 - 16:44 UTC HEPI ON / ETMX ST1 ISO - We can see the giant ~1Hz line
16:45 - 16:48 UTC HEPI ON/ ETMX ST1 ISO - H1 L4C gain 0.5 - sym filter off - ~1Hz line goes down
16:50 - 16:52 UTC HEPION/ETMX ST1/ST2 ISO - H1 L4C gain 0.5 - sym filter off
16:55 - 17:07 UTC Back to nominal
17:25 - Jim changing stage 1 Rz blend to noL4C
17:38 - Jim changing stage 1 Y blend to noL4C
18:00 - Jim measuring Rz olgf
Ref alog 73625
The ITMY peak is also coming the a "stuck" H1 L4C. This problem is kind of subtle, the L4C only seems to misbehave when the isolation loops are on and the accelerations on the L4C are low. Because tripping ETMX seemed to fix that L4C, I tried it on ITMY. To do this, I put the ISI in damped, ramped on an offset on the ST1H1 actuator, then turned it off with a zero ramp time. I did this for 2k, 3k and 4k counts of drive. All of these caused a bigger signal than the ETMX trip earlier, but the ITMY H1 L4C is still "stuck". Attached asds compare the corner 1 and corner 2 l4c-to-t240 local tfs before and after the whacks. No difference before vs after, but brown and pink are before, red and blue are after.
But, changing ITMY Y and RZ St1 blends to 250mhz blends that don't use the H1L4C makes the 1.something hz peak on ITMY go away. This also worked on ETMX. I've set both ISIs to not use their H1 L4Cs, we'll watch for a while and re-evaluate next week. At this point, only ITMX is still using it's H1 L4C.
The two lines around 1.3 Hz are gone from DARM.
Unfortunateely it's hard to tell if this improved the nosie above 10 Hz, because there are a lof of scattered light events (4 Hz cryobaffle?)
