F. Clara, J. Kissel
FRS Ticket 9683

In order to convince ourselves once-and-for-all that the problem with H1 SUS ITMY's R0 RT OSEM is because of the in-vacuum short of the cathode signal of the PD (see LHo aLOGs 40134 and 40099 and above cited FRS Ticket 9683), Fil and I have done a quick cable-swap study, where we inter change the cables of the 4-OSEM clusters at the chamber side of the satellite amplifiers. 

Attached are the corresponding ASDs of the test. Recall that there are three clusters to read out the two top-masses of the quad -- M0 F1F2F3SD, M0LFRT/R0LFRT, and R0 F1F2F3SD. These are cables ITMY_SUS-7, ITMY_SUS-8, and ITMY_SUS-9, respectively as shown in D1100022. With the cables connected as designed, the problem is on the 4th channel of ITMY_SUS-8. The ASD plots attached show what happens when you move the connection of ITMY_SUS-8 to the sat-amp that normally reads out R0 F1F2F3SD, SUS-H2-R6-6 (reference traces, showing badness on R0 SD), or when you move the connection of ITMY_SUS-8 to the sat-amp that normal reads out M0 F1F2F3SD, SUS-H2-R6-4 (live traces, showing badness on M0 SD). 
One can see that the drastically elevated broadband noise with a ton of frequency combs that is from the R0 RT OSEM follows the cable, regardless of which readout chain into which it's plugged. During both steps of the test, the opposing cable (either ITMY_SUS-7 or ITMY_SUS-9) was plugged into the sat-amp that normally reads out M0RTLF/R0LFRT, SUS-H2-R6-5. In both cases, the M0LFRT/R0LFRT channels show no problems reading out the OSEMs on either SUS_ITMY-7 or SUS_ITMY-9, which exonerates that readout chain of problems.

This puts a nail in the coffin, and confirms that the noise arises from the in-chamber short, and we'll need to vent the corner in order to fix this. 

Whether we *will* vent *just* to fix this is bigger question that requires many team's input. However, there are a few windows of opportunity in the next few months: 
  - 90% of these sorts of problems have been at the chamber feedthrough, so the "vent" may be as simple as bringing the corner up to atmosphere, popping off only that feedthrough, fussing with the in-vac connection at the feedthrough, popping the feedthrough back on, and pumping back down.
  - Gate-vales are already closed, so we need not worry about the additional cycling on the gate vales.
  - We will eventually *have* to bring the chambers to *almost* atmosphere when we attempt to discharge the ITMs.
  - We anticipate the PSL up/downgrade to the 70W amplifier, and the mid-station cryo-trap decommissioning to demand there's little we can do with the IFO until early April-ish.
We'll discuss amongst ourselves and see which path we should take.