I and Dan confirmed that shells of ALL in-vac cables that are connected to CDS side are grounded to the metal parts of ISI and TMS. I'm talking about all cables related to ETMY, ERMY and TMSY. There is a huge potential risk of ground loop(s).
The cause of this is NOT in four ISC cable chains (picomotor, beam diverter, and two QPD cables). This was confirmed by disconnecting all ISC cable chains from the CDS side at the moc feedthrough: When we did this, the shells of disconnected in-vac cables were isolated from the metal parts of ISI/TMS.
We haven't exonerated the two BOSEM cables of TMS yet, as well as all ETMY and ERMY cables.
What is likely happening here is:
- There is one or more crappy cable clamp which physically grounds the cable shielding to the metal in-vac parts.
- Cable shields are connected to pin 1 in-vac, which is connected to pin 13 out-of-vac.
- Pins 13 are connected to each other on CDS side.
- Thus, the shell of any good cable is connected to the metal in-vac parts via CDS and crappy cable.
Even though there seems to be no DC-coupled "loop" as far as ISC table/cable is concerned in the above picture, basically the ISC cable shields on the ISC table are within 1/32 inches or so of the table surface for the entire length and there certainly is a capacitive coupling. And if there are two crappy cables, they form DC-coupled loop, even though that's not ISC. This is bad.
One crappy cable might be the cause of the entire thing. I can continue testing the remaining two TMS cables, but if we're unlucky we might need to check each and every in-vac cable including ETMY, ERMY, TCS and ISI cables, and I need cooperation from all parties.
We fixed one crappy cable clamping today.
One of the peek cable clamps was squeezing four ISC cables really tightly, and this was causing some problem to the picomotor cable. Either the shielding of the cable was shorted to the shielding of one of the QPD cables, or it was directly shorted to the ISI table. (The symptom was that the shell of the picomotor cable was grounded to the ISI table even though the picomotor cable was not connected to CDS.)
Due to the position of the picomotor cable in this cable clamp, we cannot confidently say which was the case, but anyway once we made the spacing of the peek clamps a bit wider (second picture), the problem was gone.
How is IY? Have SEI/SUS people confirmed that there's no potential ground loop there?