aLIGO LHO Logbook

H1 ISC (ISC, SUS)

keita.kawabe@LIGO.ORG - posted 22:41, Monday 13 December 2021 - last comment - 15:26, Tuesday 08 March 2022(61034)

HAM6 old FRS ticket items investigations (Betsy, Keita)

I and Betsy did investigate and attempted to address open FRS tickets for HAM6. Everything was very unsatisfying but in the end OM1/2/3 and ASC-AS_C look OK. OMC PZT remains a mystery.

For flange layout refer to D1002877:

1: OM1, 2 and 3

Two tickets were relevant to these.

Optic (feedthrough)	FRS 10037 Backshells of In-vac cabling at Feedthrough Short Against Feedthrough	FRS 10518 HTTS (Tip-Tilt) Micro-D Connectors Electrically Ground to BOSEM Brackets	Ground loop?
OM1 (D6-F10)	Fix (smaller head screws) implemented	Fix (kapton sheets) implemented	No
OM2 (D6-F11)	Fix implemented	Won't fix	No
OM3 (D6-F12)	Fix implemented	Fix implemented	No

The reason why we didn't work on the OM2 Micro-D is because it's eventually replaced by SAMS, and the shimming using aluminum foil (done by me before O3) looked good enough for this one.

Interestingly, we observed ground loop for OM2 after replacing the backshell screws for the DB25-DB25 cable from outside of the chamber. Pin 13 of the DB25 has shorted to the chamber ground. We didn't test it right before that work, but it was good before O3. Anyway, Betsy noted that the DB25-DB25 cable was bunched up under the DB25 connector bracket at the back of OM2 cage where the cable could touch the LL and LR OSEM printed board. She "barely touched it" and the ground loop was gone. We couldn't reproduce the ground loop.

Very unsatisfying. But OK.

2. ASC-AS_C

FRS 10802 (AS_C QPD Anode 4 has In-vacuum Short w/ unused Dual Transimpedance Amp Pin)

Before starting to work, we checked from outside of the chamber that the pin 3 and 4 of in-air DB25 were shorted as was described in alog 42281. Then we disconnected the cable inside the chamber to check that the pins weren't bent or anything on the flange, and they were fine. Just because we can, we did swap the backshell screws on the in-vac DB25 on the cable as in FRS 10037 just like we did for OM1/2/3. We plugged the DB25 back in and the short between pin3 and 4 was gone.

Very unsatisfying. But OK.

3. OMC PZT

FRS 12445 (Grounding and shielding of OMC PZT)

At first I tried to measure the continuity between pin 13 of in-air DB25 and the chamber ground, but failed. I heard the beeping so you'd think that it's shorted, but the resistance was positive or negative 70 Ohm depending on the polarity the DVM was connected. Turns out that the voltage of the chamber ground relative to pin 13 was -22mV and the DVM was probably thrown off by this.

I discharged the PZTs by measuring the voltage between pin1 and pin 14 of in-air cable for LV PZT as well as pin2 and 15 for HV. (HV and LV seem to be swapped in the cabling diagram D1300589-v1.) But that didn't have any effect on the voltage between pin 13 and the chamber ground.

We disconnected the in-vac PZT cable from the vac-air feedthrough and measured the voltage of the chamber ground relative to the shell of in-vac DB25, and it was -22mV. Confused.

Just because we can, we also changed the backshell screws on the in-vac DB25 connector (FRS 10037 just like we did for OM1/2/3) and it did nothing.

Very unsatisfying. Will be asking questions to people.

Comments related to this report

keita.kawabe@LIGO.ORG - 15:26, Tuesday 08 March 2022 (62111)

Link

Update: No change after pumping down (alog 62092).

rich.abbott@LIGO.ORG - 12:06, Tuesday 14 December 2021 (61043)ISC

Link

The weird readings Keita and Betsy saw when looking for grounded pins are due to stray electrical potentials (either thermally driven via dissimilar metals, or some facility related potential).  You see this on a typical Fluke ohm-meter as positive or negative resistances when the meter leads are reversed during a measurement.  By taking the rough average of the two (sometimes a bit unstable) readings, you can reasonably gauge if there's a short for cases where a connection is supposed to be open.

The PZT connection issue for the OMC manifests as an unexplained extra ground for a portion of the OMC PZT cable shield inside the vacuum system.  There was a diagram linked in a March 2019 alog detailing the issue.  I have no idea where this problem is generated, but I would be suspicious of the in-vacuum Dsubs just inside the feedthrough for this chain.  Screws sticking out, or inappropriate connector metalization are possible candidates.

A related issue that makes this even more confusing is that there is a break in the in-vacuum shield for this cable run.  Nobody has any idea how or why this exists, but it could be an avenue for noise coupling if the shield break is substantial or if there are multiple in-vacuum chamber shorts creating a ground loop.

betsy.weaver@LIGO.ORG - 12:29, Tuesday 14 December 2021 (61044)

Link

Pictures of OM1 and OM3 with kapton sheet cut and slid inbetween the BOSEM body and connector where ground loops have been seen to be a problem.

Images attached to this comment

betsy.weaver@LIGO.ORG - 12:35, Tuesday 14 December 2021 (61045)

Link

Attached is a picture of the earlier "fix" by Keita on OM2 whereby he used some foil as a shim to lift the BOSEM connector to open up the gap where the ground loop was observed to be a problem. These have been left in place on OM2.

Images attached to this comment

keita.kawabe@LIGO.ORG - 21:15, Thursday 16 December 2021 (61095)

Link

OMC PZT cable fixed its grounding on its own.

Very unsatisfactory.

As of now, after dicsonnecting in-air cable from the feedthrough for OMC DCPD (D6 F1), OMC PZT (D6 F2) and OMC QPD (D6 F3) on HAM6, measured from outside of the chamber, OMC PZT pin 13 (connected to DB25-DB25 in-chamber shielding) is

NOT connected to the chamber ground,
connected to OMC QPD pin 13 (DB25-DB25 in-chamber shielding) because the shielding is connected to the DB25-MightyMouse cable shielding and because of unintended design feature that all MightyMouse connector shells are tied together on a bracket,
connected to OMC DCPD pin 15 and 16 (signal ground of one of the DCPD preamps in chamber on DB25-DB25) because that ground is connected to the shielding of MightyMouse-DB25 and because of unintended design "feature" mentioned above,
- same thing should be going on for the other DCPD but I didn't check,
NOT connected to OMC DCPD pin 13 (DB25-D3B25 in-chamber shielding).

Attached is the summary of the above.

This is different from Rich's alog from March 2019 and his extremely helpful diagram in that there's no short of OMC PZT shielding to the chamber ground, and there's no mystery break of the shielding.

The most unsatisfying thing is that the in-chamber DB25-DB25 PZT cable absolutely had the ground problem on Tuesday, but it was somehow good today. I know this because, on Tuesday, I completely disconnected the DB25-DB25 PZT cable from both ends. The connector at one end of the cable was put on a clean Vectra wipe so that it won't touch the ISI surface, the other end was hand-held by me, and there was a connection from pin 1 (which is connected to the shield) to the chamber ground as well as the connector shell. After that measurement I connected both ends back and left.

Today I disconnected all three relevant in-air cables from the feedthrough, then disconnected the DB25-DB25 PZT cable from the cable bracket on top of the OMC cage, did some measurements to check DB25-MM section and nothing was wrong. Just because I could, I checked the connection from the connector shell of DB25-DB25 to the chamber ground while one end was still connected to the feedthrough, and to my surprise there was no connection.

Everything was connected back again (except that I forgot to connect the HV supply cable back to the PZT driver).

The problem might have solved itself but it could come back if you look at it funny or something. We'll still have to do the ground check again before closing HAM6.

Images attached to this comment