aLIGO LHO Logbook

H1 ISC

jenne.driggers@LIGO.ORG - posted 18:43, Wednesday 05 June 2024 - last comment - 18:47, Wednesday 05 June 2024(78267)

Attempting to relock, maybe yaw alignment issues?

[Jenne, Sheila, Jennie, RyanS, Ibrahim]

The interferometer has remained fussy all day. I suspect we may have some alignment issue somewhere, since the AS_AIR camera looks a little bit lobe-y and wrong in yaw. Twice now, if we've gotten past the DARM_OFFSET state and convinced the OMC to lock (which it will do with the autolocker once the darm offset is at the full value of 9e-5), we're able to at least start powering up.

One time, as RyanS mentioned, we succeeded by slowly stepping the DARM offset up to the value of 9e-5, after all ASC for full IFO (Including SOFT) loops had converged. I'm not sure why we lost lock on this one, but it's possible that we're more sensitive to motion on the LVEA floor, and Robert was out taking photos.
One time, that wasn't enough, and we were more successful with an offset of +0.6 in place in the SRC1 yaw loop both during DRMI ASC and re-engaging it during full IFO ASC. This current lock, with that technique, once the OMC was locked I slowly set that offset back to its nominal value of zero, and we're powering up okay. More specifics on what I did:
- With the ISC_LOCK at CHECK_DRMI_ASC, I turned on an offset of +0.6 in SRC1 yaw. I then let the ASC converge and set ISC_LOCK to PREP_ASC_FOR_FULL_IFO, which turned off the offset (during a time when SRC1 was not engaged).
- I set ISC_LOCK to ENGAGE_ASC_FOR_FULL_IFO, and it was clear that the alignment was getting a bit worse with the SRC1 yaw offset off, so I re-turned it on again to +0.6.
- I set ISC_LOCK to PREP_DC_READOUT_TRANSITION, and the OMC was able to lock, we didn't lose lots of PRG, everything seemed overall fine.
- I ramped the SRC1Y offset back to 0, and then requested NomLowNoise. The alignment definitely saw the move, but it didn't seem terribly dramatic, so it seems fine.
I am hopeful that this was a fluke, and we don't need to actually have a SRC1 yaw offset.

My hypothesis of why we might be needing this alignment offset, is that increasing the DARM offset is letting more carrier light go to the AS port, and the the AS WFS loops are seeing that and responding poorly. So, this alignment offset helps kepe them in the right spot. That's not a very satisfying story though.

Earlier, I had tried moving SR3 by +/- 1urad on the slider in yaw (reverting afterward to the previous slider val), but that didn't really help. I also tried offsets in AS_C and AS_A pointing, but neither of those had the same improvement in the AS camera or POP90. They would improve momentarily, but after the rest of the ASC caught up, we still had the lobe-y look on the AS camera.

Jennie also noted earlier (and it happened at least both of these last two power ups) that we're saturating the OMC suspension when we power up (before moving spots). But, after all the ASCs catch up, it's no longer saturated. So, we're close to the edge of what the OMC suspension can do, but it seems surviveable.

Attached are screenshots at CHECK_DRMI_ASC of the AS air camera, without and then with my hokey offset of 0.6 in SRC1 yaw. You can see that with the offset in place, the camera image is much more centered and nice looking.

For posterity, here are the mattermost versions of the notes, starting with this message:

Jennifer Wright, 4:17 PM: We managed to get past DARM offset because Jenne slowly increased the DARM offset up to its nominal value. The OMC managed to lock but was starting to saturate during power up, we made it through to Power 25W but may be we want to relieve the OMC sus alignment during our next commissioning period so we have more range.

Jenne Driggers, 5:18 PM: Ibrahim is seeing that SRC1 and SRC2 respond to the DARM offset, and it seems like they are pulling things to be misaligned and that's why we're losing PR gain. I don't really have a great solution for this though. I've tried putting in some offsets into AS_C, but when we engage the DARM offset, the same bad behavior happens. I also tried putting in yaw offsets in AS_A yaw, but that time the bad behavior happened fast enough that we lost lock before we could turn off the darm offset

Jenne Driggers, 5:30 PM: It still feels like we have an alignment fiasco going on. The AS_AIR camera looks yaw-ish when we have DRMI locked. When DRMI ASC is engaged, I can move SR3, and before SRC1 and SRC2 can catch up, the AS camera looks round again, and the POP90 buildup goes down while POP18 goes up. But, once SRC1 and SRC2 catch up, we're back to the same ol' story.

Jenne Driggers, 6:21 PM: Ugh. Putting in a SRC1 Y offset of +0.6 makes the AS Air camera better, makes POP90 go down and POP18 go up (not much POP18 though). And, when we do DARM offset, we don't see the PR gain drop. Once the OMC was locked, I slowly removed the offset, and we're currently powering up

Images attached to this report

Comments related to this report

jenne.driggers@LIGO.ORG - 18:47, Wednesday 05 June 2024 (78269)

Link

There may be some additional low frequency noise, which could be due to whatever is causing our alignment woes. At this point, I'm going to let Ibrahim bring down the violin modes and put us in Observing, and hopefully we'll be able to address this tomorrow during commissioning.