aLIGO LHO Logbook

H1 ISC

elenna.capote@LIGO.ORG - posted 17:00, Tuesday 03 June 2025 - last comment - 21:12, Tuesday 03 June 2025(84769)

Strange Oscillation during DRMI ASC

Elenna, Georgia, Craig, Stefan, Oli

We saw a very strange oscillation that appears to be beamsplitter related during the engagement of DRMI ASC. I tried turning down various DRMI ASC gains: SRC1, SRC2, MICH P, and it made no difference. We were getting BS saturation warnings. Finally, I just moved from"engage drmi asc" to "turn on BS stage 2" and the oscillations appeared to reduced. We saw large peaks with harmonics in the corner LSC signals starting near 18 Hz. We are wondering if it's an errant beamsplitter bounce mode?

Comments related to this report

craig.cahillane@LIGO.ORG - 17:24, Tuesday 03 June 2025 (84770)

Link

Seems to be the SRCL1 offset change from 800 cts to zero counts.
The ringup frequency is 16.2 Hz and is seen in every loop very strongly, especially MICH.  

I thought the MICH2 BR0604 filters might have something to do with this, but those are focused more along 17.8 Hz.

Elenna points out that the 16.2 Hz ringing is present in all LSC prior to to the SRLC1 offset change.  
It is clear that the SRCL1 offset change makes things 100 times worse over a period of around 5 seconds.

We could consider slowing down the offset change, as it currently happens in only one second.  
We could also investigate these bounce roll notches to ensure they are at the correct frequency for the BS.
We could check the DRMI LSC OLGs at acquisition as well, as one may be on the edge of stability during a messy acquisition.

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elenna.capote@LIGO.ORG - 21:12, Tuesday 03 June 2025 (84783)

Link

This has caused another DRMI lockloss but it was too fast to determine where it was coming from.

We haven't been able to lock DRMI since this particular lockloss, so we haven't had a chance to measure OLGs to see if it's a loop instability.