sheila.dwyer@LIGO.ORG - posted 17:34, Monday 31 August 2026 - last comment - 22:38, Monday 31 August 2026(91755)
trying to locking ALS DIFF on ETMY
Louis, Sheila, TJ, Camilla, Oli
We have been working on changes to lock ALS DIFF on ETMY.
- We put the negative sign in the LSC output matrix to account for the DARM sign flip from switching arms, we will try to use the same bias and filter signs elsewhere because that seems easiest to keep track of.
- On Friday we added a flag to lscparams: no_ETMX_actuation to be set to true when we don't want any actuation on ETMX (basically only for this week when we have a rubbing issue), which also sets a flag that determines which ETM to use for locking ALS DIFF, ALS_ESD = 'Y'. We will need to clean this up a bit, but there are two slightly different purposes for the two flags, because we may want to run ALS on ETMY even once the ETMX suspension is fixed and can run slow feedback and WFS.
- Filiberto got the ESD high voltage driver working: 91750
- Oli and TJ re-ran the baffle align scripts to reset the alignment of the TMSs and ITMs, since we may have wandered off with the X arm alignment with some ETMX shifts that happened on Friday: 91747
- the ETMY LR electrode does not work: 43053. We changed the EUL2ESD matrix to only drive LL and UR to reduce the cross coupling to pitch from our length drive. (screenshot attached).
- ALS COMM locked without any issue. This stayed locked while we locked and unlocked ALS diff many times, this is a nice feature compared to locking DIFF by actuating on ETMX.
- ALS DIFF we were able to keep locked by manually setting ALS Y to NO_SLOW_GR_WFS, and setting the DARM gain to 4. It should be set to 400.
- Ramping the gain up to 8 rings up a 1Hz instability after ~10 seconds.
- ramping gain to 40 or 400 instantly breaks the lock, probably saturating the ESD.
- We tried copying the L1 L2P from ETMX, and turned off the L1 L2Y (was a scalar -0.002) neither of these things seemed to change the outcomes.
- We can leave the DARM1 gain at 4 and increase the L3 drivealign gain to 2. This may be because our UGF is so low that L3 doesn't come above unity gain.
- We got a few points of OLG measurement with the gain at 4, attached.
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I was able to use an old template for measuring the ALS L1/L3 cross over with ALS DIFF semi locked with a DARM gain of 4. Since the gain is so low this is more like an OLG measurement than a cross over.
I was also able to turn off the input to L3 drivealign with no discernable impact on the lock. I then did an excitation on the L3 drivealign L2L, and measured the response at ETMY DRIVEALIGN in with the input turned off. I got this measurement for both ETMX and ETMY, so we can compare them. The ESDs have the same strength and different signs as expected.
File locations are visible at the top of each screenshot.
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