Anamaria, Georgia, Danny, Corey, Craig Locking has been a problem since we keep saturating our ETMX PUM drive. The problem is a combination of this oft-referenced 4.1 Hz ringing somewhere in our pitch ASC (from the control signal, CHARD P is the most likely culprit), in combination with high microseism finding it's way into the PUM length drive. Together, these two effects ask too much of the PUM stage and causes locklosses. Similar to last night, we've sacrificed range for stability to get more observing time for ER14. We decreased L2 LOCK gain from 23 to 15, increased L1 LOCK gain from 1.06 to 1.16, and left all quad coil drivers in the "1" high noise state rather than the "3" low noise state. These changes are in guardian. We have not tried putting the coil drivers into low noise with the gain changes. We updated and checked the calibration, it seems fine from 25 Hz up, so the high noise is real. The first attachment is the PCAL BB injection. The second is the ETMX PUM Coil MASTER OUT channels before changing the suspension hierarchy around. The third is the same plot after. It is clear our coils were saturating before, and now are not coming close to their rails. More thought required on why our LSC microseism is showing up so strongly in PUM.
Looking at the sensemon range FOM, it seemed like we were losing ~10 Mpc when we locked with lower ETMX L2 gain and higher range on our coil drivers.
I had a look at the DARM spectrum during a few locks over the last few days, it seems like in locks with low ETMX L2 gain there is additional broadband noise in DARM from 28 - 80 Hz. There's at least one lock with the coil drivers in their high range state (state 1) , and nominal L2 gain, and the noise in DARM is lower (brown trace).
We had a lockloss at 1237370491 due to the 4.2 Hz ringup again, even after we changed the ETMX_L1_LOCK and ETMX_L2_LOCK gains. Looking at the drive to ETMX L2 (bottom row, second-from-left in attached plot), our gain changes are doing their job - the low frequency drive to L2 is small - but the 4.2 Hz is still too big (most noticeable in CHARD_P, the pale green trace, but dominating all the arm ASC DOFs). Also of note is that this signal shows up at twice double the frequency in the OMC DCPD SUM channel (second-from-top row, leftmost plot).
This noise is very interesting, I wonder if we can chop between state 1 and 3 in low noise. It looks very similar to the L1 "sunday noise" situation, where noise would come up in this frequency band in random locks. This noise has not appeared again since one of the ITM coil drivers was switched out.
With the higher microseism it seems L2 gets too much length drive and saturates the DAC; the addition of the angular cancellation in the "spot position" through A2L filters on top of that makes the DAC saturate easier but I don't think we can afford very different spot positions. This is why we have to be in state 1 right now. But we should rethink the DARM filter so L2 offloaded properly at low frequencies.
Furthermore, it also seems that the 4.2Hz is worse when ETMX is in state 1 versus state 3, which might be related to how accurate the analog compensation filters are in the digital path. Regardless, Craig thinks it might be coming from the spring change in the DARM plant once we added SR3 heater so maybe it's in line with Sheila's previous fix and some retuning of that filter would help.
I had a closer look at the two times Georgia listed where the L2 LOCK gain was changed for the same coil driver state. The difference in the noise is just because the MICH feedforward became mis-tuned when the L2 gain was changed. The attachment shows that the MICH coherence was high in the frequency band where we saw excess noise when the gain was lowered.
