Naoki, Vicky
To try damping the 80 kHz PI recently causing locklosses (LHO:70434), today we installed a new path on PI28, with drive sent to ETMX. To phase-lock the ESD damping drive to the PI mode, we bandpassed the DCPD signal around 80.296 kHz (foton here). This frequency was chosen based on the DCPDs full-spectrum signal around 80.3 kHz, which today we saw the 3 peaks in red here between 80.295-80.301 kHz in full lock. It seems like our problem is the bigger peak around *296, where pink cursors are centered. We are trying to damp on ETMX first, since it seems like PI29 80kHz damping on ETMX could impact this mode.
The new path for PI28 has been updated and damping is guardianized, but this PI28 damping is untested, and there are no verbal alarms for PI28 yet.
Summary with the current status of PI damping:
| PI frequency | PI damping mode number | Test Mass | PI Guardian status | _DAMP_GAIN |
| 10.428 kHz | 24 | ETMY | automated | 1000 |
| 10.431 kHz | 31 | ETMY | automated | 1000 |
|
80.302 kHz (LHO:68760) |
29 | ETMX | automated, likely working (LHO:70243) |
50000 |
|
80.296 kHz (LHO:70443) |
28 | ETMX | guardianized, testing now |
50000 |
SDFs were recoinciled after, see screenshots -- mainly, we un-monitored guardian-controlled things like the damping phase and PLL integrator.
I've added in a test for PI mode 28 into verbal alarms with an RMS threshold of 1 for now (the same as mode 29).
Just checking in, looks like PI28 does at least see a mode come through ~1-2 hours into lock (RMSMON spikes just before NLN b/c OMC whitening, the second peak is the real one), but this hasn't run away yet. Then ~10 min after PI28, looks like PI29 sees something pass through.
Looking at disaggregated temperature trends across the EX VEA over the last year, it might be a bit misleading to only look at the "average EX VEA" temperature. Temperatures in some parts of the EX VEA seem to have drifted by up to 0.5-2 deg F over the past few months. Temps now seem to be returning to where they were about 6mo - 1 year ago.
I think Robert and Aidan have both made the point that, to understand temperature drifts, it's helpful to look at the individual temperature sensors across the VEA, rather than the average VEA temperature. For example, see Aidan's alog LLO:25785 where he also thought about stabilizing the VEA temperatures to a different sensor, which is better correlated with the test mass temperature. For this 80 kHz PI though, Aidan's also said that the temperature dependence of the mechanical mode freq is about 80ppm/K, so for ~0.5K (delta~1degF), the HOM frequency changes by ~3 Hz for the 80 kHz mechanical mode, and it seems unlikely we're just within 3 Hz of the PI going unstable -- so it's not totally convincing that our recent 80 kHz PI ring ups are simply b/c VEA temperature drifts. But at least, at LLO, he found their ETMY is most correlated with the EY VEA_202B sensor.
I'm not sure which of LHO's EX VEA sensors are most correlated with ETMX. But, it may be worth considering more than the average VEA temperature. Especially since the individual temperature sensors have seen some drifts over the past year, which aren't seen by trends of the average VEA temperature.
