evan.hall@LIGO.ORG - posted 02:41, Sunday 19 July 2015 - last comment - 01:14, Monday 20 July 2015(19738)
Lockloss extravaganza
Rana, Matt, Evan
Low-noise commissioning was derailed by various CARM-reduction locklosses:
- Locklosses at and around the transition of CARM from ALS to the arm transmission signals.
- Locklosses at and around the transition of DARM from DIFF to AS45Q. At the very least, Rana has removed the EX saturations that happen around this time. Unclear whether this will also stop the locklosses. Please make a note of any locklosses near this transition step.
- Locklosses at and around the transition of CARM from the arm transmission signals to the normalized PDH signal. Even for the successful transitions, there is significant instability and we seem to survive only because we have shortened as much as possible the amount of time spent in this state. Additionally, the successful transitions lead to resonance with good recycling gain (>37 W/W), indicating that this is not a sufficient condition for ensuring a stable transition.
During the ALS DIFF to DARM RF transitions, there were saturations of the ETM ESD actuators which caused occasional unlocks. This was due to the poor sensing noise in the ALS and so any change in the dark noise or EMI at the end stations would slow down the whole lock acq sequence,
We put the following low pass filter in DARM: started with the RED RLP80, but found the BLUE RLP180 was good enough so that the control signal is now dominated by low frequency stuff instead of the hash at 1 kHz. The script now turns this on by default and turns it off to recover the 100 Hz phase margin as soon as we have transitioned to a low noise DARM RF signal.
* also, while writing this, we had the second Guardian crash of the night using the latest build with the 'no double main()' fix. Jamie has been notified.Today we've had >5 locklosses of the 3rd kind: where the CARM handoff from using arm transmission to REFL PDH fails. Attached is a lockloss plot of one of these events.
@ -1.2 seconds, CARM is on normalized REFL9_I
@ -1.3 seconds, it is starting the transition and the arm powers are starting to dive
Why are the arm powers diving here? I suspect that the normalized REFL signal may have some instability depending upon the L2A -> power coupling, so we might try freezing the normalization at that time.
Also, since we only normalize by Y arm power for CARM, we're getting a DARM-> CARM coupling at f < 0.1 Hz...