(kiwamu, nic)
There was a very large ~10Hz spike in the ALS DIFF control signal. The signal was also visible in the optical lever signals of both ETMs, and they looked coherent with each other
We were only feeding ALS DIFF to ETMX, so Kiwamu hypothesised that this indicated that the source of the motion was ETMY and the DIFF loop was impressing motion onto ETMX.
We waited for a long time for the mode to damp itself, but it was very persistent.
We weren’t sure what caused the mode to ring up, it may have been the bad L2P filters that caused the DAC to saturate. The only drive signals on ETMY were OSEM and optical lever damping. So we also put a notch in the OL damping loop in case that was ringing it up.
Finally we decided to just damp the mode using the “DARM ERR DAMP FILTERS” on the SUS screen (thanks LLO!). Using the H1:SUS-ETMY_L2_DAMP_MODE1_INMON filter bank and with the output matrix feeding to ETMY L2 pitch, we put a narrow bandpass around 9.73Hz, as well as a velocity damping filter. Then we just varied the output gain until we could see the mode changing amplitude. Then we flipped the sign and further increased the gain until the mode was clearly ringing down. It had a time constant of about a minute. Once we had a good gain, we dumped the gain into a filter module.
Now the correct gain in H1:SUS-ETMY_L2_DAMP_MODE1_GAIN to damp the mode is -1.0.
Bounce mode coupling (to the length) is a factor of 80 larger at EY than at EX due to local gravity VS IFO z-axis angle (alog 14876).
Unlike roll, bounce mode should have a predictable, consistent phase relation to the length, i.e. bounce up = longer arm.
You can see the bounce mode rung up by looking at top mass V (attached) for EY, but not for EX.
This should mean that the bounce could be damped by a factor of few using the top mass (which might or might not be able to stop the ringing up).
Also, if we have a path to route the LSC signal to the top V, we should be able to damp the mode easier (as in easy filter design) than feeding back to L2 stage.
Just in case my elog wasn't clear.
We found the mode rung up while locking ALS DIFF. We were not sure of the reason for it to be rung up.
We were able to damp it using the DARM ERR damping filters that feed to L2, these had not been used in the past. We commissioned them for the first time. Our initial guess of the sign was wrong (50/50 chance) so we flipped it to damp the mode.
Once having the new filters and gain, the mode was damping. We don't yet know if the coupling is stable (requiring retuning) over long time scales. I think LLO has seen that these are stable.
The error signal is DARM ERR so it can only be turned on when on ALS DIFF or full lock. With ALS DIFF, the gain that gives good damping is -1.0.