(Alexa, Evan, Stefan, Matt)
The ETMX bounce mode has been limiting us in locking ALS DIFF, and causes us to lose lock when the ESD saturates. The first attachment shows the coil outputs of the stages, and there is a clear peak in the ESDs (note this screen shot was taken with a much lower gain in DIFF and with the boosts off; this peak causes saturation when the DIFF gain is nominal). The second attachment shows the bounce mode appearing in the oplev spectrum. You can see that this mode starts to appear Friday and gets worse into today. Thursday night the mode looks fine, which explains why we were able to lock DIFF Thursday night. At some point Friday afternoon we turned off the ETMX L2 oplev damping; however, we see this mode ringing up before this was switched off.
We managed to damp this bounce mode using the optical lever signal. This has the advantage of not needing anything to be locked, so it is very robust, which is important since it took about 2 hours to damp. The first image shows the L2_OLDAMP_P filter configuration we were using, and a time series of the resulting coil signal. The second image is a trend of the OLDAMP_P_OUT signal, which shows the damping in action (the envelope of the signal slowly decreases with time as the mode rings down, and then jumps up each time we increase the gain). After a couple of hours of damping, we were able to reduce the mode amplitude by a factor of 10, and then return to locking. Note - the bounce mode frequency is 9.775 Hz
I want to give some more detail on the filters we used for the bounce mode damping: We used the ETMX_L2_OLDAMP_P filter, and had only FM1 (0:0.5) and FM6 (Pass 9.8) engaged. The OLDAMP pitch gain was -1.