Dan, Evan, Kiwamu, Lisa, Sheila (and earlier advice from Keita)

Today we turned on the DARM boost filter described here, to increase the suppression in the 1-6Hz band.  The boost itself was no problem, but we realized that changing the phase of the open-loop gain around 10Hz changes the sign of the ETMY and ITMX bounce modes in the DARM error signal, which means the sign of the damping loops need to change as well.  This led to a few hours of bounce mode excitement, but the new signs and phases are now in the Guardian.

The first plot is a comparison of the OMC TRANS intensity noise before and after the boost filter was engaged.  The RMS before the boost was 2e-2 /rt[Hz], now it is 3e-3 /rt[Hz].  The OMC TRANS camera is much calmer.

The boost filter also reduced the coherence between the OMC length error signal and DARM.  Yesterday, Keita pointed out that the OMC length loop was far, far too coherent with DARM between 1-6 Hz, and this noise was being upconverted around the dither line at 3.3kHz.  The upconversion mechanism is not clear; if the OMC length is properly servoed the dither should not generate any RIN (or, very little) and should not upconvert any low-frequency RIN noise to sidebands around the dither frequency.  But, that's what's happening.  The new boost filter reduces the coherence at low frequency (second plot) and reduces the sidebands at 3.3kHz (third plot), but the coherence at low frequency and at 3.3kHz is not zero (second, fourth plots).

As a precaution against OMC LSC --> DARM coupling, we have reduced the gain of the OMC length loop even further: the OMC LSC boost is now off, and there is a 30Hz rolloff filter (FM3) in the OMC length loop.  The UGF is a little more than 10Hz, as described by the green trace from the first figure in this entry.

Keita also recommended changing the dither frequency and changing the dither amplitude and OMC LSC loop gain.  We haven't had a chance to do these things tonight.  We need to investigate why this noise is being upconverted - note that the amplitude of the 3.3kHz dither line in the OMC RIN increased after the OMC LSC boost was turned off (third plot).

The good ETMY and ITMX bounce mode damping settings with the DARM boost ON are (in M0_DARM_DAMP_V):

The good settings with the boost OFF are:

This switching is handled by the Guardian.

By the way, an ongoing mystery is the source of a 300Hz line in DARM that is very large at the beginning of a lock but steadily decays to the level of the noise floor over a timescale of ~10-20min.  The rate of decay is somewhat matched to the BS butterfly mode and the earlier twin peaks noise, but the line is present when we're locked on RF readout, so it doesn't seem to be coming from the OMC.  We've checked the IOP inputs that are sampled at 65k and we don't see a line at 16384+/-300Hz that could be aliased, but we should check again at the very start of a lock when the 300Hz line is very prominent.