[Carl, Haoyu]

We propose that the violin modes could be actively damped.  Also that notching violin mode freqencies in control paths could reduce mode excitations.
This could potentionally give a factor of 10 improved sensitivity between 176Hz and 186Hz during some recent lock stretches.  First attached figure top panel shows violin mode spectrum during some recent lock stretches (Dec and Jan).  The blue trace shows a trace where a factor of 10 to 100 reduction in mode amplitude could gain significant bandwidth sensitivity in DARM. The most recent trace from 22 Jan is similar to the red trace with low violin mode amplitude.
The violin modes are often small in amplitude in the figure, however we identify several times when the amplitude of individual modes grows dramatically during lock. One such transient is shown as a sequence of PSD (fig 1 bottom panel).  It grows in amplitude in a few seconds then rings down over a minute of so (03:13:00 31/12/2024 UTC).
We guess this must be the result of some control loop driving the modes (or intentional driving), notching violin mode frequencies in control loops that actuate on IM (probably pitch vertical and roll) may be a way to avoid these transients in violin mode amplitude and active damping may be able to reduce amplitudes faster and to a lower level after excitation.

In klog31963 excitation and ring down measurements of violin modes is described.   We understand the drive signal to excite the modes is IM pitch.

We therefore propose that the DARM signal could be fed back to IM pitch to actively damp violin modes.
A model change would be required to send the DARM signal to a VIS model to damp one violin mode.  Cartoon of where this change would be required in the RCG is shown in the second attached figure. The VIS models would also require DARM from IPCx_PCIE if this is available on VIS computers.  This type of damping has been used at LIGO Livingston for violin mode and bounce and roll mode damping.   Also similar notching of control signals (in addition to existing notches in damping filters) may decrease excitation of lower frequency suspension modes from glitches.