ALS DARM was locked and lasted for more than 2 hours!!
We designed feedback filters from ALS DARM signals to ETMX.
During O3, we used only TM and MN for mass feedback but in this time, we used all three stages:TM, IM, and MN.
Since we haven't measured OLTF, we don't know UGF, cross over frequencies, and so on but designed values are as follows:
UGF: 20 Hz
COF IM-TM: 7 Hz
COF: MN-IM: 3 Hz
Figure 1 shows the spectra of ALS DARM signals with (red) and without (blue) mass feedback.
RMS becomes more than 10 times smaller.
We have already implemented ALS DARM lock in LSC_LOCK guardian and can lock it by requesting ALS_DARM_LOCKED state for the guardian, if all mirrors' alignments are good.
Red spectrum is smaler than blue one between 10 Hz and 100 Hz but this is not due to DARM feedback.
We are not sure the source but somehow spectrum at these frequency regions are breathing.