Cheryl (over phone), Rahul
This morning we made changes to the damping filter for ITMX mode 2,3,4. We narrowed down the band pass filter and moved them away from Interfering from each other. Next we loaded the coefficients and made the gains to be zero on lscparam (followed by svn commit). Jeff. B then attempted to lock the IFO and during this time I found a good gain settings for ITMX mode 3 (gain of -20) and was tinkering with IMTX mode 2 (with a very small gain of -0.1). We lost lock after some time hence I was not able to find a good value for mode 2.
During the second attempt of locking, the modes were rung up and we lost lock very quickly. Next I reverted all the damping filter settings (including lscparams) back to how it was this morning at 8am. In the third attempt I requested Jeff. B to hold on at PREP_ASC_FOR_FULL_IFO, while I was manually damping the rung-up modes. We stayed at this state for an hour or so, and the rung up modes were damping well. Hence I requested Ed (who took over form Jeff. B) to start moving up the ISC lock state - here we lost the lock once again.
In the fourth attempt, the violins looked fine and we are currently locked and Observing (at 0:14 UTC).
Plots of the three attempts at reaching NLN that did not make it, which include the ITMX violin modes 2, 3, 4, 5, 6, and 7. New filters were tested on modes 2, 3, and 4. One unexpected event was that damping mode 4 caused mode 5 output to increase, suggesting modes 4 and 5 are the same fiber. A couple of the lockloss events resulted in ITMX L2 DAMP pitch changing bu about 10urad, which doesn't seem to be the case for every lockloss, however the third lockloss can be attributed to violins, since some were rung up to a level where the RMS was 6.5 to 7.
Plot showing ITMX mode 5 ringing up. This is about 60 seconds of data, showing damping on modes 2, 3, 5, and 7
Trends are color coded:
Phase plots of the old and new damping filter is attached below.
Given below are the phase information (old vs new) for the band pass filter designed for ITMX mode 2,3,4. I have also attached 3 figures which shows the bode plot from foton. One can compare the old and new filter design using these plots.
ITMX mode 2 (504.887 Hz)
Phase old: -412 (+360-412= -52 degrees)
Phase new: -619 (+360-619 = -259 degrees)
This filter has a Phase difference of -207 degrees, hence will require a sign change by 180 degrees.
ITMX mode 3 (504.719 Hz)
Phase old: -360 (+360-360 = 0 degrees)
Phase new: -719 (+360-719= -359 degrees)
This filter has a Phase difference of 1 degree, hence will not require any phase change.
ITMX mode 4 (504.852 Hz)
Phase old: -402 (+360-402= -42degrees)
Phase new: -818 (=360 – 818 = -458 degrees, i.e. -98degrees)
This filter has a Phase difference of -57 degrees, hence will require a sign change by -60 degrees.