I tried to keep the PRFPMI_RF_LOCKED state and ENGAGE_ETM_DIFF_WFS state (10 min in each state), then request OBSERVATION state.
Interferometer seems rather stable(?)
DARM OLG is smaller, this is same as yesterday(Fig.1.).
I found one glitch in 37 Hz and its harmonics(Fig.2.).
Lock continued about 20 min.
Temporary I changed the OMC lock with 30 mW.
Also turned off the beacon control
Check the stability
Fig.1. showed the spectrum of DARM displacement [a.u.] including(red) and not including(blue) 37 Hz glitch occurred in 3rd May.
I found similar(but not same) structure when we tapped around the gate valve between SRM and OMC area(Fig.2.).
Could someone check the gate valve and its system work well now?
The 37 Hz glitches might come from the instability of DARM loop itself due to the optical gain reduction.
In that case, the work reported in klog25200 could be a reason why the glitches disappeared.
Curently stable operation can be done if DARM UGF is between 50 Hz to 140 Hz (phase margin at this region is over 30 degrees).
Before the gain tuning, DARM UGF becomes 40 Hz (unstable region) by only 2dB optical gain reduction.
On the other hand, even if 4 dB gain reduction happens, UGF is about 50 Hz curently.
So, the stability of IFO is now much better than that before the earthquake due to the modification of DARM control.