Cheryl, Evan
The 0.43 Hz pitch instability strikes again. I had to turn up the dHard pitch gain from 10 to 20 in order to prevent a runaway. Back into the guardian it goes!
It takes tens of minutes to ring up, and even after tweaking loops, it never exactly rings down (the fuzz in the attached striptool is almost all from this 0.43 Hz feature).
Attached are error and control signals for the arm pitch loops at several different powers: 10.4 W (blue), 15.2 W (yellow), and 19.8 W (green).
Is it strong enough to show up in the suspension OSEMS?
Are the OPLEVs still on? They suppress the ASC gain below 1Hz, and might be the cause of instability.
A trend of the oplev PIT loop buttons over the last 12 hours show the:
ITMY OL damping OFF
ITMX OL damping ON
SR3 OL damping ON
PR3 OL damping OFF
ETMX OL damping OFF
ETMY OL damping OFF
All above YAW loops OFF.
BS OL PIT and YAW damping ON.
THe sttached screenshot is from the few minutes before Evan turned up the DHARD PIT gain last night. You can see that there is an oscillation in the Y arm only, and it seems to be soft. The same oscillation shows up in MICH but it is much smaller.
As Betsy pointed out, ITMX oplev damping was on last night, while ITMY oplev damping was on. Both ITM oplev loops are currently turned on in the INCREASE power state of the ISC_LOCK gaurdian, which still seems to be necessary for stability. Based on OLG measurements of DHARD, the Oplevs should not impact the stability of DHARD. However, we haven't been able to get good measurements of CHARD since rephasing the REFL WFS and hopefully getting a reasonable amount of gain from these loops. (just because of other activities).
Hopefully we can spend some time on CHARD today, which may shed light on why we need the ITM oplevs and hopefully reduce the large optical gain fluctuations we have.
