Anamaria, Georgia

This evening we struggled to get past LOWNOISE_ESD_ETMX several times. The ~4.2 Hz instability kept ringing up and killing the lock almost instantly. We see this instability in ETMX L2 drive, the ASC arm degrees of freedom, and in DARM. We're not sure why this problem suddenly became so egregious. Earlier today we changed the coil drivers state for L2, maybe this is related?

(Recall: in the TRANSITION_FROM_ETMX state we transition DARM control from ETMX L1/L3 to ETMY L1/L2 and ITMX L3. In LOWNOISE_ESD_ETMX we transition all 3 stages back to ETMX. )

Things we tried:

• We tried to transition only L1 and L2 back to ETMX, leaving L3 on ITMX, but we lost lock due to the 4.2 Hz instability in L2, so the ESD is not the problem.
• We tried to take some PUM/UIM crossover measurements, but had problems getting sensible coherence.
• We tried reverting the changes made at the end of February (a gain increase and a boost at 4.2 Hz in ETMX_L2_LOCK filters), and saw the same sort of lockloss we had back then (L2 and L3 ringing up at the same frequency but out of phase).
• We tried only lowering the ETMX_L2_LOCK gain, while leaving the boost in. This seems to have done the trick.

We lowered the ETMX_L2_LOCK gain to 17 (from the nominal value of 23). Anamaria noted that the 4.2 Hz peak was still visible in DARM and occasionally dominated the RMS, so we lowered it again to 15. I put this in guardian and loaded it.

I updated the front end calibration actuator filter (CAL-CS_DARM_FE_ETMX_L2_LOCK_GAIN) to match the new actual gain.