Alexa, Sheila, Kiwamu,

We are concluding that the DRMI seems to have a high rate of the SRC mode hops when the arm cavities are present. At this point, it is unclear why.

(A comparison test with and without arm cavities)

We locked the DRMI while holding the arm cavities at a off resonance point (750 Hz away from resonance in IR) by ALSs. The number of the mode hop was as high as once a second or so. As usual, a SRCL offset of -800 cnts could help reducing the mode hops and also aligning the DRMI helped it as well. However we could not completely get rid of the mode hops at the end. So we quickly switched back to a simple DRMI by misaligning ETMs to determin whether if it is an issue only when the arms are resent. Indeed, the simple DRMI locked and stayed without a mode hop. So the arm cavities really increase the rate of the mode hops.

Note that there was a good DRMI stretch starting at Oct-29 7:00 UTC.

We also tried engaging the ASC loop that had been tuned up for a simple DRMI. This did not look very promising as it was not able to reduce the number of mode hops. It may have been a situation where the ASC was not able to correctly work because of too many hoppings. We don't know what exactly was going on yet.

(Some ETM suspension adjustment for DIFF loop)

The roll mode rang up tonight as well in ETMY. We decided to use only ETMX until the roll mode settles down. Also we changed the L3 gain from 0.8 to 1.1 in ETMX because we saw an oscillation at the cross over frequency at around 2-ish Hz when the gain was at 0.8 and ETMY was disabled. We could not push the UGF of the DIFF loop to 10 Hz because of saturation in the ETMX ESD supposedly due to the roll mode dominating the rms. We decreased the overall LSC UGF by a factor of 2 or so in order to avoid the saturation.

At some point, we were suffering by an issue of the X arm being misaligned as the LSC feedback got bigger. This turned out to be a bad L2P decoupling in the L2 stage which Sheila turned off (alog 14428) because the filter was thought to be bad. We have no idea what has changed since then, but it was obvious tonight that the L2P decoupling filter really helped the decoupling and improved the stability of the DIFF loop a lot. We now enabled it.

(SR3 opelv damping loop was off)

When locking the DRMI, we noticed that a pitch mode of an optic was too large that it was inducing more mode hop in the SRC. We identified it to be SR3 whose oplev pitch loop had been switched off. So we switched it back on and this helped the stability of the DRMI.