Yuta, Stefan, Kiwamu

This morning, we newly started using the BS M1 longitudinal actuator to make the PRMI lock more solid. This works well.

BS M1 stage feedback:

Our next target is to get the PRC length measurement done. In order to do that, we needed the PRMI locked for a sufficiently long time (more than 20 min). This morning, we found that the PRMI dropping the lock every 5-10 minutes. After some investgations, we found that the lock loss was due to the BS M2 actuator hitting the DAC range. There was a large motion in the Michelson over a time scale of  20 sec. So we decided to use the top mass of the BS to help the M2 stage actuator at the low frequencies.

The attached is the current-emperically-optmized top stage feedback. It seems helping the middle stage a lot at the low frequencies, resulting in a long PRMI stretch. As of now, it stayed locked for more than 30 minutes and is still locked. This feedback is implented in the guardian so that it automatically engages it when the PRMI becomes locked. We didn't measure the crossover frequency, though it should be around 0.1-ish Hz.

Limitter in PRM M2 stage:

We put a new feature in our lock acquisition sequence in which a limiter in PRM M2 stage limits the feedback when the interferometer is in the acquire state. It seemed that the most shaky optic was PRM during the lock acquisitoin. The limitter reduces the kick in PRM and hope this increases the chance to grab the PRMI fringe. We will see how this goes.

Schmitt trigger in guardian:

We implemented a Schmitt trigger in the LSC guardian so that now the unlocked and locked thresholds are different.