Stefan, Kiwamu,

We spent this afternoon to continue working on the PRMI lock. After an adjustment of the MICH gain, it became able to reliably lock.

Even though it stays locked for typically 2 minutes or so, it looses the lock for unknown reason.

(The gain adjustment)

To get the PRMI stably locked, the MICH gain had to be significantly reduced. The original gain setting by guardian was 16, but this was too big to have a UGF at around 10 Hz. At the end of the exercise, the gain ended up to be 2. I don't understand why it needs to be such low. Anyway, once the gain was adjusted, it locked repeatedly and no gain peaking or saturation were found. Good. I didn't change the PRCL gain at all -- it was fine with the original gain of 1.4. The attached is a screen shot of the PRMI buildup in striptool. The power build up observed by POP18 was 70 at highest and typically between 40 and 60. This is almost the same as before (see alog 10472). This corresponds to a power recycling gain of approximately 6.

(Lock losses)

For some reason, it typically unlocks after a couple of minutes. According to the feedback as well as error signals, it is not due to servo instability. Also I didn't see a sign of saturation in PRM, PR2 or BS. A funny thing is that the MICH loop acts odd before it looses lock. The attached below is time series of various feedback signals when the PRMI lost lock. It unlocked at t=29 sec. As clearly seen in the plot, the MICH feedback suddenly became quiet at t=28sec -- it seems as if a low-pass filter was enabled. I looked into two lock loss events and both events exhibited the same behavior. I believe that this is the last obstacle which prevents us from robust lock. This needs to be fixed.