Evan, Kiwamu,

In this afternoon, we closed the ISS third loop which are meant to damp the arm power fluctuation.

We confirmed that it suppressed the common arm power fluctuation at around 0.5 Hz by 10 dB nominally. We tested this loop at various PSL power from 2 to 25 W. Further sophistication of the loop shape can be done if necessary.

[nominal settings]

• Hit the SECONDLOOP_CLOSED on the upper right in PSL_CUST_SECONDLOOP.adl
  • This enables the thrid loop signal to change the 1st loop error point
• Set the servo to the nominal setting as shown in the first attached.
  • Also the second attachment shows a measured open loop transfer function, but before adding FM4 which is designed to reduce the gain below 0.2 Hz by a factor of 0.25. By the way, we do not fully understand the behavior of the phase.
  • Probably the current ramping time of 10 sec is too long and can be shorten, but it helped reducing unnecessary kicks to the first loop.
• Open the third loop when changing the PSL power.
  • We had one incident where having the third loop engaged during the power up for some reason kicked the first loop enough that it eventually unlocked and the first loop and subsequently the interferometer.
  • Later, Evan found that the power normalization had an unnecessarily long ramping time of 3 sec. We reduced it to 0.5 sec. So maybe the situation is better now.

[injection point changed]

The original plan was to inject the third loop signal to either TRANSFER1 or 2 input. However it turned out that they have seem to have a factor of 10 attenuation, we ran into a range issue where the DAC output and SR560 (which is meant as a dewthitening filter) saturated before getting to the nominal servo gain. We instead moved the injection point to the one for the second loop by disconnecting the second loop and plugging the third loop cable.

Also, there is a low-pass-ish response somewhere in the second loop injection path which let us put a lead filter in FM3.

[Instability at 25 W]

With a 25 W PSL, we see an instability that occured at 0.24 Hz. Similarly to the usual arm power instability at 0.4 - 0.5 Hz, it modulated the arm power, but the frequency seems to be a bit low and it coupled to the HARD mode. This instability seems to happen regardless of whether the third loop is closed or not. A possible source might be an unstable ASC loop. Currently  under investigation.