[Tanaka, Komori]

Abstract:

At this stage, we have mitigated the DSOFT Y issue by waiting for the BPC to stabilize and adjusting the input matrix of the X trans QPD combination.

Details:

We had been experiencing a significant phase delay in the DSOFT Y control loop, along with poor reproducibility after each IFO lock acquisition, likely due to strong dependence on the beam spot position.
Today, after allowing the BPC to stabilize, we measured the openloop transfer function using the input matrix values previously tested in klog:36329 (red).

We found that the phase delay was recovered to the same level as in the earlier measurement (brown), indicating that this input matrix configuration provides good reproducibility once the BPC is stable.

Therefore, at this point, we have decided to switch the ASC from the low-bandwidth mode to the high-bandwidth mode only after confirming that the BPC has stabilized.