Abstract:

Current low frequency noise sensitivity, which is better than before warming up, seems due to the coupling reduction from feedback signals to DARM.
At this moment, I have no idea why the coupling is reduced.

Detail:

Currently, low frequency sensitivity (10 - 30 Hz) is slightly (but significantly) better than before warming up.
So, I investigated what happens by comparing the TF measurement results reported in klog35621 and klog36363.

Figure 1 shows the result of the transfer function measurement from ETMX_MN_DAMP_L_OUT to DARM at room (red) and cryogenic (blue) temperature.
The TF gain seems significantly smaller at room temperature.
Since actuator efficiency change should be much small according to the change of OLTF of local control loops after warming up, I have no idea why the coupling from feedback signals to DARM is reduced so much.