We succeeded to lock FPMI for more than 2 hours with low noise suspension controls for BS and Type-A suspensions.

Detail:
We locked FPMI and requested OBSERVATION state for all Type-A suspensions and BS to lower the suspension local control noises.
Current UGF of DARM feedback is abot 38 Hz (fig1).
After the measurement, we updated the calibration parameters and measure the displacement noise spectrum.

Figure 2 shows the current best sensitivity.
Pink line represents the sensitivity during O3GK, and blue line shows the current best sensitivity.
Brown line shows the 3-Mpc sensitivity curve (JGW-T2113624-v3), which is our target for the O4a observing run.
Purple line represents AS dark noise in the current configuration.

As you can see, high-frequency noise above 34 Hz is limited by AS dark noise.
Low noise contributions should be characterized but the sensitivity around 10 - 20 Hz is already 10 times better than O3GK sensitivity.
So, current low noise controls for Type-A suspensions and BS seems much better than before.

Note:
No ASC is engaged during the FPMI lock except for IMC ASC.
Y arm transmission were decreasing a lot during the lock (fig3) maybe due to turning on the FFUs around BS area in this morning.
ETMX once oscilated a lot and recovered (fig4). We are not so sure the reason but it seems related to the reduction of Y arm transmission; so ASC is necessary.
IMMT1 jumped at t = -30 min but we have no idea of the reason.
During the lock, GVs of BS-ITMX/Y, PRM-PR3, and IMC-IFI were still closed. So, some of the peak might come from clipping, scattering, and so on.