[Tanaka, Komori]

Abstract:

We attempted to achieve the 10-W DC readout, but this has not yet been successful.
We will stop pursuing the 10-W DC readout for now and proceed with increasing the ASC bandwidth.

Detail:

We had difficulty increasing the input power above 6 W because of an oscillation of EX length at 0.6 Hz.
After tuning the coil balance (klog:36227, klog:36228, klog:36229), we succeeded in increasing the input power up to 10 W without the oscillation, suggesting that the coil balancing is effective.

We then attempted to move to the DC readout state; however, the lock was lost immediately after handing over the DARM control from EX to EY in order to switch the coil driver from the high-power one to the low-power one.
After the handover, EY started to oscillate at 0.6 Hz.
We suspected that this oscillation might originate from an actuator imbalance between the EY and EX test masses, which could lead to instability in the hierarchical DARM control of the multi-pendulum system.

The attached figure shows the results of the actuator efficiency measurements of both test masses using the ALS DARM signal.
The efficiency difference is below 1 dB, indicating that this is unlikely to be the cause of the oscillation.

Although further investigation is needed to identify the underlying reason, we will stop attempting to achieve the 10-W DC readout at this stage, since the current 5-W DC readout is sufficient for the work planned in the near future.
We will instead proceed with increasing the ASC bandwidth.