Ushiba, Hirose, K. Tanaka
### Abstract
We tried to lock PRFPMI in the state of increasing the injection power from 1 W to 3 W. We succeeded in PRFPMI RF lock with ASC when IMC output was ~3 W, but some suspensions oscillated at 1.7 Hz whose this frequency is Type-A Yaw resonant freq., especially ITMY and ETMY YAW. We measured the OLTFs related to LSC (DARM, CARM, PRCL, and MICH) and confirmed these controls have enough phase margin so they are stable. And we found this oscillation doesn't occur (for at least ~30 min) when any BPC turns off because we turned off all BPC during the OLTF measurements. We tuned the ITMY and ETMY MNOLDAMP controls. The situation was improved a little. So we may need to damp this frequency more robustly with local (or global control?).
### What we did
- We turned the HWP in PSL to 29 degrees and increased IMC output to ~3W.
- We modified the ASC_LOCK guardian so that it sets the value of K1:ASC-WFS_GAIN to 0.3 in order to decrease all WFS controls' gain because the injection power got 3 times.
- We requested the LSC_LOCK guardian to PRFPMI_RF_LOCK, then succeeded without any problem. However, after a while, some suspensions oscillated at 1.7 Hz, especially ITMY and ETMY YAW (Fig. 1).
- We measured the OLTFs related to LSC (DARM, CARM, PRCL, and MICH) after we turned off any BPC because BPC prevents the measurement of LSC OLTFs. Fig 2, 3, and 4 are DARM, PRCL, and MICH OLTFs, respectively. (I hope CARM OLTF may be posted by Ushiba-san later.) These gains of each OLTF seem to be small changes, but each phase margin is enough. So these controls themselves are stable.
- And we found that 1.7 Hz oscillation did not occur during the LSC OLTF measurements for 30 mins. So we turned on PR2 BPC, then 1.7 Hz oscillation started. However, the oscillation didn't stop even though we turned off BPC. So we couldn't distinguish the cause of the oscillation by BPC or timing.
- We tuned the ITMY and ETMY MNOLDAMP controls to damp Yaw resonance. The situation was improved a little. The situation was improved a little. So we may need to damp this frequency more robustly with local (or global control?).
