We continued the several trial for obtaining best performace.

## what we did

### guardian implementaion to engage dewhitening filters in TypeA TM stages

Currently, this engagement of 3-stage dewhitening filters for each TM stage of ITMX, ITMY, ETMY and 2-stage dewhitening filrers for ETMX is done automatically in the LOW_NOISING_SUS_FOR_PRFPMI state of LSC_LOCK.

Also, in ITMX and ETMY cases, the dither excitation for BPCs becomme amplified by the increasing the number of the filter. So we lower the dither amplitude by turining on the 1/10 gain filter in FM1 of K1:VIS-{ITMX, ETMY}_TM_DITHER_{P, Y}, respectively.

### increase CARM IN1GAIN

To reduce the seinsing noise of CARM, we increased the CARM CMS IN1GAIN from -25dB to -13dB, and also, to compensate the overall gain of the CARM loop, we decreased the CARM CMS FASTGAIN from 4dB to -8dB and K1:LSC-REFL_SERVO_SLOW_GAIN. This gain adjustment is done after increasing input power to 10W. we implemented the adjustment in the INCRASING_LAS_POWER of LSC_LOCK (fig.1).

Actually, it seems to be necessary to increase CARM IN1GAIN more but the IFO lost the lock on chaging the FASTGAIN from -8 dB to -9dB maybe due to the CMS switching issue as reported previously (klog29112). So we need the same modification in klog29460 to FASTGAIN as IN1GAIN.

### check VRPnotch frequency in TM_LOCK_{P,Y}

I checked the frequencies of the VRPnotich filtres in TM_LOCK_{P,Y} whether these frequencies reflect the eigenmode frequencies of the resonances of V, R, P at the current temperature. The measurement seems to have been done on 2025.02.21 according to the date in the measurement filename, {I,E}TM{X,Y}_WNI_MN{V,R,P}_202502211804.xml. Therefore, as for ITMX, ITMY, and ETMY, I used the eigenmode frequencies' result from the latest files. As for ETMX, I made a mistake and ran the measurement. So I used the eigenmode frequencies from today's files. There seems to be no changes, except for ITMY Y and ETMX P and Y. I adjusted the frequencies of notich filters in ITMY TM LOCK Y and ETMX TM LOCK {P, Y}.

### Tune the REFL HWP during PRMI lock

This night, PRMI lock acquisition often failed. I found that the power on REFL PDs seems to lower than the one before 9 days (fig.2) even though the REFL HWP angle is the same. So I tuned the REFL HWP angle from 164 deg to 160 deg so that the REFL power get the same during PRMI lock acquisition. Then the succeces rate of PRMI lock acquisition seems to be improved.

## Next to do

• Tune MICH/PRCL Feed forward control
• make OMC lock acquistion faster

## Note

OMC lock acquisition takes too loooong time just after the initial alignment to OMC...

I prepared a new model library of the common mode servo on which the latch system was applied to all IN1, IN2, and FAST gains (see also Fig.1).

The latch system was already applied only to IN1 for IMC and CARM servo as shown in Fig.2 and it works fine. So this new model should help to mitigate a loud glitches coming from a timing gap between turning on and turning off of BIO and probably allow to change a gain also for IN2 and FAST as 16dB <-> 15dB and 7dB <-> 8dB with keeping IR lock.

For applying this new model to CARM servo (= k1alspll), we need to take care about Dolphin issue. So I haven't installed the new model yet. It can be applied on next maintenance day.

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An amount of the timing gap is slightly different in each BIO card. So a waiting time on the latch system was implemented as a variable named as K1:***-SERVO_BO_DELAY_N in the past work. Though I had considered to prepare independent 3 variables for IN1, IN2, and FAST gains, one variable should be enough because one common mode servo is driven by using one BIO card. So in this update, an amount of the waiting time is a common variable for IN1, IN2, and FAST gain.