KAGRA Logbook

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VIS (PR3)

kentaro.komori - 3:00 Thursday 01 August 2024 (30659)

Reduction of PR3 length RMS

[Ushiba, Komori]

We continued our efforts to reduce the PR3 length RMS as reported in the klog:30638, and we have now successfully installed a narrow-band damping filter.

Ushiba-san measured the precise susceptibility of the PR3 length with a 1 mHz resolution (Fig. 1) to identify the pendulum mode around 0.5 Hz, which we aim to damp, as well as the parasitic resonances nearby.
Based on this measurement, a narrow-band damping filter, including two notch filters, was implemented and tuned to reduce the length motion.

The closed-loop transfer function (1/1+G) with this filter is shown in the top-right panel of Fig. 2.
It demonstrates that the resonant peak is suppressed without significant gain peaking.

I started long-term measurement of the PR2 and PR3 lengths tonight.
The PR3 RMS appears to be even smaller than PR2 at the 0.43 Hz resonance.
I will report on the result later.

Images attached to this report

30659_1722442582_Screenshot from 2024-08-01 00-58-03.png

30659_1722442599_Screenshot from 2024-08-01 00-55-01.png

Comments to this report:

kentaro.komori - 13:00 Thursday 01 August 2024 (30669)

The displacement spectra and corresponding RMS of the PR2 and PR3 lengths were measured overnight, and the results are shown in the attached figure.
These suspensions were in the lock acquisition state with the BF length damping.

The displacement RMS above 0.1 Hz for both suspensions was around 0.1 µm.
PR3 had larger noise at lower frequencies, resulting in a higher total RMS for PR3 compared to PR2.

The motivation for this work is to avoid degradation of the DARM sensitivity when the micro-seismic activity is high, as observed in O4a, likely due to larger PRCL causing scattering noise.
However, this measurement was performed on a very quiet night.
Therefore, we will measure these spectra again in the near future when the micro-seismic motion is high and compare it to the data from O4a.

Images attached to this comment