Abstract:

I remeasured TF from MICH to DARM and made a noise budget of MICH control.
Measured TF was about 2 times larger than the previous one and current sensitivity from 60 Hz to 130 Hz is limited by MICH control.
So, we need to finely adjust MICH FF for improving the sensitivity around 100 Hz.

Detail:

Today, I injected 100 Hz peak from excitation channels of MICH1 and PRCL1 filter banks.
Then, I found that the peak in MICH control noise projection is roughly two times smaller than the peak in DARM (PRCL is also 1.5 times smaller but not so problematic because PRCL control noise is much smaller than MICH one).
So, I remeasured the TF from MICH to DARM between 50 Hz and 200 Hz with sqept sine (fig1).
This time, I used swept sine measurement for obtaining better coherence.

Compared with the previous measurement (klog24152), gain is 2 times larger.
So, I implemented new TFs into diaggui to see the current noise contribution.
Figure in the following link is current MICH noise contribution to DARM.
https://dac.icrr.u-tokyo.ac.jp/KAGRA/DAWG/Detchar/klog/klog24239
As you can see, current sensitivity is limited by MICH control from 60 Hz to 130 Hz, except for several peaks.
So, we need to improve MICH FF or reduce control noise itself for changing control filters.

Discussion:

There are several possibility that the current TF from MICH to DARM is different from the previous measurement.
1. MICH FF performance was changed.
2. Previous measurement had some systematic errors.
MICH FF performance is basically determined by only actuator efficiency of BS and ITMX, so it is not likely due to performance changes because actuator efficiency change of a factor of 2 seems too large.
Another possibility is measurement errors and since white noise injection measurement increases RMS of error signals, effective optical efficiency during the measurement can be maller than real one.
So, it seems better to use swept sine measurement result for noise projection of MICH and PRCL control noise.

Note:

The TF data is stored at /users/Commissioning/data/MICH/2023/0302/TF_MICH2DARM_20230302.xml.