KAGRA Logbook

with Washimi-san, Tomaru-san, Takahashi-san (online), Ikeda-san

Conclusion

・The spectral structure between 80 and 200 Hz of the mirror on mount similar to IFI STM 2's mount can be changed depending on how the clamps are fixed and how many clamps we use to it.

Details

Introduction

We measured the displacement of a mirror mounted on a structure similar to the IFI STM 2 mirror mount under various experimental conditions. The experimental conditions were as follows:
1. Relationship between the tightening levels of set screws and the mirror’s spectrum
2. Relationship between number and type of clamps and the mirror’s spectrum
3. Relationship between clamps position and the mirror’s spectrum
4. Relationship between tapping locations and the mirror’s spectrum
Displacement sensor's resolution is 0.15 nm.

1. Relationship between the tightening levels of set screws and the mirror’s spectrum

Using the same setup as before (pic1), we varied a tightening levels of a set screws on side of the mirror mount (pic2) and measured the displacement spectrum of the mirror. However, it was difficult to quantify the tightening levels, as the rotation angle of a hex wrench was not visually discernible, and an adjustment relied on tactile feedback. The results (pic2) showed no significant changes, suggesting a minimal relationship between the tightening levels and the spectrum.

2. Relationship between number and type of clamps and the mirror’s spectrum

In the previous Klog report, we used only one clamp, which differed from those used in actual vacuum chambers named Neji clamp (pic3). For this experiment, we included clamps believed to be used in the IFI chamber referred to as L clamp (pic4), examining the relationship between their number, position, and the spectrum.
The experiment was conducted with no clamps, one clamp, two clamps, and three clamps. The clamp types included Neji clamps and those presumed to be used in the IFI chamber. Details of the setups are shown in pic5, and the results in pic6.
The results indicated that the spectral structure in the 80–200 Hz band changed depending on the number of clamps. However, no significant changes were observed below 200 Hz when comparing two and three clamps, whereas changes were observed above 200 Hz.

3. Relationship between clamps position and the mirror’s spectrum

We investigated the impact of clamp positions on the spectral structure and evaluated the reproducibility of these results.
Clamps are circled in red in pictures.
- Single clamp
The setup for varying clamp positions with one clamp is shown in pic7, and the results in pic8. The results confirmed a relationship between clamp positions and the spectral structure in the 100–200 Hz range. Additionally, comparing the results of position 1 and position 1again (clamped again at the same position) demonstrated reproducibility.
- Two or three clamps
The setups for varying clamp positions with two or three clamps are shown in pic9, and the results in pic10. No changes in the spectral structure were observed in the 100–200 Hz range. However, changes were observed above 200 Hz.

4. Relationship between tapping locations and the mirror’s spectrum

To identify which resonance peaks, correspond to specific parts of the structure, we measured the spectrum while tapping different locations. The tapping locations and their corresponding results are attached.
The findings from this experiment are summarized below.
- The resonance frequency of the mirror-mount-pedestal assembly is 335 Hz (and possibly at higher frequencies?). The peak around 130 Hz is not from the assembly.
- The resonance frequency of the optical table is approximately 119.5 Hz. However, the peak usually observed at 120 Hz has a much narrower bandwidth, suggesting it may not be the resonance of the optical table (possibly power supply noise?).
- The resonance frequencies of laser displacement sensor are 155 Hz and 260 Hz.

・Transfer function measurement using a shaker and accelerometer (Is it necessary?)
・Background measurement with a SUS block as the target
・Cleaning, packing, and dismantling (Setup put by IFI chamber as well)
・Others?