[Aso, Miyoki, Ushiba]

Abstract:

We performed tapping test around IFI chamber and found that it is affected to DARM a lot, especially several peaks above 100 Hz.
Also, we can observed IMMT1 trans QPD signal enhancement during the tapping test.
So, it is very likely that jitter is generating inside the IFI chamber.

Detail:

According to the noise estimating from IMMT1T QPD (klog24187), jitter could be a current main noise sources above 100 Hz.
Main jitter generator can be IMC suspensions, STM1, and STM2 but STM1 and STM2 are more likely to be a noise sources because they are not suspended mirrors.
So, to confirmed the sources are STMs, we performed tapping test arouind IFI chamber.
We tapped following places and can see several reaction at DARM.

1. Copper frange on IFI chamber for high power beam dump.
When we tapped Cu frange, we can see enhancement of DARM sensitivity above 200 Hz.

2. Outer frange handle of Cu frange for high power beam dump.
The largest enhancement of DARM sensitivity above 200 Hz was observed.
The sound is obviously higher than that when tapping Cu frange.
DARM spectrum when tapping can be seen in the following link.
https://www.dropbox.com/work/KAGRA%20Dropbox/Personal/Aso?preview=2023-02-28+15.51.49.mp4

3. KF400 franges close to Cu frange.
We can also see enhancement of DARM sensitivity above 200 Hz.

4. Legs of IFI chamber
No significant changes.

5. Bellows legs of IFI chamber
DARM sensitivity became worse slightly above 200 Hz but not so significant.

6. Franges for REFL beam.
We can see enhancment of DARM sensitivity but it was smaller than that when tapping Cu frange, its handle, and KF400.

7. REFL table.
No significant changes

Discussion:

The changes of IFI chamber from O3 period are beam dumps.
Especilly, high power beam dump was installed with thick heat link cables and connected to Cu franges on IFI chamber.
Though STM1 and STM2 inside IFI chamber was isolated by the double stack, this heat link might be a shortcut of vibration to STMs.
Since the tapping close to the Cu frange affects DARM sensitivity larger, it is very suspicious.
We will consider how to solve this issue as soon as possible.

Note:

According to klog13033, coherence between DARM and IMMT1T QPD PIT sometimes appeared also during O3 era and disappeared somehow.
So, it might be able to reduce by doing fine alignment of IFO.