Following Thursday measurement, today more tapping tests were performed in the central area in order to find sensitive spots on the vacuum chambers and links. After this identification, a more quantitative measurement is needed in order to better characterize the noise impact on the sensitivity.

We focused on the remaining perimeter of the central area, and we found some sensitive spots, which are highlighted in yellow in the first attached plot. In the same plot, the red arrows show the areas which were investigated in the last measurement campaign.

In particular, what we found pretty critical was the link between SR and DET. Figure 2 and 3 show the DARM spectrum during the tapping test performed on the DET West flange compared to the same spectrum during a "quiet" period. This test was performed twice during two different locks, and they gave similar result (figure 2 and 3).

In figure 4 the tapping is performed on the DET link, very close to the flange. In both the cases, the induced effect is an increase of a forest of lines in the frequency region between 200 Hz and 1 kHz. Since this area seems to be pretty critical, a sistematic noise injection with a shaker has to be performed.

The third region highlighted in the first figure is the SW flange of the NI tower. We started doing tapping tests also here, but we realized that the whole noise floor was growing, independently on the tapping itself.

During these tests, we got the feeling that the same point is more sensitive to noise injections when the sideband power is low, which seems to suggest that the ITF working point is less stable, and the noise floor is higher. This point has to be further investigated, but seems to point out that we have to carefully check the relevant signals (sidebands, alignment, etc.) are ok before considering meaningful the outcome of the tests.