Following up the tests performed on Sept 5th (39382), this morning the vacuum team worked on B1 mini-link aiming to dump its mechanical vibrations. Each of the two bellows has been wrapped with one layer of silicon rubber (2mm thick) placed in contact with the pipe and on top of it one layer of lead (1mm thick sheet) the whole gently tightened with a metal strip, see pictures 1 (before) and 2 (after).

With one mini-accelerometer sticked to the pipe (sensing in the direction perpendicular to the pipe axis) we measured the mechanical effect of this installation, the plots refer to quiet noise condition:

• before (blue line in Figure 3) the first mode of the link was around 51Hz, after it moved a bit down at around 44Hz and it reduced in amplitude (Fig. 3 purple line)
• other peaks around 84Hz, 255Hz (triplet) and 405 (doublet) Hz are no more visible in the quiet vibration spectrum (see also Figure 4 and Fig 5, for zoom in)

We also tried tapping the link before and after the istallation: while before the tapping immediately excited one resonant mode seen by the GEO on SDB2 F0 (Figure 6) in particular a resonance at 52Hz which occasionally happened to match the pipe resonance (as foreseen by A.Bertolini in 39397) eventually saturating the geophone signals, in the new configuration the resulting excitation of the suspension measured by the geophones is less pronounced and did not saturate (Figure 7). However this test was better repeated with the ITF locked, as described in 39547.

Finally, we also measured the resonant modes of the other pipes, for characterization purposes (that is we sticked the mini-acc on each one in turn and measured a quiet spectrum). We annotate the frequency of the first mode:

• B1p pipe (Figure 8): 30 Hz
• HS beam pipe (Fig. 9):  24 Hz
• B5 pipe (Fig. 10): 45 Hz