Scattering from beam tube baffles is more likely to be a noise source in aLIGO than it was in iLIGO. With this in mind, the motion of the beam tube was studied. Figure 1 shows beam-line, horizontal and vertical motion for accelerometers mounted midway up the side of the beam tube near one of the fixed supports. This spectrum was taken at 988m Y from the beam splitter, when the wind was less than 5 MPH. Figure 2 shows beam-line motion spectra for different locations along the beam tube and at different wind velocities. These results are similar to results at LLO here.
The beam tube consists of a series of modular segments, with the great majority being 39 m long and weighing about 4000 kg (A+B segments). The segments are supported close to the middle with a fixed support, and each end hangs from a guided support. The beam tube segments are connected at the guided supports to the next segment in line by bellows that have a measured spring constant of 8.2e5 N/m. Figures 3a and b show that, in the 5 – 20 Hz band, the segments act as rigid bodies, with beam-line motion at the middle and at the ¾ point of a segment being virtually identical, highly coherent and with zero phase difference. Figure 3b also shows that, in contrast, the motion of adjacent segments is not very coherent. This is likely better news than if adjacent sections moved coherently.
The calculated beam-line resonance of a beam tube segment, considering only the bellows springs, would be about 3 Hz, while the actual lowest resonance in the figures is at about 8 Hz. This demonstrates that the central support is the stiffest spring. Near the stations, some of the segments are shorter; Figure 3 shows that the lowest beam-line resonant frequency of a 33.5 m segment is about 9.3 Hz instead of 8 Hz. The frequency goes approximately as mass rather than the square root of mass, suggesting, as one might expect, that there is a torsional component about the central support. A modal explanation of the 8 and 14 Hz resonances awaits modeling.
Since the beam tube segments are moving as rigid bodies in the 10 Hz region, the insulation may not damp the motion much at these frequencies. We may get a chance to test this as John is going to remove some insulation.
