The AURIGA-RUN2 suspensions

The tiny bar vibration induced by a passing g.w. is easily masked not only by the thermal noise (thus making necessary to cool down the bar) but also by the mechanical and acoustical noises. The acoustical noise coming from the laboratory environment is suppressed by placing the bar in a vacuum chamber: in AURIGA two vacuum chambers (the OVC and the IVC that houses the bar) provides for this insulation.

A more difficult task is to reduce the mechanical noise: this is due to floor vibrations, caused for instance by any nearby moving vehicle or walking person and by the intrinsic seismic noise. The AURIGA cryostat is placed on the top of a special 200tons heavy concrete platform, separated from the rest of the laboratory. The platform sits on a sand layer which reduces by -20dB the mechanical noise at the floor: on the platform this is measured to be 2x10^-14m/Hz^½ around the AURIGA bar resonant frequency (which occurs at about 900 Hz). Unfortunately this noise figure is still too high for a g.w. experiment as AURIGA: other insulation stages are necessary so to reduce the mechanical noise felt at the bar input down to a negligible level.

Figure 1

Global view of the AURIGA suspension for RUN2: the outer blue shell shown is the liquid helium dewar, inside the cryostat. Not shown are the outer layers (identical to those in RUN1) and the room temperature stage. Also shown is the transducer assembly.

Click on the picture for explanation of the parts

Figure 1 shows the suspension design for AURIGA in RUN2. It is identical to the suspension of the RUN1 from the top (yellow outer shell) down to the liquid Helium vessel, shown in blue in the figure. The inner part of the cryostat has undergone a major change. The necessary mechanical isolation is now given by the 4 columns that hung from the dewar: each column consists of 6 loaded springs, shown in more details in figure 2. The springs are machined from a single piece of a special aluminum alloy, namely Alumold 1-500; the loads are made out of brass.

The 4 columns support, via a couple of compression springs, a mass made out of the same aluminum alloy as the bar (namely Al5056). From this mass a CuBe cable is hanging that supports the bar from its center of mass: see figure 3 for a detailed view. The cable is fixed to the mass by means of bayonet coupling: any screws would have the effect of lowering the mechanical quality factor of the bar, thus lowering the performance of the AURIGA detector. The top end of the cable can be thermally anchored to the cold finger of the refrigerator: the cable is used to extract heat from the bar thus cooling it.

The suspension design has been carried out by means of a commercial Finite Element Methods (FEM) software, which allows the possibility of running a static and a dynamic simulation. The static simulation was used to monitor the stressed parts of the suspension: one requirement in the design was that the maximum applied stress in any piece is below 30% of the material yield stress. The dynamic analysis was used to study the modal behavior of each stage of the suspension: one requirement in the design was the absence of mechanical resonance in a broad frequency range centered on the bar resonance (about 900Hz).

The total expected gain at the bar resonance (ie at about 900 Hz) is -240dB in the vertical direction.

Figure 2	Assembly of the columns: they consist of 6 aluminum springs (shown in yellow) loaded by bronze masses (shown in green). Each spring is machined from a single piece of material. The photo shows one of the realized springs.

Figure 3	Drawing of the bar suspension: it is suspended by a cable (shown in yellow) inserted through a hole in the bar's vertical diameter passing through the center of mass. The top of the cable is fixed by bayonet coupling. The compression springs are shown in blue.