The AURIGA 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/Hz½ around the AURIGA bar resonant frequency (which occurs at 920 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.
The fundamental idea for the suspension employed in AURIGA is to take advantage of the properties of a damped harmonic oscillator: for frequencies higher than the the oscillator's resonance, the displacement transfer function decreases with a slope of -6dB/octave. The basic suspension stage is indeed a spring-mass system, whose resonant frequency is chosen to be much smaller than the bar resonance (i.e. the frequency of interest). By the way in a real spring-mass system one must take into account its internal modes which add structure to the transfer function and complicate the design of the suspensions for AURIGA. This is further complicated by the requirement of preserving the bar high mechanical quality factor and of the detector overall volume being not too large. The solution adopted in AURIGA is to take advantage of the thermal shields that needs to be employed in the cryostat in order to cool down the bar. For what concerns the suspension, the thermal shields act as the masses in the spring-mass systems; the cables that support the shields in cascade constitute the spring. The last suspension stage (i.e. the last spring-mass system) is provided by the cable that suspend the bar and the bar itself: the bar is suspended horizontally by a belly copper cable around the bar middle section. Except for the little cables for the transducer and the calibrator, the belly cable is the only connection of the bar with the rest of the world! |
Fig. 1 is a schematic of the suspension design in AURIGA: suspensions and cryogenics are fully integrated. More details and suspension performances are listed in the following table (they refer to room temperature measurements in the frequency range 800Hz÷1000Hz). The total mechanical gain of the overall suspension is -240dB at bar resonance.
| Mechanical filter | mass [tons] | suspension | mean vertical gain |
| Al5056 bar | 2.3 | Cu-OF belly cable | -100 dB |
| Cu inner shield | 0.5 | 4 rods of Ti64 | -45 dB |
| Cu middle shield | 0.5 | 4 rods of Ti64 | -40 dB |
| Cu outer shield | 1.6 | 4 rods of Ti64 | -30 dB |
| Liquid He vessel | 2.0 | 4 rods of Ti64 | -25 dB |
