central area, the heart of the interferometer, is a huge optical table placed
under ultra high vacuum. Each optical element is suspended to a seismic isolation
system contained in a vacuum tower. The towers are linked by vacuum
tubes. The tubes linking the two towers of the Fabry-Perot cavities are 3 km
long and 1.2m diameter.
Because the presence of a residual gas would perturb the measurement, the path
of the light beam propagating between mirrors has to be evacuated down to the
extremely low pressure of 10-10 mbar thus making the Virgo vacuum chamber the
largest ultra-high vacuum volume in Europe (6800 m3). To attain this very low
value, special metallurgical processes such as hydrogen desorption at 400o C
have been developed for the fabrication of the vacuum parts. In addition, to
eliminate the water vapor, , all vacuum parts are initially heated at 150 degrees
for several days at the beginning of any vacuum cycle. Despite its thermal isolation,
each 3 km vacuum tube requires a power close to 1MW during this operation!
The vacuum tubes are located inside tunnels which must be kept very stable over
long periods. The tunnels are made of 20m long pre-fabricated concrete elements
supported on piles reaching more stable underground layers. About 1000 concrete
piles of 20 to 50 meters depth have been built for VIRGO.
high vacuum technology