The AURIGA cryogenics
|AURIGA is an ultracryogenic g.w. detector: in fact its
working thermodynamic temperature is of the order of 0.1K. The reason for
cooling down the detector is twofold:
Actually these two points are related as the sensitivity improves with the ratio Q/T. Another advantage of a cryogenic detector is that a SQUID amplifier can be used: being a superconducting device, the SQUID operates at low temperatures (say, at the liquid helium temperatures). As already explained AURIGA employs a SQUID amplifier as it is the less noise device available in the frequency range of interest.
Here you will find drawings of the transverse and longitudinal cross-section of the AURIGA cryostat; you can also have a look to the photograph taken before the cryostat was closed in 1995 and corresponding to the transverse cross-section.
The cool down of the detector (i.e. the bar and the transducer) proceeds by steps: first the temperature of the liquid nitrogen (approx. 77 K) is reached (the bar is further cooled down to 67 K by pumping on the liquid nitrogen) and then that on the liquid helium (approx. 4 K). This is achieved by filling with liquid nitrogen and afterwards with liquid helium the main helium vessel inside the AURIGA cryostat: once the vessel is filled with the liquid one has to wait for the system to achieve a steady state (obviously the liquid that evaporates must be refilled). The time scale for cooling is 3 weeks from room temperature to liquid nitrogen temperatures and 1 week down to liquid helium temperature. A further temperature decrease is obtained by means of a very special refrigerator, namely a dilution refrigerator.
Cooling the more than 2 tons heavy bar down to sub-Kelvin temperatures is a challenging task. More difficulties are added by the requirement that the thermal gradient along the bar is minimal: thermal shields and links must be carefully designed. Moreover in AURIGA the cryogenics and the suspensions are integrated: this permits a detector overall volume minimization and a better performance.
The AURIGA cryostat is organized in two vacuum chambers: the Outer Vacuum Chamber (OVC) and the Internal Vacuum Chamber (IVC). Between them a liquid vessel is placed: at regime this vessel is periodically refilled with liquid helium thus it is called liquid helium vessel. The OVC separates the laboratory environment (room temperature and atmospheric pressure) and the liquid helium vessel: this chamber is pumped down to 5x10-7mbar and houses two aluminum thermal shields, the outer at 100 K and the inner at 20 K. These shields are cooled by the cold gas evaporated from the liquid helium vessel (which is at 4 K) thanks to thermal exchangers.
The IVC houses the bar and transducer assembly: it is pumped down to 2x10-7mbar thus permitting to bias the capacitive transducer with very high electric fields. The IVC also houses 3 copper thermal shields: the SQUID amplifier is fixed to the middle of these shields.
The 2 aluminum shields, the liquid helium vessel and the 3 copper shields constitute each a full solid angle enclosure: they are inserted one into the other as a onion system.
The dilution refrigerator is vertically inserted into the cryostat: it passes through the OVC shields, the liquid helium vessel and the IVC shields. It ends at the inner shield of the IVC. The bar is thermally linked to the coldest point (named cold finger) of the refrigerator: the heat transfer from the bar to the refrigerator occurs via conduction. The cable that suspend the bar is indeed itself the thermal conductor that permits to remove the heat from the bar. This is just an example of the link between the suspension design and the cryogenics in AURIGA.
The working principle of the dilution refrigerator is based on a special mixture between the two helium isotopes 3He and 4He, with 10% content of 3He . Below 0.8 K the mixture separates into two different phases, one enriched with 3He and one with 4He. The migration of 3He from the former phase to the latter is a phase transition and occurs with a heat absorption: this is the phenomenon used for the refrigeration. The cooling power of the AURIGA dilution refrigerator is 1.3mW at 0.1K.