The cryogenics in the AURIGA RUN2
In the second run RUN2 of the AURIGA detector the operating temperature of the bar is 1.5K, as the dilution refrigerator employed in the first run is not installed. In fact this unit, that allowed to cool down the bar to 0.1K, during RUN1 had to be refilled with the liquid Helium every 1-2 days and originated much mechanical noise due to the bubbling of the cryogenic liquid.
The drawback is that the thermal noise is higher (it scales linearly with the temperature). A third run of the AURIGA detector (RUN3) is already planned: by that time we will solve the problem of the noise introduced by the dilution refrigerator 1 K pot. Consequently, the temperature will go down to 0.1K and the thermal noise will be decreased. The effect on the detector sensitivity to gravitational gain is shown in this picture.
In RUN2 a new 100 liter dewar is inserted centrally to the detector, from the top (see fig.1): this is a well characterized device that assures good thermal control and stability with a time maintenance of about 1 week. The design of this dewar allows to use it also in RUN3 where the dewar will hold the cold part of the dilution refrigerator, ie the dilution unit.
The bottom part of the dewar is in thermal contact with the top end of the cable that suspends the bar thus allowing the cooling of the bar itself (see fig.2). The thermal contact is achieved by soft OFHC copper 'spaghetti', heat treated at 500°C. See the photo album for a picture of the dewar and the 'spaghetti' thermal links.
Compare with the cryogenics in RUN1