The AURIGA-RUN2 cryogenics

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.

Figure 1	Cross-section drawing of the AURIGA cryostat in RUN2. The new dewar inserted from the top centrally to the cryostat is shown. Outer thermal shields (the one at 100K and the one at 20K) are also shown: they are the same as in RUN1. The main liquid helium vessel is also shown: the new AURIGA suspensions are supported by this vessel. The Internal Vacuum Chamber (IVC) is the vacuum enclosure that houses the bar. The thermal shields external to the main liquid helium vessel are housed in the Outer Vacuum Chamber (OVC) which is independent from the IVC.

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.

Figure 2	A view of the internal part of the AURIGA cryostat in RUN2. The coloured parts show what have been changed with respect to the previous run RUN1. The light parts show what have been preserved from RUN1. The new dewar is shown in light blue.