MiniGRAIL “Extreme Make-over” Giorgio Frossati Kamerlingh Onnes Laboratory Leiden University In collaboration with ROG group AURIGA-group

The spherical gravitational wave antenna MiniGRAIL is being prepared for the next cool-down, using the experience acquired with the last cool-down of 2005. The problems found were: 1- Lack of sensitivity of the transducers when cooled to 60mK. The thermally excited resonance were barely above the thermal noise. The best spectral strain sensitivity was ~8x10-21 /√Hz , still more than two orders of magnitude lower than the quantum limit of 4x10-23/√Hz 2- Instability of the SQUIDs due to RF noise and too high an intrinsic SQUID noise 3- Relatively low cooling power of the dilution refrigerator limiting the minimum sphere temperature to 60 mK.

Improve sensitivity  new transducers: Larger surface area Smaller mass To improve the sensitivity of the transducers we have re-designed them so as to achieve a higher capacitance by increasing the area but also to facilitate the polishing of the surfaces of both oscillating mass and electrode. This was obtained by using a central fixing bolt instead of 8 bolts on the perimeter. Figure1 shows the oscillating mass and the removable central post to which the electrode is glued. A 10 μm plastic foil is placed on the mass and the electrode is placed freely on it and glued to the central post with low contraction epoxy glue. The capacitance of the 12 cm diameter transducer is typically 5 nF, a factor 5 better than the old one. Several transducers were made and tested at room temperature on the sphere. The 6 best were chosen for mounting on the sphere after tuning. 6 new transducers

Decrease the external EM perturbations: -Super-conducting shielding -Magnetic (Glasmet) shielding -Low frequency vibrations damping To decrease the external perturbations on the SQUID the 600 mK and 60 mK shields as well as the corresponding flanges were lead-tin plated to make concentric superconducting shields. Each shield and flange was covered with a layer of glasmet foil that should strongly reduce the trapped magnetic field seen by the transducers. Silodamp foam will be placed under the concrete support of the sphere to decrease the low frequency vibrations. Six new transformer boxes were made, with smaller diameter that the previous ones so that they all fit easily on the last damping copper mass. A new SQUID multi-channel electronics was bought from Star-Cryoelectronics together with 6 SQUID array amplifiers that will be placed after the input DC SQUID. Experiments are being done to find out which of the available DC SQUIDS has the smallest noise. 6 new transducers

super-conducting and magnetic shielding Improve transducer read out: super-conducting and magnetic shielding 6 new transformers To decrease the external perturbations on the SQUID the 600 mK and 60 mK shields as well as the corresponding flanges were lead-tin plated to make concentric superconducting shields. Each shield and flange was covered with a layer of glasmet foil that should strongly reduce the trapped magnetic field seen by the transducers. Silodamp foam will be placed under the concrete support of the sphere to decrease the low frequency vibrations. Six new transformer boxes were made, with smaller diameter that the previous ones so that they all fit easily on the last damping copper mass. A new SQUID multi-channel electronics was bought from Star-Cryoelectronics together with 6 SQUID array amplifiers that will be placed after the input DC SQUID. Experiments are being done to find out which of the available DC SQUIDS has the smallest noise. 6 new transducers

super-conducting and Mu-metal shielding Improve SQUID sensitivity: super-conducting and Mu-metal shielding 2-stage Twente 6 new transformers New double SQUIDs To decrease the external perturbations on the SQUID the 600 mK and 60 mK shields as well as the corresponding flanges were lead-tin plated to make concentric superconducting shields. Each shield and flange was covered with a layer of glasmet foil that should strongly reduce the trapped magnetic field seen by the transducers. Silodamp foam will be placed under the concrete support of the sphere to decrease the low frequency vibrations. Six new transformer boxes were made, with smaller diameter that the previous ones so that they all fit easily on the last damping copper mass. A new SQUID multi-channel electronics was bought from Star-Cryoelectronics together with 6 SQUID array amplifiers that will be placed after the input DC SQUID. Experiments are being done to find out which of the available DC SQUIDS has the smallest noise. 2-stage Trento 6 new transducers

Super-conducting and mu-metal shielding new dilution refrigerator new dilution refrigerator unit: Super-conducting and mu-metal shielding 6 new transformers new dilution refrigerator large surface mixing chamber and heat exchangers expected cooling power of at least 50 microW at 30 mK A new dilution refrigerator unit with large surface mixing chamber and heat exchangers was made and installed. We expect a cooling power of at least 50 microW at 30 mK that should allow a lower sphere temperature to be obtained. In November we will do a quick cool-down of the system without the sphere, but with complete transducers (with different SQUIDs) mounted on the last mass so as to test 3 complete measuring chains, the effect of the shielding and the performance of the dilution refrigerator. If all is well we will then mount the sphere with the best SQUIDs and 6 transducers. Figure 2 shows the present experimental set-up. 6 new transducers