MiniGRAIL“Extreme Make-over”

Giorgio FrossatiKamerlingh Onnes LaboratoryLeiden 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.

6 new transducers

Improve sensitivity new transducers:

• Larger surface area

• Smaller mass

6 new transducers

-Super-conducting shielding-Magnetic (Glasmet) shielding

-Low frequency vibrations damping

Decrease the external EM perturbations:

6 new transducers

6 new transformers

super-conducting and magnetic

shielding

Improve transducer read out:

6 new transducers

6 new transformersNew double

SQUIDs

super-conducting and Mu-metal shielding

Improve SQUID sensitivity:

2-stage Twente

2-stage Trento

6 new transducers

6 new transformers new dilution

refrigerator

Super-conducting and mu-metal shielding

new dilution refrigerator unit:

• large surface mixing chamber and heat exchangers

• expected cooling power of at least 50 microW at 30 mK