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Acoustic properties of a prototype for a hollow spherical gravitational antenna. M. Bassan, S. Giannì , Y. Minenkov ^, R. Simonetti Dip. Di Fisica, Università di Tor Vergata e INFN, sezione Roma Tor Vergata With crucial help from L. Quintieri * , A. Rocchi,.

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## Acoustic properties of a prototype for a hollow spherical gravitational antenna

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**Acoustic properties of a prototype for a hollow spherical**gravitational antenna M. Bassan, S. Giannì, Y. Minenkov^, R. Simonetti Dip. Di Fisica, Università di Tor Vergatae INFN, sezione Roma Tor Vergata With crucial help from L. Quintieri*, A. Rocchi, (^) Laboratori Nazionali del Gran Sasso dell’INFN (*) Laboratori Nazionali di Frascati dell’INFN ILIAS - LondonOct 27th 2006**Summary**• Advantages • Comparison bulk- hollow Shperical Antenna • General features • Suspensions • Experimental results Bulk Sphere • Realization of a cavity • Bonding methods • Fabrication of a hollow sphere • Experimental results Hollow Sphere • Comparison of results: bulk-hollow • Conclusions Discussion**Who cares about these tests ?**• People interested in making high Q resonators of VERY large size will have to deal with the issue of bonding. • We have addressed here the problem of preserving mode shapes and Q factors in brazed metallic resonators.**it is omnidirectional**• it can determine the direction of incoming g.w. • it can determine the polarization state of the g.w. • it has a larger cross section of a bar at the same frequency • it has a wider bandwidth Advantages • it has the largest mass • its cross section for the 1st spheroidal quadrupole mode is larger • it is difficult to construct and cool • its bandwidth is still too narrow wrt interferometers Bulk Sphere • its cross section for 1st spheroidal quadrupole mode is somewhat smaller than the bulk sphere • it is an easier object to fabricate and cool • using both 1st and 2nd mode we can recover both bandwidth and overall cross section Hollow Sphere Why a Spherical Antenna ?**Cross section for the 1st and 2nd modes**bulk shell Why a hollow sphere ? • Larger surface/ volume => easier cooling • The cross section is smaller wrt bulk, but it can make up at the • n=2 mode • Choice of thickness can be used for centering two bandwidths**Why a hollow sphere ?**• Larger surface/ volume => easier cooling • The cross section is smaller wrt bulk, but we can make up at the • n=2 mode • Choice of thickness can be used for centering two bandwidths**Problems with a Hollow Sphere**(A) How do we produce it ? • casting • fabricating from plates • welding two half-spheres (B) How do we suspend it ? • Can’t suspend it from center of mass • Would surface suspension affect Qs ? (C) Effects of bonding on modes and Qs ? • Will elastic continuity be retained across the welding interface ? • Will the bonding affect Q ? Need to practice and investigate on a small size sample**Bulk Sphere in CuAl6%(kindly provided by Minigrail)**Our Benchmark: General Features**Effect of suspension on the Q of the quadrupolar modes of**the bulk sphere Testing suspension : surface vs center of mass**Measuring Apparatus**T=300 K ÷ 4.2 K T=300 K Excitation : PZT or mag. hammer ReadOut : PZT or accelerometer**Bulk Sphere in CuAl6%**Experimental results Tests: T=4.2 K, T=77.4 K, T=300 K Frequencies Quality factors Q**“Hollowing” the sphere**Machining 22 mm**(B) Bonding methods:**Electron Beam Welding • Tested by Minigrail people. • Really bad results: • poor beam penetration • cracks • Uneven welding Diffusion • Test on a hollow cylinder CuAl6%:m=4.261Kg, L=0.228m, Φ=56mm,thick. 22mm, fosp=8312Hz, τ<1s • Results: • OK at T=300K • Degraded after thermal shock at T=77K Furnace Brazing • Satisfactory results: • OK a T=300K • OK thermal shock a T=77K**Hollow sphere in CuAl6%**General features**Hollow sphere in CuAl6%**T=300 K T=300 K ÷ 4.2 K Same experimental set-up as for the bulk sphere**(C ) Hollow sphere in CuAl6%Experimental results**Q vs Temperature T= 4.2 K ÷ 300 K Tests: T=4.2K, T=77.4 K, T=300 K Frequencies Quality Factors Q**Mode location and splitting**BULK SPHERE T=300 K Frequencies HOLLOW SPHERE**f (Hz)**13313 Good Agreement with theory ! fpiena=13313 Hz fcava=7537 Hz 7537 5025 Validating Lobo’s Calculations Det (Ap)=0**Q Bulk 300 K**Q Hollow 300 K Q Bulk 77.4 K Q Hollow 77.4 K Q Bulk 4.2 K Q Hollow 4.2 K Comparing Hollow vs Bulk Sphere**CONCLUSIONS**• Good agreement with elastic theory: potential gravitational antenna • Good results from furnace brazing: homogeneity is mantained and Q degradation is small • For the future: additional tests on different brazing techniques and procedures. • Investigation of alternate bonding methods: diffusion welding and electron beam welding.

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