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Magnetic shield for SPIRAL2

H long. Sensitivity: 0.1-0.2 nOhm / mG Goal: H residual <20 mG (Rbcs~2.5 nOhm @4K and 88MHz ) Typical values for Earth’s magnetic fields H long = 430 mG H trans = 200 mG Formula (single-layer) S trans = 1+µ.e/2R; e=thickness, R=shield radius S long proportionnal but < to S trans

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Magnetic shield for SPIRAL2

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  1. Hlong • Sensitivity: 0.1-0.2 nOhm/mG • Goal: Hresidual<20 mG (Rbcs~2.5 nOhm @4K and 88MHz) • Typical values for Earth’s magnetic fields • Hlong= 430 mG • Htrans= 200 mG • Formula(single-layer) • Strans= 1+µ.e/2R; e=thickness, R=shield radius • Slongproportionnal but < to Strans • Easier to shield horizontal cavities like elliptical rather than vertical ones Magnetic shield for SPIRAL2 Htrans

  2. Magnetic shield for 4K application  Cryoperm • Guarantied values from suppliers • µ=12000 (SEKELS) • µ=16250 (AMUNEAL) 4K

  3. Cool down • 1.5 mm thick Cryoperm, single layer • In contact with He vessel: cool down by conduction + copper braids • Bad thermalization: magnetic shield temperature still > 150K when Tcavity is < 9K

  4. Cool down • New cool down with copper wool between the magnetic shield and the He vessel on the bottom of the shield only  better thermalization • Bottom part @50K but top and middle parts are still > 150K

  5. New design • Dual-layer inCryoperm (1mm) • Pre-cooling: SS tubing between the two layers (welded on the inner shield) • Outer shield screwed on copper spacers • Expected Hresidual divided by 2 as compared to single-layer option • Main concern: cavity could stays for hours (how many?) between 150K and 50K while the shield temperature is getting down to 4K LHe shield inlet LHe cavity inlet LHe shield outlet

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