Improving a magnetic shield: what works and what does not

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Improving a magnetic shield: what works and what does not. 26 March 2013 Kiril Marinov. Cylindrical shell in an external homogeneous field. A ferromagnetic cylinder in an external homogeneous field B 0 =0.1T. Is there anything else other than using a thicker shielding box?. B in max

Text of Improving a magnetic shield: what works and what does not

  • Improving a magnetic shield: what works and what does not26 March 2013Kiril Marinov*

  • Cylindrical shell in an external homogeneous fieldA ferromagnetic cylinder in an external homogeneous field B0=0.1TBin max
  • Suggested ideasAdding a second layer of mu-metal besides the steel?Adding steel only where the flux density is higher?Using co-axial cylinders with gaps (zero gauss chambers)? Keep size within reason.

  • Mu-metalTwo BH curves obtained from different sources, similar but not identical.

  • Mu-metal vs. 1010: permeability

  • 7 cm steel and 1.8 cm mu-metal vs 7 cm of steel7 cm steel and 1.8 cm of mu-metal7 cm steel onlyImprovement is visible but is the mu-metal layer really working?

  • 7 cm steel and 1.8 cm mu-metal vs 8.8 cm of steel7 cm steel and 1.8 cm of mu-metal8.8 cm steelAll-steel, 8.8 cm-thick shield preforms better

  • Permeability distribution7 cm steel and 1.8 cm of mu-metalThe mu-metal layer is fully saturated. Bringing mu-metal in contact with or close to strongly magnetized steel results in mu-metal saturation.The permeability of the mu-metal layer is lower than that of the steel layer. This results is poor shielding Introducing gaps between the two materials does not eliminate the problem.

  • Boundary conditionsIf 2/1>100 and B1~1.5T is B2>150T? An interface between two magnetic materials: H|| must be continuous across the interfaceThe mu-metal has to saturate. This results in 2
  • Adding steel where the flux density is higher5 cm-thick can and a second, 5cm layer, 1cm away, covering half of the surface area of the can. Mirror symmetry w.r.t. both X and Y axes, 5 cm-thick can acting alone9-fold reduction of Bmax ; 25% lighter than a 10 cm can. If we get the shield thickness wrong we can still fix this by adding steel at the appropriate places. No need for a good contact between the two layers.

  • Adding steel where the flux density is higher5 cm-thick can and a second, 5cm layer, 1cm away, covering half of the surface area of the can. Mirror symmetry w.r.t. both X and Y axes, 10 cm-thick can acting aloneIf we get the shield thickness wrong we can still fix this by adding steel at the appropriate place(s). No need to worry about good contact.

  • Co-axial cylinders with air gaps5 cm-thick can, 1 cm air gap, 4cm steel Solid 9 cm-thick can acting aloneThe 1 cm gap results in lowering the field in the shielded region (by 13%) without increasing the weight of the shield.

  • Co-axial cylinders with air gaps5 cm-thick can, 2 cm air gap, 4cm steel Solid 9 cm-thick can acting aloneThe 2 cm gap results in lowering the field in the shielded region (by 26 %) without increasing the weight. The gap results in the outer layer carrying higher flux density thus allowing higher permeability and lower flux density in the inner layer.

  • SummaryThree different strategies for improving shield performance have been considered:Adding a second layer of mu-metal to the steel?Adding steel only where the flux density is higher?Using co-axial cylinders with gaps (quasi-zero-gauss chambers)?Does not work at flux density levels typical for the MICE shielding problem. Works. Allows corrections to be made at a later stageWorks. Could be implemented, if needed.If you can recommend a good Physics article on zero-gauss chambers please, e-mail me. Thanks.