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Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

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Page 1: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Improving a magnetic shield: what works and what does not

26 March 2013

Kiril Marinov

1

Page 2: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Cylindrical shell in an external homogeneous field

A ferromagnetic cylinder in an external homogeneous field B0=0.1T

Bin

max<<<B0

Is there anything else other than using a thicker shielding box?

Page 3: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Suggested ideas

Adding 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.

Page 4: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Mu-metal

Two BH curves obtained from different sources, similar but not identical.

Data has been read from the plot and then smoothed to produce the red curve in the LHS plot.

Page 5: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Mu-metal vs. 1010: permeability

Page 6: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

7 cm steel and 1.8 cm mu-metal vs 7 cm of steel

7 cm steel and 1.8 cm of mu-metal

7 cm steel only

Improvement is visible but is the mu-metal layer really working?

Page 7: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

7 cm steel and 1.8 cm mu-metal vs 8.8 cm of steel

7 cm steel and 1.8 cm of mu-metal

8.8 cm steel

All-steel, 8.8 cm-thick shield preforms better

Page 8: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Permeability distribution

7 cm steel and 1.8 cm of mu-metal

The 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.

Page 9: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Boundary conditions

If μ2/μ1>100 and B1~1.5T is B2>150T?

An interface between two magnetic materials: H|| must be continuous across the interface

The mu-metal has to saturate. This results in μ2<μ1 and the boundary conditions can now be satisfied.

μ1, B1||

μ2, B2||

20

||2

10

||1||

BBH ||1

1

2||2 BB

||111

22||2 B

B

BB SAT

SAT

Note, that the steel can be far from saturation.

Mu-metal and steel should only be combined with care.

)( 22

2

SAT

SATair B

BB

0.7T

100-200

mumetal

steel

Shielding factor is low

Page 10: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Adding steel where the flux density is higher

5 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 alone

9-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.

Page 11: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Adding steel where the flux density is higher

5 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 alone

If 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.

Page 12: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Co-axial cylinders with air gaps

5 cm-thick “can”, 1 cm air gap, 4cm steel

Solid 9 cm-thick “can” acting alone

The 1 cm gap results in lowering the field in the shielded region (by 13%) without increasing the weight of the shield.

Page 13: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Co-axial cylinders with air gaps

5 cm-thick “can”, 2 cm air gap, 4cm steel

Solid 9 cm-thick “can” acting alone

The 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.

Page 14: Improving a magnetic shield: what works and what does not 26 March 2013 Kiril Marinov 1

Summary

Three 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 stage

Works. Could be implemented, if needed.

If you can recommend a good Physics article on zero-gauss chambers please, e-mail me. Thanks.