# Discussion Forum

## Shielding from magnetic field of the earth

Hello,

I am completely new to comsol. I've created a geometry of cocentric cylinders (with some openings) in solidworks and have imported this geometry to comsol.

I'd like to simulate how well these cylinders shield from the earth's magnetic field, though I'm unsure how to generate a constant and uniform magnetic field of the earth or assign the material properties.

If you can point me to any relevant resources it would also be appreciated.

5 Replies Last Post Sep 18, 2020, 4:58 PM EDT

Posted: 4 months ago

First, you might want to read this: https://en.wikipedia.org/wiki/Mu-metal

You can create a domain filled with a uniform magnetic field in the Comsol magnetostatics application (mfnc) very easily. E.g., in 3D, create a simple rectangular box, set two opposite facing walls to have to two different magnetic potentials and set the other 4 walls to magnetic insulation. Then the computed magnetic field lines will go straight from one mag potential wall to the other. (Just like a parallel plate capacitor, but with magnetic instead of electric fields.)

Posted: 4 months ago

Hi Robert,

Thanks for your guidance, it has been helpful. I have been able to set up a uniform magnetic field on a box surrounding my cylindrical shield.

Of course I have some new questions:

1. I've set up the field by applying different "magnetic scalar potential" to two opposite facing walls and "magnetic insulation" to the other 4 walls. I can only do this if I choose my rectangular box to be a solid, but not when I choose it to be a surface. Why is this?
2. Regarding my cylindrical shield as mumetal: I have selected the shield material to be Nickel and given the relative permeabilty (mu_r) 1,000,000. Would this be enough to simulate the shield as being made of mumetal?

Alvaro

Posted: 4 months ago
1. Your box (aka, the "computational volume" or "computational space") must be a solid (3D) because you are modeling the magnetic fields inside it, using finite elements. This is fundamental to the way the numerical method works. In contrast, if you only modeled the walls, you would be modeling a 2D finite universe of 6 connected rectangular surfaces. (And believe me, that isn't what you want.)
2. In regard to specific material properties, whether of mumetal or other high permeability materials, I recommend that you do internet searches for them and see for yourself what the manufacturers/retailers of these materials say/claim about them. Good luck.

Posted: 4 months ago

Keep in mind that mumetal and the like get saturated easily. So you may want to model that nonlinear with an appropriate BH curve. You may need to inquire with the manufacturer to get that. Also a practical limit to the screening efficiency is residual magnetization in the material resulting from mechanical strain. This is very difficult to take into account. For optimum results the complete screen assembly is heat treated as a whole and utmost care is taken not to deform it when mounting it.

Cheers Edgar

-------------------
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com

Posted: 4 months ago

Thank you Robert and Edgar for your input. I've gotten the datasheet, including BH curve, from the manufacturer. I'll be getting some help on this from a couple of colleagues. If more questions arise later I may ask for your help again.

Thanks!

Alvaro