  # Plotting Spatial Derivatives of the Magnetic Field

March 5, 2014

Computing the spatial derivative of the magnetic field or magnetic flux density is useful in areas such as radiology, magnetophoresis, particle accelerators, and geophysics. One of the most important use cases is the design of magnetic resonance imaging (MRI) machines, where it is necessary to analyze not only the field strength but also the spatial variation of the field. Today’s blog post demonstrates how to compute and plot the gradients of the magnetic field in electromagnetic simulations in the COMSOL Multiphysics® software.

Editor’s note: This blog post was updated on 1/21/2020 to reflect new functionality and information.

### Background and Objective

When you are computing magnetic fields induced by currents, computing electric fields induced by time-varying magnetic fields, or solving 3D electromagnetic wave problems, COMSOL Multiphysics uses so-called curl (vector or Nédélec edge) elements in all three cases. The curl element is also used in 2D and 2D axisymmetric magnetic field simulations that involve in-plane current distributions. Within the AC/DC Module, the curl element is typically used to compute the magnetic vector potential, A. However, the higher-order spatial derivatives are not available for the curl element.

Check out the blog post What Is the Curl Element (and Why Is It Used)? for an elaborate introduction to the curl element.

The magnetic flux density B, magnetic vector potential A, and magnetic field H are related through the magnetic permeability μ, as given by the following:

\textbf{B} = \nabla \times \textbf{A}\\
\textbf{B} = \mu\textbf{H}

The equations show that the magnetic flux density and the magnetic field are functions of the first-order spatial derivative of the magnetic vector potential. Since the second-order spatial derivative is not defined on curl elements, the gradients of B and H cannot be derived from the curl element directly.

The technique demonstrated here shows how each component of the magnetic field H = [Hx, Hy, Hz] can be mapped to an auxiliary field by adding an extra equation that has three unknown variables (one scalar field for each component of H). These new variables use the second-order Lagrange element. Since both first- and second-order spatial derivatives are defined on this kind of element, it now becomes possible to obtain the spatial gradients of the B and H fields.

Note that when solving for 2D or 2D-axisymmetric magnetic field problems involving out-of-plane currents, or static magnetic field problems without any current flow within the model, the Lagrange elements are used to solve the governing equations, which makes the second-order spatial derivative available. The method shown in this blog post applies only to cases where the curl elements are used to compute the fields.

### Step-by-Step Tutorial Using a Helmholtz Coil

Let’s look at an example where we can demonstrate the technique introduced above. We will show how you can compute and plot the spatial derivatives of the magnetic field produced by a Helmholtz coil. A detailed description of this model, along with step-by-step instructions to simulate the coil, can be found in the Application Gallery.

A Helmholtz coil is a pair of parallel circular coils separated by a distance equal to one coil radius where the current through both the coils flows in the same direction. Some known uses of this kind of configuration are canceling Earth’s magnetic field and generating controlled magnetic fields for experiments.

This model uses the Magnetic Fields physics interface and the Coil feature to model the two coils. Each coil has 10 turns and a 0.25-mA current circulating through it. The magnetic field norm and the magnetic field direction in the Helmholtz coil.

#### Plotting the Gradients of the Magnetic Field

We will start with the solved example from the Application Gallery and see how we can determine the derivatives of the magnetic field.

The first step is to map the magnetic field on Lagrange elements. For that, we add the Coefficient Form PDE interface with three dependent variables (one per component), which uses the Lagrange element by default. Here, we also set the appropriate units for the dependent variables. Settings of the Coefficient Form PDE interface.

The second step is to correctly set up the coefficients of the PDE, in order to get the appropriate expression for mapping the field components. Configuration of the Coefficient Form PDE coefficients.

We set all coefficients other than “a” and “f” to zero, thereby creating an equation of the following form:

a\textbf{u} = \textbf{f}

Where a = 1, u is the auxiliary vector field, and f represents the components of the magnetic field.

As a consequence, when solving for this Coefficient Form PDE, the three components of the magnetic field are directly mapped onto the three scalar fields u1, u2, and u3, as follows:

u1 = mf.Hx
u2 = mf.Hy
u3 = mf.Hz

Concerning the study setup, we first compute only the magnetic fields problem in Study 1, and in the second study, we solve for the variables u1, u2, and u3. In order to take the values from the electromagnetic solution, we set the feature Values of variables not solved for using the study settings shown below. Configuration of Study 2, with user-defined setting for Values of variables not solved for.

It should be noted that the components of the magnetic field do not map perfectly onto the Lagrange elements, causing a small discrepancy between the two (even at the mesh nodes). It is a mapping, not an identity. Therefore, it is a good practice to check abs(H-u) before evaluating spatial derivatives.

In order to compute the gradients of the magnetic field, we append to the variable name the spatial direction in which we want to compute the partial derivative. These suffixes for the spatial derivatives are available for all degrees of freedom, and come directly from the shape functions. For example, u1x would compute the gradient of the x-component of the magnetic field with respect to the x direction. Plot of the gradient (with respect to the x direction) of the x-component of the magnetic field. The y-component of the magnetic field and its gradient (with respect to the y direction) along the centerline of the Helmholtz coil.

### Next Steps

Download the Magnetic Field of a Helmholtz Coil model by clicking the button below, which will take you to the Application Gallery. To download the MPH-file, you need to be logged into a COMSOL Access account and have a valid software license.

#### Categories ##### Tuan-Anh Le
July 2, 2016

But when I checked it, I see u1 and mf.x is different. Plz expain it. Thank you ##### Tuan-Anh Le
July 2, 2016

But when I checked it, I see u1 and mf.Hx is different. Plz expain it. Thank you ##### Rahmetullah Cagil
October 2, 2017

I applied a similar procedure for magnetic flux density and just like Tuan-Anh, I see that mf.Bx and u1 are different, they are almost identical in shape but u1 is scaled down by three orders of magnitude. What could be the cause for this error? ##### Tijani Aziz Feki
October 10, 2017

Hi all
I did all steps about mf.Bx but the results from second study is different from what we can obtain from first study, what is the reason and how should I fix this problem?
thanks. ##### Caty Fairclough
October 13, 2017

Hi Tuan-Anh, Rahmetullah, and Tijani

Online Support Center: https://www.comsol.com/support
Email: support@comsol.com ##### kai zhang
November 9, 2017

We alse can select the “dode” interface and set “f=u1-mf.Hx” and so on. It seemed this way will be more accurate for “u1” in the result for the transient study, such as “ecore transformer.mph” in the library. ##### Simon Houis
August 7, 2018

Hello,

Thank you very much for this interesting post. I managed to compute the gradient of a stationary study (1 single current applied). Now I would like to do the same but for a frequency sweep. I computed a model over 10 frequencies and I ‘d like now to compute spatial gradients for each frequency. The idea is vizualize on a single plot the magn gradient over x for all frequencies. Nonetheless when I applied the method you described, I am only able to vizualise results for the last frequency and not all of them.

Could you tell me how to proceed to compute gradient for all frequencies?

Regards. ##### Caty Fairclough
August 7, 2018

Hi Simon,

Thanks for your interest in this blog post!

For your questions, I would suggest contacting our Support team.

Online Support Center: https://www.comsol.com/support
Email: support@comsol.com

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