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Mass Spectrometry Mechanical Design

"Inside-out" Helmholtz coil

In the inside-out configuration, the iron cores are positioned within each coil, occupying the central region of each coil. The coils are still arranged in parallel, with their centers aligned along a common axis. The iron cores extend along the axis of the coils.

This configuration can provide certain advantages, such as a more compact setup and potentially better magnetic field uniformity within each coil. Placing the iron cores inside each coil helps to enhance the magnetic field strength within each coil individually.

However, it's important to note that the magnetic field produced by each coil will interact with the neighboring coil, and the overall magnetic field distribution in the space between the coils may differ from the traditional Helmholtz coil configuration. The specific behavior will depend on the dimensions, positioning, and magnetic properties of the iron cores and coils.

If you are considering using the inside-out configuration, it is recommended to carefully analyze and model the magnetic field distribution to ensure that it meets your specific requirements and objectives.

Types of iron:

In the context of enhancing the magnetic field in a Helmholtz coil, you can use various types of iron or iron-based materials. The choice of material depends on factors such as the desired magnetic properties, availability, and cost considerations. Here are a few commonly used types of iron or iron-based materials:

Soft Iron: Soft iron is a common choice for enhancing magnetic fields. It has good magnetic permeability, which means it can easily magnetize and demagnetize in response to an external magnetic field. Soft iron is often used as a core material in electromagnets and transformer cores.

Iron Alloys: Various iron alloys can be used to enhance magnetic fields. One example is permalloy, which is an iron-nickel alloy with high magnetic permeability. Permalloy is commonly used in applications requiring high sensitivity to magnetic fields, such as in magnetic sensors and magnetic shielding.

Ferrites: Ferrites are ceramic materials composed of iron oxide and other metallic oxides. They have high magnetic permeability at high frequencies and are commonly used in applications such as inductors, transformers, and electromagnetic interference (EMI) suppression.

When selecting the type of iron or iron-based material, it's important to consider factors such as the desired magnetic properties (e.g., permeability, saturation, coercivity), operating conditions (e.g., temperature, frequency), and the specific requirements of your application. Consulting with experts or conducting further research on the magnetic properties of different iron materials will help you choose the most suitable one for your specific needs.

NOTE: we will use the soft iron in our project.


Characteristics of soft iron:

Soft iron is a specific type of iron with certain characteristics that make it suitable for various applications, particularly those requiring high magnetic permeability. Here are some key characteristics of soft iron:

High magnetic permeability:  soft iron exhibits a high magnetic permeability, which means it can be easily magnetized and demagnetized by an external magnetic field. This property makes it ideal for applications where high magnetic field strength is required, such as in magnetic cores for transformers and electromagnets.

Low coercivity: Coercivity refers to the resistance of a material to changes in its magnetization. Soft iron has low coercivity, which allows it to be easily magnetized and demagnetized without requiring significant energy input. This characteristic makes soft iron suitable for applications that involve frequent magnetization and demagnetization cycles.

High saturation magnetization: Saturation magnetization refers to the maximum magnetic moment that a material can achieve. Soft iron has relatively high saturation magnetization, meaning it can achieve a strong magnetic field when magnetized. This property is beneficial in applications requiring high magnetic field strengths, such as in magnetic shielding.

Low residual magnetism: Residual magnetism, also known as remanence, refers to the magnetic field that remains in a material after the external magnetic field is removed. Soft iron has low residual magnetism, allowing it to quickly return to a non-magnetic state once the external magnetic field is removed. This characteristic is advantageous in applications where precise control of magnetization is necessary.

Low electrical resistivity:  soft iron typically has low electrical resistivity, making it suitable for applications that involve the flow of electrical currents. This property enables efficient magnetic induction and reduces power losses in magnetic circuits.

It's important to note that the properties of soft iron can vary depending on factors such as impurities, processing techniques, and alloying elements. Therefore, the specific characteristics of soft iron may differ slightly based on the specific grade or formulation used.


Expression of the magnetic field:

The expression of the magnetic field in a “inside-out” Helmholtz coil is:  

B= μᵣ B /    B0 = (μ0 * I * N * R^2) / (2 * (R^2 + (x - d/2) ^2) ^ (3/2))


B is the magnetic field at the point of interest

μᵣ is the relative permeability of the ferromagnetic material (in this case, iron).

B0 is the magnetic field without ferromagnetic.


When working with a Helmholtz coil and incorporating iron inside the coil, it is generally recommended to isolate the iron from other materials, such as stainless steel, to prevent any potential electrical or magnetic interactions between them. The use of an insulating material, such as plastic or non-magnetic materials, between the iron and stainless steel can help minimize such interactions.

The reason for isolating the iron is to prevent the formation of unwanted electrical currents or magnetic coupling between the iron and the stainless steel. When different materials with different electrical conductivities or magnetic properties are in close proximity, there can be induced currents or magnetic fields that could affect the desired behavior of the Helmholtz coil.

Therefore, to ensure the accuracy and consistency of your magnetic field setup, it is generally best practice to use an insulating material between the iron and stainless-steel elbow. This will help maintain the integrity of the magnetic field produced by the coil and prevent any unintended interactions or disturbances.


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Version Oct 2022


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