Halbach Array of magnets

The Halbach Array is a magnetic arrangement structure. Before understanding this structure, let's first take a look at the magnetic field line distribution of some common permanent magnets.

From this diagram, it is easy to find that the direction and arrangement of the magnet will directly affect the distribution of magnetic lines, that is, it will affect the form of magnetic field distribution around the magnet.

Part I: Concept of Halbach Array

Halbach Array (Halbach Array, Halbach permanent magnet) is a kind of magnet structure, in 1979 the American scholar Klaus Halbach to do electron acceleration experiments discovered this special permanent magnet structure and gradually improve, and finally formed the so-called "Halbach" magnet, It is an approximately ideal structure in engineering, it uses a special arrangement of magnet units to enhance the field strength per unit direction, the goal is to use the smallest number of magnets to produce the field strength per unit direction. The so-called "Halbach magnet", which is an approximate ideal structure for engineering, uses a special arrangement of magnet units to enhance the field strength per unit direction, with the goal of generating the strongest magnetic field with the smallest number of magnets.

The array is composed entirely of rare-earth permanent magnets, and by arranging the magnets in different magnetising directions in a regular pattern, it is possible to obtain a relatively ideal unilateral magnetic field by converging the magnetic lines on one side of the magnet and weakening them on the other. This is of great significance in engineering, and the Halbach?array is widely used in industrial fields such as nuclear magnetic resonance, magnetic levitation, and permanent magnet special motors because of its excellent magnetic field distribution characteristics.

On the left is a single magnet, with all north poles facing upwards. From the color, it can be seen that the strength of the magnetic field is located at the bottom and top of the magnet.

On the right is a Halbach?array, with a high magnetic field at the top of the magnet and a relatively weak field at the bottom.

(Under the same volume, the strong side surface magnetic field intensity of the Halbach?array magnet group is about √?2 times (i.e. 1.4 times) that of a traditional single magnet, especially when the thickness of the magnetization direction of the magnet is 4-16mm.)

The most common example of a Halbach array may be a flexible refrigerator sticker. These thin, soft magnets are usually printed and attached to refrigerators or the back of cars. Although their magnetism is weak compared to neodymium iron boron (only 2% -3% strength), their low price and practicality make them widely used.

PartII: Forms and Applications of the Halbach Array

Linear Arrays

Linear arrays are the most basic form of Halbach arrays. These arrays can be considered as a combination of radial and tangential arrays.?

Linear Halbach arrays are currently used mainly in linear motors. The levitation principle of magnetic levitation train is that the interaction between the moving magnet and the magnetic field generated by the current it induces in the conductor produces the levitation force, accompanied by magnetic resistance, and improving the buoyancy to resistance ratio is the key to improving the performance of the levitation system, which requires light weight of the on-board magnets, a strong magnetic field, a uniform magnetic field, and a high degree of reliability. The Halbach array is horizontally mounted in the centre of the vehicle body, which interacts with the windings in the centre of the track to generate propulsive force, maximising the magnetic field with less magnet dosage, while the magnetic field on the other side is less, so that passengers can avoid being exposed to a strong magnetic field.

Circular array

A circular Halbach array can be seen as a circular shape formed by combining the head and tail of a linear Halbach array.

In permanent magnet motors, permanent magnet motors using Halbach array structures have a more sinusoidal distribution of air gap magnetic field compared to traditional permanent magnet motors. When the amount of permanent magnet material used is the same, Halbach permanent magnet motors have a higher air gap magnetic density and lower iron loss. In addition, Halbach ring arrays are widely used in permanent magnet bearings, magnetic refrigeration equipment, and magnetic resonance equipment.

Part III: Manufacturing and production methods of Halbach array

Method 1: Based on the topology of the array, use glue to bond the pre-magnetized magnet segments together. Due to the strong mutual repulsion between each magnet segment, a mold should be used for clamping during adhesion. This method has low manufacturing efficiency, but is easier to implement and is more suitable for use in laboratory research stages.

Method 2: Firstly, a complete magnet is manufactured by filling or pressing the mold, and then magnetized in a special fixture. The array structure processed by this method is similar to the figure below. This method has high processing efficiency and is relatively easy to achieve mass production. But it requires specialized design of magnetization fixtures and formulation of magnetization processes.

Method 3: Use a specific shaped winding array to achieve Halbach type magnetic field distribution, as shown in the following figure.