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Using high frequency Helmholtz coils

03 Feb 2016  | KC Yang

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High frequency Helmholtz coils are usually employed to generate uniform but time varying high frequency magnetic fields for a number of applications such as magnetic field susceptibility, calibration, and scientific experiment. A high frequency Helmholtz coil driver is needed to generate the required magnetic field. Because the magnetic field density is proportional to electrical current, to generate high magnetic field, high current is needed. However, at high frequency the coils impedance is also became high impedance.

For a given driver voltage amplitude, the coil current is inversely proportional to the coil impedance. Therefore the two opposing factors that affect magnetic field are current and frequency. Achieving high frequency magnetic field is very difficult. This article discussed three techniques to produce high magnetic field for high frequency Helmholtz coils.

High Frequency Helmholtz Coils Basic
Helmholtz coils, named after the German physicist Hermann von Helmholtz, is consisted of two identical electromagnetic coils place in parallel and aligned their centres in the same axis like a mirror image as shown in figure 1. When electrical current pass through the high frequency Helmholtz coils in the same direction, it creates a highly uniform magnetic field in a 3-dimension volume of space inside the coils. These Helmholtz coils are often used for cancel background (earth's) magnetic field, measurements and calibration, and magnetic field for susceptibility testing of electronic equipment.

Figure 1: One-axis high frequency Helmholtz coils is consisted a pair of coils with radius R and separated by a distance equal to R.

Helmholtz Coils Design and Construction
High frequency Helmholtz coils are constructed by two coils. Because the two magnetic coils are designed to be identical, uniform magnetic field is achieved when the coil radius is equal to the separation distance. The two coils are connected in series such that identical current feeding both of them creates two identical magnetic fields. The two added fields achieved uniform magnetic field in a cylindrical volume of space in the centre between the two parallel coils.

This cylindrical-shaped volume space uniform field is approximately equal to 25% of coil radius (R) and a length equal to 50% of the spacing between the two coils. High frequency Helmholtz coils are available in 1, 2, or 3 axes. Multiple axis magnetic coils generate magnetic fields in any direction in the three dimensions space inside the Helmholtz pair. The most common high frequency Helmholtz coils are circular. Square Helmholtz coils are also commonly available.

Helmholtz Coils Magnetic Field Calculation
Each Helmholtz coil is constructed by loops of electrical (copper) wires. When electrical current is passing through it, magnetic field is generated. The magnetic field density is proportional to current. The Helmholtz coils magnetic field equation is given below.

From Equation 1, smaller radius coil generates higher magnetic field density. Also the more number of turns in each coil the stronger the magnetic field.

High Frequency Helmholtz Coils Model
Helmholtz magnetic field is generated either using AC or DC current. Most Helmholtz coils applications are static (constant) magnetic field using DC current. Some applications such scientific experiments required non-static magnetic fields at high frequency (kHz to MHz). This article is mainly discussing high-frequency Helmholtz coils.

Figure 2: Circuit model of two Helmholtz coils connected in series.

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