Magnetic compensation coils are often used to improve the effectiveness of a magnetically shielded room (MSR), but a distortion of the field is caused by the shielding layer consisting of high-permeability material, which leads to a reduction in compensation capacity. To solve this problem, a new method is presented in this article based on a target-field method, an image method, and particle swarm optimization. By this way, a magnetic compensation system composed of biplanar coils is designed and fabricated to compensate the background field for a wearable magnetoencephalography device inside the MSR, and the coupling effect between the magnetic shielding material and the compensation field is effectively avoided. Experimental data show that these compensation coils produce uniform fields and field gradients with the inhomogeneity of less than 3.8%, consistent with the simulation results. In addition, this system realizes a real-time compensation to the residual field and field gradient in the MSR.

The uniform field and field gradient coils can be utilized for nulling the background field inside a magnetic shielding room (MSR), but the desired field produced by the coils is inevitably distorted due to the coupling effect of the magnetic shielding material. To overcome this problem, an improved method is proposed based on the image method and target field method (TFM). Directed by this method, a series of biplanar coils (BCs) are designed to produce the uniform fields and field gradients for an active compensation in an MSR. Then, their finite element models and practical experiment are established to verify the effectiveness. Compared with the conventional TFM, the proposed method shows significant effects for both uniform field coils and field gradient coils. Uniformities of the B _x and B _z increase by 79.9% and 72.5%, and the dB _x / dz and dB _z / dz improve by 89.6% and 63.2%, respectively. It demonstrates that this method is adopted to design the BCs fixed in MSR and holds great promise for the active compensation of environmental magnetic field in the measurement of magnetoencephalogram (MEG).

A novel method to design highly uniform axial coils inside a closed magnetically shielded cylinder (MSC) is presented. The proposed method has two major advantages over existing methods. First, based on the coupling model of coil and MSC with finite permeability and finite thickness, the image method and target-field method (TFM) are introduced to provide a flexible design method for the coil inside the MSC. Second, a backpropagation neural network (BPNN) model and particle swarm optimization (PSO) algorithm are used to predict and optimize the coil parameters, thus reducing the magnetic field deviation caused by the use of discrete wires to approximate the continuous current density. Simulations indicate that this method reduces the magnetic field deviation within the target region from $2.63 \times 10^{-3}$ to $1.13 \times 10^{-4}$ compared with the coils designed using the TFM without consideration of the coupling effect. The magnetic field deviation is reduced from $1.42 \times 10^{-4}$ to $1.13 \times 10^{-4}$ compared with the coils designed in the ideal magnetic conductor. Moreover, the field deviation is reduced to $2.43 \times 10^{-5}$ after optimization using the BPNN and PSO methods. The experimental results also verify the effectiveness of the design method and its practicability in compensating for the residual field of the MSC. The method proposed in this research has major significance for the establishment of practical magnetoencephalography and magnetocardiography measurement systems.

The MINI-magnetically shielded room (MINI-MSR) offers a near-zero magnetic environment for the high performance of optically pumped atomic magnetometers (OPMs) in magnetoencephalography (MEG) tests. It has the advantages, such as being small-sized, portable, lightweight, offering efficient space utilization, and being cost-effective. However, owing to its small size and narrow internal space, the MINI-MSR can only provide a limited uniform region with larger gradients and irregular magnetic noise distributions. Moreover, it is affected by large external background fluctuations. Once the external interference exceeds the magnetic shielding capacity of the MINI-MSR, the precision of OPMs is compromised. Therefore, an active magnetic compensation system (AMCS) composed of biplanar coils (BCs) is introduced to enlarge the uniform region, maintaining the inhomogeneity errors below 1% to meet the measurement accuracy. However, conventional algorithms for designing BCs struggle with searching for the regularization coefficient ω that determines the coil’s shape. The process lacks ergodicity and efficiency, posing challenges in obtaining maximum volume and uniformity within the target region. In this study, a new hybrid method (NHM) composed of the target field method (TFM), image method (IM), and novel artificial fish swarm algorithm (novel ASFA) is proposed to design BCs for the AMCS. The aim is to suppress background field fluctuations, enhancing the signal-to-noise ratio while addressing coupled errors to reduce larger gradients and magnetic noise levels. Experimental results show that the maximum inhomogeneity error of BCs is less than 1.23%, which is consistent with simulation results. The sensitivity of the AMCS at 0.01 Hz ranges from 180 to 0.16 pT/Hz ^1/2 , realizing the noise suppression ratio of 30.51 dB. Based on the closed-loop AMCS, the signal-to-noise ratio in the alpha rhythm generation task is effectively improved.

Magnetic-shielding technologies play a crucial role in the field of ultra-sensitive physical measurement, medical imaging, quantum sensing, etc. With the increasing demand for the accuracy of magnetic measurement, the performance requirements of magnetic-shielding devices are also higher, such as the extremely weak magnetic field, gradient, and low-frequency noise. However, the conventional method to improve the shielding performance by adding layers of materials is restricted by complex construction and inherent materials noise. This paper provides a comprehensive review about the enhancement of magnetic shielding in three aspects, including low-noise materials, magnetization control, and active compensation. The generation theorem and theoretical calculation of materials magnetic noise is summarized first, focusing on the development of spinel ferrites, amorphous, and nanocrystalline. Next, the principles and applications of two magnetization control methods, degaussing and magnetic shaking, are introduced. In the review of the active magnetic compensation system, the forward and inverse design methods of coil and the calculation method of the coupling effect under the ferromagnetic boundary of magnetic shield are explained in detail, and their applications, especially in magnetocardiography (MCG) and magnetoencephalogram (MEG), are also mainly described. In conclusion, the unresolved challenges of different enhancement methods in materials preparation, optimization of practical implementation, and future applications are proposed, which provide comprehensive and instructive references for corresponding research.

To achieve the nearly zero-field environment, demagnetization is an indispensable step for magnetic shields composed of high-permeability material, which adjusts the magnetization of the material to establish magnetic equilibrium with the environmental field and improve the shielding performance. The ideal demagnetization can make the high-permeability material on the anhysteretic magnetization curve to have a higher permeability than on the initial magnetization curve. However, inappropriate parameters of degaussing field cause the magnetization state to deviate from the anhysteretic magnetization curve. Therefore, this article proposes a new assessment criterion to analyze and evaluate the parameters of degaussing field based on the difference between the final magnetization state after demagnetization and theoretical anhysteretic state of the shielding material. By this way, the magnetization states after demagnetizations with different initial amplitude, frequency, period number and envelope attenuation function are calculated based on the dynamic Jiles–Atherton (J–A) model, and their magnetization curves under these demagnetization conditions are also measured and compared, respectively. The lower frequency, appropriate amplitude, sufficient period number and logarithmic envelope attenuation function can make the magnetization state after demagnetization closer to the ideal value, which is also consistent with the static magnetic-shielding performance of a booth-type magnetically shielded room (MSR) under different demagnetization condition.