Linear electromagnetic actuators (LEAs), which conduct linear physical movements driven by electromagnetic force, are widely used in various applications. However, LEAs are very sensitive to magnetic fields because of their ferromagnetic materials. The failure of LEA under magnetic field interference is a potential threat to the system reliability and safety. Therefore, it is significant to reduce magnetic field interference on LEAs. In this paper, a novel method to increase the immunity of LEAs against disturbing magnetic field by simply adding two small iron legs is proposed. The iron legs change the magnetic field distribution in LEA, and significantly reduce the magnetic field interference of LEA. Compared with conventional magnetic shielding, the proposed method shows great advantages of higher failure threshold, smaller space occupation and lower material cost. Experimental tests are also performed to verify the theoretical analysis, and the results fit well, which prove the effectiveness of this method. In general, this paper provides a simple and effective method to improve the immunity of LEAs against disturbing magnetic field, and the idea can also be extended to the antimagnetic design of other similar devices.

Ultra-low homogeneous magnetic fields are essential for many areas of scientific research. To enable the realization of ultra-low homogeneous magnetic field, magnetically shielded room (MSR) is employed to shield the geomagnetic and environmental magnetic noises. A group of homogeneous magnetic field coil (HMFC) enclosed by the MSR is then used to generate ultra-low homogeneous magnetic field in a certain zone. Since the presence of MSR will significantly influence the field distribution of the HMFC, proper cooperation between MSR and HMFC is crucial for reducing the volume of the entire facility. In this paper, the theoretical magnetic field analysis model of HMFC based on MSR was constructed by the image method. By adopting a new optimization design method, the space-efficient configurations of HMFC based on MSR were proposed, and the validity of the analytical model was numerically verified. The optimized system parameters were normalized to scale the given results for any desired ultra-low homogeneous magnetic field. We show that to generate the same ultra-low homogeneous magnetic field, the system volume of the proposed structure is only 1%–83.3% of that of the traditional Merritt coil configurations. In general, this work paves the way for the space-efficient apparatus design for ultra-low homogeneous magnetic field generation.

Homogeneous magnetic field is widely used in various applications, and Helmholtz and Merritt coils are the most commonly used homogeneous field generators. However, only the field homogeneity at the center point is concerned in their design, which leads to restriction for most applications, where a certain volume of homogeneous field is required. This indicates that traditional coil configurations can be further optimized. Based upon a comprehensive definition of homogeneous field, improved three-s and four-coil configurations are proposed in this article. Practical formulae with normalized coil parameters are given to provide a convenient tool for designers. These formulae establish relationships between coil parameters and the required homogeneous field. The accuracy of the formulae is verified by the finite-element analysis. Comparisons with Merritt coils show that, when getting the same homogeneous field, the improved coils can reduce both power loss and conductor mass by up to 24.8%. Through a comparison among main coil configurations, the improved four-coil configuration is recommended for homogeneous magnetic field generation. For high-intensity magnetic field applications, the winding cross-section effect is analyzed. At last, experiments are performed to verify the theoretical analysis, and the reason why the improved coils have better performance than traditional ones is revealed.

Electromagnetic (EM) relay is a commonly used electronic control component, which is extremely sensitive to magnetic field. With the increasing magnetic field around tokamak devices, the reliable operation of EM relay is greatly threatened. Therefore, it is necessary to study the failure mechanism of EM relay under disturbing magnetic field. In this paper, a fully coupled analysis model of EM relay, including electric circuit, magnetic field and mechanical motion, is established. Taking plunger-type DC EM relay as an example, the influence of disturbing magnetic field of EM relay is studied. The relationship between the disturbing magnetic field and the action time, holding force, and some other parameters of the relay is given. Moreover, the failure magnetic field threshold is obtained. An efficient method to suppress the impact of disturbing magnetic field on the relay is proposed. The work presented in this paper has a great significance to improve the reliability of EM relay in the disturbing magnetic field and ensure the stability of tokamak devices.

For disruptions without mitigation, a large amount of thermal energy and poloidal magnetic energy will be dissipated inside the vacuum vessel (VV). A slow current quench may be accompanied by a large halo current, while a fast current quench often causes large eddy current, which will result in electromagnetic force. At the same time, fast current quench will induce strong toroidal electric field, which will result in a large fraction of runaway current and the hitting of runaway beam on first wall. The disruption mitigation is essential for large scale tokamak. The existing methods to mitigate disruptions, such as massive gas injection and resonant magnetic perturbations, are aimed at increasing the runaway generation threshold or the lose rate of runaway electrons. It may not work for ITER with Ip = 15 MA operation. The root of runaway generation is the large toroidal electric field induced by fast current quench and the large avalanche factor with high plasma current. The reduction of toroidal electric field is favor for the runaway suppression. The magnetic energy transfer (MET) based on electromagnetic coupling for disruption mitigation has been proposed on J-TEXT. It has the advantage of transferring the magnetic energy to outside of vessel by the electromagnetic coupling. It accelerates the current quench (CQ) rate and reduces the toroidal electric field at the same time. The runaway current has been suppressed by the MET system on J-TEXT. The experimental results show that the MET can reduce the energy dissipated in the VV by 20 % through transferring of energy to outside of VV. The MET can increase the CQ rate about 50.7 % and decrease the loop voltage about 35.3 %. The MET provides a new idea to transfer the magnetic energy and to suppress runaway current for disruption mitigation in future devices.

The HUST field-reversed configuration (HFRC) is a field reversed plasma research device based on colliding and merging under design. To increase the parameters of the initial plasma formed in the theta-pinch, the theta-pinch is designed with very high parameters, namely a chamber diameter of 0.6 m, a bias magnetic field of -0.16 T, a preionization frequency of 150 kHz, and a main magnetic field of 0.6 T. To control the plasma formation and ejection, the discharge regulating accuracy for the theta-pinch is required within several microseconds. Thus, a high voltage, high current pulsed power supply with high controllability is required to satisfy the physical design. This article mainly presents the challenges and solutions in the design of the power supply system. First, 18 theta-pinch coils are used for the field generation to reduce the current on each coil, which results in design parameters of 70 kV/86 kA for the power supply. Then, a new topology with two power supply modules connected in series is adopted to feed the coil, which reduces the system voltage level and increases the operational reliability. Besides, hydrogen thyratron is adopted as the pulsed switch, and a corresponding trigger circuit based on insulated gate bipolar transistor (IGBT) is designed to ensure the precise control of the sequential discharge. Experimental tests are also performed on the thyratron and its trigger circuit, and the breakdown delay time is proven to be less than 1 mu s. In conclusion, the designed power supply system can satisfy the requirements of high operation parameters and high controllability.

The electromagnetic interference caused by the strong stray magnetic field generated by a tokamak device and its coil power supply will greatly threaten the reliable operation of electrical and electronic devices nearby, and a magnetic field immunity test is the most commonly used method to qualify the sensitive equipment and ensure the reliability of the system. Thus, building a high-intensity magnetic field immunity test platform is essential for the stable operation of a large tokamak device in the future. This article presents the preliminary design of a large-scale high-intensity magnetic field immunity test platform, including dc static test platform, dc transient test platform, and power frequency test platform. Referring to the actual magnetic field environment around the International Thermonuclear Experimental Reactor (ITER) tokamak device and its magnet power supply, the design parameters of the subplatforms are determined. Then the scheme design of the subplatforms and the integration design of the whole platform are briefly introduced. As a part of the large-scale superconductor rest facility (LSTF) in Chinese Academy of Science, Institute of Plasma Physics (ASIPP), the design takes full advantages of the other devices planned in the facility. This test platform will be the largest in the world and enable the test of various equipment under different magnetic field environments, and more importantly, play an important role in the reliable operation of China Fusion Engineering Test Reactor (CFETR) and future large tokamak devices.

During disruptions runaway electrons (REs) often drift from high field side to low field side in J-TEXT. It may cause severe damage to the plasma facing components when REs strike them with high energies. In order to mitigate the damage, a novel method called magnetic energy transfer (MET) based on electromagnetic coupling is proposed. A set of extra coils with a high coupling coefficient with plasma are installed on the high field side of the device, and a toroidal current can be induced in the coils during disruptions which can transfer the plasma poloidal magnetic energy out of vacuum vessel. Flowing in the same direction as the runaway current, the induced current can attract the runaway current to high field side, control the displacement of the RE beams and prolong runaway current plateau. Experiments are carried out on J-TEXT to verify the mitigation method, and the influence of different induced current on REs horizontal displacement is studied by changing the electrical parameters of energy absorbing unit. The experiment results show that the increase rate of RE beams' horizontal displacement can be significantly slowed. The runaway current plateau can be prolonged by 4-5 ms and the control effect becomes better as the induced current in the MET coils increases. Moreover, MET also has a good effect on displacement control of plasma during disruption when no RE beams are produced.

Due to high rated current and special Fault Suppression Capacity (FSC) criterion, 12 thyristors connected in parallel are applied in one bridge arm of ITER Poloidal Field (PF) converter. A large amount of parallel thyristors will give rise to significant difficulties in the current sharing design due to the electromagnetic coupling between different parts of the bridge arm. This paper presents a new method to analyze the influence of the electromagnetic coupling on current sharing performance and guide the bridge arm structure design. At first, the bridge arm is decomposed to many segments and the stray inductances are extracted. Then, an equivalent circuit of the decomposed model is built and a mathematical model is established to analyze the circuit. It will result in the current waveforms of parallel thyristors and thus provide the current sharing information. Based on the mathematical model, the optimization design of PF converter is conducted along with conventional methods. Experimental tests are also performed, and the results fit well with the analyzed ones. The current sharing coefficient of the 12-parallel thyristors is over 0.8, which is far beyond the value required by ITER Organization. In a word, the method presented in this paper provides an effective tool for the current sharing structure design for high-current fusion power supplies.

The low-frequency magnetic field immunity test usually requires a homogenous magnetic field inside a cubic space to demonstrate the interference field. However, the 3-dB homogenous magnetic field region generated by the commonly used the Helmholtz coil is not a cubic space, which indicates a suboptimal design. This paper proposes an improved two-coil configuration for low-frequency magnetic field immunity tests and other similar applications, and discusses two key issues related to its field inhomogeneity, namely winding cross section and installation misalignment. Three simple formulas are concluded to establish a relationship between the parameters of the improved coil and the required homogenous magnetic field. These formulas enable the parametric and fast design of the improved coil. A comparison with the Helmholtz coil shows that the improved coil can reduce over 10% conductor mass cost when the same homogenous magnetic field is required. In addition, the effects of winding cross section and installation misalignments on the field homogeneity are analyzed, and valuable suggestions are provided for the design of a practical coil system. At last, an experimental test and a finite-element analysis are performed. The results are compared with the theoretical analysis, and the validity of the main conclusions in this paper is proved.

Planar transformers (PTs) are becoming increasingly popular in high-power density, high-frequency SMPS in recent years due to their unique advantages including low profile structures and excellent thermal properties. This study focuses on detailed investigation of the effects of coil current distribution within each winding layer on leakage inductance and AC resistance of a PT, and an analytical derivation based on variational method is given. Then the optimal current distribution is proposed and verified through 3D finite element analysis simulation and physical experiments. The results show that the leakage inductance and AC resistance can be reduced further by optimising the current distribution. Accordingly, a practical implementation method is proposed to control the current distribution within a winding layer by adjusting the widths of conductors.

Due to the space limitation and safety concerns, the non-same-phase anti-parallel connection structure is applied in the International Thermonuclear Experimental Reactor (ITER) Poloidal Field (PF) converter unit. Consequently, the electromagnetic interference caused by the strong stray magnetic field has been one of the main concerns in its design. And this paper mainly presents the stray magnetic field analysis of ITER PF converter unit. At first, an isolated phase bus (IPB) is designed for the shielding of the alternating magnetic field generated by the ac busbars to avoid overheating on ferromagnetic materials in the fire wall. The stray magnetic field on the fire wall is analyzed to verify the shielding effectiveness of the IPB. Then, the stray magnetic field generated by the converter bridges and dc reactors which is very hard to be shielded is analyzed separately. The results show that the stray magnetic field exceeds the limit value and will be a potential threat for the instrument and control (I&C) system. All the analysis presented in this paper will provide useful inputs for the electromagnetic compatibility design and test of the I&C system.