The flat-top pulsed magnetic field (FTPMF), which has the dual advantages of high strength and high stability, is one of the most significant tools for the modern physics research, such as the magnetization effect, magnetoresistance effect and electrical transport. The Wuhan National High Magnetic Field Center (WHMFC) in China has built the experimental stations of the nuclear magnetic resonance (NMR) and the current-voltage (I-V) based on FTPMF, which require the strength of FTPMF to be higher than 40 T, the flat-top stability to be the order of 0.01%, and the flat top duration to be longer than 5 ms. Up to now, only the Institute of Solid State Physics (ISSP) in Japan can achieve an FTPMF of this level based on a DC pulsed generator. However, it is very expensive and prolonged to construct a large DC pulsed generator. In this paper, according to the requirements of the NMR measurements and I-V measurements, a high-stability FTPMF scheme was proposed, which combines the coupled double capacitor bank circuits and the linear compensation of the magnetic field. The coupled double capacitor bank circuits can generate the FTPMF with the stability of about 1%, and the linear compensation circuit was used to precisely adjust the magnetic field to achieve the high stability.
Firstly, the circuit topology including two capacitor circuits coupled by an air-core transformer, a main magnet, a compensating magnet embedded into the main magnet and the linear regulating circuit of the compensating magnet was expounded. Then, the discharge process of the double capacitors was discussed based on the circuit state equations. An optimized method of the charging voltage and discharge sequence of the two capacitors was proposed for generating the background magnetic field. After that, a field-compensation magnet of 1 T was designed. The compensation magnet consists of three windings, there are two decoupling windings in addition to the compensating windings. The mutual inductance between the main magnet and the compensation magnet is reduced from 100 μH to 10 μH by using this design. Moreover, a linear power supply based on the current control characteristics of the IGBT in the active region was proposed. The IGBT was connected in series with the compensation magnet. An IGBT driver with current and voltage feedback was designed to control the loop current through the gate voltage of the IGBT in the active region. Next, a corresponding controller combined with feedback and feedforward was designed for the high-precision compensation of the background magnetic field. The parameters of the controller were discussed in detail and the stability of the system was analyzed. The phase degree and amplitude margin of the system are 38° and 50 dB respectively.
Finally, a system of FTPMF was constructed. The optimized method of the charging voltage and discharge sequence of the two capacitors was verified. The average relative error between experimental waveform and simulation waveform is about 3%. The dual-capacitor coupled discharge system can realize a background magnetic field with the strength of 40 T, flat-top duration of 10 ms and stability of 1%, which lays a foundation for the high precision linear regulation. A FTPMF with the strength of 45.2 T/ 8 ms / 0.02% was achieved by the linear compensation, which can meet the requirements of relevant solid-state NMR experiments and so on. The method of prolonging the duration of flat-top was also pointed out by analysis.