Houxiu Xiao

Development of a High-Stability Flat-Top Pulsed Magnetic Field Facility

A high stability flat-top pulsed magnetic field has both high field strength and high field stability simultaneously, which can be used for the high precision scientific experiments. In this paper, we present such a high stability flat-top pulsed magnetic field facility developed in the Wuhan National High Magnetic Field Center. A passive bypass circuit is developed to regulate current up to tens of thousands amperes, as well as a 25 T/200 ms flat-top pulsed magnetic field with a ripple of 250 ppm (0.025%). The main circuit scheme of the facility as well as the preliminary experimental results will be presented in this paper.

Magnet Development Program at the WHMFC

In 2008, the Wuhan National High Magnetic Field Center (WHMFC) was set up at the Huazhong University of Science and Technology, Wuhan, China. The objective of the pulsed magnet program is to produce pulsed magnetic fields in the range of 50 to 80 T. The following pulsed magnets were initially conceived: 1) 50-80 T monolithic magnets energized by one or a few modules of the 1 MJ capacitor bank, 2) a 50 T/100 ms flattop long pulse magnet energized by the 100 MJ/100 MVA pulse generator, 3) a 60 T/200 ms long pulse magnet energized by the 12 MJ capacitor bank, and 4) an 80 T dual coil system energized by two capacitor bank modules. In addition, a 16 T PPMS and a 7 T SQUID-VSM systems were acquired. Currently, work on a 40 T long pulse magnet energized by lead acid battery units and the acquisition of a 400 MHz NMR spectrometer are in progress.

Study on a Highly Stabilized Pulsed Power Supply for High Magnetic Fields

This paper proposes a pulsed power supply scheme to generate highly stabilized high magnetic fields (ripple less than 10 ppm), which are needed for some kinds of scientific experiments (such as the nuclear magnetic resonance experiment). The power supply presented in this paper consists of a large-capacity pulse-forming network (PFN) power supply and a small-capacity power electronics power supply (PEPS). The PFN provides a background field, and the PEPS compensates for the flattop of the background field to get a highly stabilized magnetic field.

Design and test of a flat-top magnetic field system driven by capacitor banks

An innovative method for generating a flat-top pulsed magnetic field by means of capacitor banks is developed at the Wuhan National High Magnetic Field Center (WHMFC). The system consists of two capacitor banks as they are normally used to generate a pulsed field. The two discharge circuits (the magnet circuit and the auxiliary circuit) are coupled by a pulse transformer such that the electromotive force (EMF) induced via the transformer in the magnet circuit containing the magnet coil is opposed to the EMF of the capacitor bank. At a certain point before the current pulse in the coil reaches its peak, the auxiliary circuit is triggered. With optimized parameters for charging voltage and trigger delay, the current in the magnet circuit can be approximately kept constant to obtain a flat-top. A prototype was developed at the WHMFC; the magnet circuit was energized by seven 1 MJ (3.2 mF/25 kV) capacitor modules and the auxiliary circuit by four 1 MJ modules. Fields up to 41 T with 6 ms flat-top have been obtained with a conventional user magnet used at the WHMFC.

Short and Long Pulse High Magnetic Field Facility at the Wuhan National High Magnetic Field Center

The pulsed high magnetic field facility funded by the Chinese National Development and Reformation Committee has been developed at the Wuhan National High Magnetic Field Center (WHMFC). Magnets of short pulse, long pulse and the combination of both with bore sizes from 12 to 34 mm have been developed and are operational for electric transport, magnetization, magneto-optics and electron spin resonance at temperatures in the range from 100 mK to 350 K. The power supplies for these magnets consist of a capacitor bank with 12 modules of 1 MJ/25 kV each and 2 modules of 0.8 MJ/25 kV each, a 100 MVA/100 MJ flywheel pulse generator and a 771 V/180 kAh battery bank. A dual-coil magnet driven by the capacitor banks has successfully generated 86.3 T field with a total pulse duration over 350 ms in a 12-mm bore. A dual-coil long pulse magnet wound from soft copper wire energized by the flywheel generator produces 50 T peak field with a 100 ms flat-top and a total 1.1 s pulse duration in a 22 mm bore. A battery bank driven long pulse magnet composed by the series connection of two nested coils generates 32 T peak field with the pulse duration of 1.5 s in a 21-mm bore.

Fabrication and Test of a 40 T Long-Pulse Magnet Driven by Battery Bank

A 40 T long-pulse magnet has been constructed and tested to 32 T with total pulse width of 1.5 s at the Wuhan National High Magnetic Field Center (WHMFC). The magnet is energized by a battery bank power source consisting of 900 12 V/200 Ah lead-acid battery cells. The magnet consists of a polyhelix inner coil and an outer coil wound with copper foil. We made some improvements by adding axial slots on the helices due to the eddy current electromagnetic force, and each layer of helices is reinforced by Zylon-epoxy composite. The copper foil coil is insulated with polyimide film extending over the sides of the foil. The copper bar electrode is silver brazed on the ends of copper foil and has been proved to be quite feasible. In this paper, the calculated results of the electromagnetic force caused by eddy current will be presented. The magnet manufacture techniques will be described and the magnet test results will be listed and analyzed.

High-Performance Pulsed Magnetic Fields at the Wuhan National High Magnetic Field Center

The Wuhan National High Magnetic Field Center has passed the hnal evaluation organized by the Chinese government and has been open to the external users for experiments since 2014. Pulsed magnets for different held waveforms are in operation. A dual coil magnet driven by the capacitor banks has successfully generated 90.6-T record held with total pulse duration over 350 ms in a 12-mm bore. Fast cooling magnets with bore size of 21 mm provide 65-T peak held every ~40 min. Driven by the pulse generator power supply, the dual coil magnet produces 50-T long pulse held with 100-ms flat-top in 22-mm bore. An innovative method for generating a flat-top pulsed magnetic held by means of capacitor banks has produced helds up to 52 T with 6-ms flat-top with a conventional user magnet. The combination of the lead-acid battery and a monolithic coil magnet produces a free running held waveform with peak held of 35 T at total pulse duration of 1.5 s. By adjusting the voltage across the magnet with a bypass circuit, held at 25 T with stability better than 0.02% for 200-ms flat-top held has been achieved.

Detecting and Positioning the Insulation Failure of Pulsed Magnets

Pulsed magnets are usually damaged because of the insulation failure, and the insulation failure will eventually trigger the ultimate structure failure and ends up with explosion. Hence, the insulation failure investigation is very important for pulsed magnets. The investigation has to begin with detecting and positioning the fault point to know when and where the insulation fault occurs. Detecting and positioning the insulation failure will provide experimental data for revealing the mechanism of the insulation breakdown. On the other hand, detecting and positioning the insulation failure is an important technique to predict the lifetime of a magnet and to avoid the risk of magnet explosions. A detection coil matrix that is able to detect and locate the insulation failure as well as the preliminary experimental results is presented in this paper.

Design and Test of a Long-Pulse Large Current Sensor With a Hall Probe Installed Inside

A high accuracy large current sensor which can directly output the current signal without integral has been designed, fabricated, and tested. It is composed of a coreless helix coil cut by pure copper tube and a hall probe installed inside. It has excellent linear performance due to the absence of iron core. The ability of resisting disturbance is much improved because the hall probe is installed inside. A calibrated prototype sensor has been applied on a 40 T long pulsed magnetic field system to output the current data as feedback signal. A long pulse magnetic field with 25.6 T/200 ms flat-top and ripples less than 0.02% was achieved.

Design of a Current-Controlled Power Supply for High-Stability Flat-Top Pulsed Magnetic Field

A flat-top pulsed magnetic field is of great interest for scientific experiments that need both high strength and high stability in the magnetic field. A battery bank with advantages of large energy storage and relative voltage stability is an ideal power supply for a flat-top pulsed field. However, due to the change of magnet resistance during discharge progress, the battery system is unable to maintain a flat-top magnetic field alone. This paper presents a current-controlled power supply based on batteries for a high-stability flat-top pulsed magnetic field at the Wuhan National High Magnetic Field Center. An active parallel circuit is proposed as an auxiliary power supply integrating the present battery system to be a current-controlled power supply, which can sustain a 35-T/270-ms flat-top pulsed field with the ripple less than 100 ppm. The prototype of the scheme has been developed, and experiments will be carried out in early 2016 on schedule. After introduction of the circuit topology and control strategy, digital simulation results are discussed in this paper.