Wuhan National High Magnetic Field Research Articles

Quench analysis of a no-insulation REBCO magnet based on the ADI method considering the coupling effect of the cryostat

A no-insulation (NI) REBCO superconducting magnet is under development at Wuhan National High Magnetic Field Center, China. The magnet with the liquid helium cryostat system has a compact structure to reduce the space required for operation. During a quench, the fast-changing spatial magnetic field around the NI magnet may induce a strong eddy current in the conductive parts of the cryostat. The eddy current and its associated Lorentz force will generate mechanical stress on the cryostat, especially on the thermal shield (TS). The mechanical strength of the cryostat needs verification in the preliminary design. Furthermore, the degree to which the electromagnetic coupling between the cryostat and NI magnet might impact the quench behaviors of the NI magnet remains uncertain. In this paper, a multi-physics quench model is newly developed for the NI REBCO magnet, and the alternating direction implicit method is employed for the solver of the thermal model to improve computational efficiency. This simulation model can consider the electromagnetic coupling effect between the NI magnet and cryostat by constructing a partial element equivalent circuit. A quench analysis has been performed and we found that: (1) The cryostat can function as a secondary shorted circuit to the NI magnet and slow down the quench speed to a certain extent. (2) During the rather fast inductive quench phase, the cryostat will experience an attraction force towards the quench propagation frontier. (3) A quench propagation from one end of the magnet can cause a significant z-axis unbalanced force on the TS. (4) Cryostat materials with drastically changed electrical conductivity can significantly affect their mechanical responses during a quench. However, the eddy current density and maximum Von Mises stress on the TS are barely affected by the thickness of the TS and the contact resistance of the NI REBCO magnet.

The design of YBCO binary current leads and its electromagnetic-thermal coupling analysis based on PEEC and finite volume method

A superconducting magnet system is developed for the application of high-frequency gyrotrons at Wuhan National High Magnetic Field Center, China. The operating cost for the magnet is dominated by refrigeration power. To reduce the heat load for cryogenic systems, a pair of YBCO binary current leads is designed by consideration of the industrial concentration of type II high-temperature superconducting YBCO material. In this paper, a simulation model is proposed to perform the electromagnetic-thermal coupling analysis of these YBCO binary leads under different operating conditions by combining a partial element equivalent circuit method and finite volume method. The simulation code can improve computational efficiency and enable high-accuracy coupling between the electromagnetic field and heat transfer process. In the steady state, the heat leakage at the 4.2 K cold end of the YBCO binary leads depends mainly on the geometric parameters of the YBCO tape, especially the cross-sectional area of the copper layer in the YBCO tape. The cooling mode and liquid helium level also have a significant impact on the heat leakage level. The optimal outer diameter of the normal copper section is identified, and the optimum value is largely influenced by the effective cooling power imposed on the cold end of the normal copper section. In the transient state, simulations for the charging process and the loss of cooling accident are performed, along with a detailed analysis of the electromagnetic-thermal response features of the leads under these conditions. The results indicate that the YBCO binary current leads possess high thermal stability and an ample time margin for the magnet system to be demagnetized.

Development and Initial Experimental Results of a Terahertz Pulsed Field Gyrotron in the WHMFC

We report the design and initial experimental results of an 800-GHz second harmonic gyrotron prototype, in which a 15-T pulsed magnet is applied. This gyrotron prototype is developed at the Wuhan National High Magnetic Field Center (WHMFC). An in-house software for analyzing and designing terahertz gyrotrons and an experimental research platform are developed. In addition, alignment adjustments, eddy current effects, and radiation tests are discussed. The initial experimental results show that the prototype gyrotron can irradiate ~40 W at fundamental harmonic 383.7-GHz (TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,4</sub> ) waves at 14.14 T and a few milliwatts at second harmonic 0.8-THz (TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8,5</sub> ) waves at 14.86 T.

Failure Analysis of the 100 T Pulsed Magnet at the WHMFC

A triple-coil pulsed magnet aiming at 100 T (Tesla) was designed and tested at the Wuhan National High Magnetic Field Center. The magnet was powered by a 100 MJ pulse generator, a battery bank, and 25 kV capacitor banks in combination. The magnet failed at 83 T, while the whole structure remained intact. The failure process was analyzed by using the recorded waveforms of the voltage and the current, as well as disassembling and dismantling the failed magnet. It shows that the inner coil broke down first, and then the electrical breakdown occurred between the middle and outer coils. The possible reasons were also discussed. The buckling analysis results show that the inner and middle coils would buckle at 71 T and 83 T, respectively, which were consistent with the experimental results. The cross-ply reinforcement method is proposed to enhance the structural stability of high field magnets.

Globally Optimal Algorithm of Superconducting Magnet Design for Gyrotrons

This paper presentsa new hybrid global optimization method to design a 25 T All YBCO solenoid superconducting magnet with a warm-bore diameter of 52 mm for high-frequency gyrotrons in Wuhan National High Magnetic Field Center, China. This method includes 3 main routines: linear programming (LP) code to get the non-rectangular coil design, coil-picking code to get rectangular coil design, and nonlinear programming (NLP) code to get final integer coil design by using rectangular coil design as initial input. The designed magnet includes Inner, Middle and Outer coils which consist of 34, 56 and 68 double pancakes (DP) wound by 4 mm x 0.1 mm bare YBCO tape, respectively. The designed peak-peak field in-homogeneity is 50 ppm within 10 mm Diameter of Spherical Volume (DSV). The designed 5 Gauss line is restricted in a cylinder with the sizes of 10 m in axial direction and 8 m in radial direction, respectively. The maximum hoop stress is limited below 450 MPa. Layer-layer separation happens in the inner coil since non-insulation (NI) winding is adopted. The results show that this methodology is very efficient for High-frequency Gyrotron superconducting magnet design. This methodology also can be used for other solenoid superconducting magnet design, such as Magnetic Resonance Imaging (MRI) superconducting magnet and Nuclear Magnetic Resonance (NMR) superconducting magnet.

Fully redundant auxiliary system for gyrotron-based terahertz sources in long-term operation.

The Wuhan National High Magnetic Field Center is incorporating the flat-top pulsed magnetic field (FTPMF) into pulse gyrotrons. It will be the first chance to make a pulse-magnet gyrotron available for generating a long-pulse radiation of 100ms or above without affecting its high operating frequency and high radiation power. However, unlike continuous wave gyrotrons, pulse gyrotrons in long-term operation have their own challenges, namely, misalignment caused by concussions, much stronger low-frequency electromagnetic interference from the pulse magnet, and inevitable explosion. This article will focus on the difficulties faced by pulse gyrotrons in years of operation, discuss the protection and restoration from failures, and, consequently, propose a fully redundant, explosion-proof, and quickly recoverable auxiliary system for long-term operation of pulse gyrotrons. This system integrates the control unit of traditional pulsed magnets and superconducting magnets so that it can be compatible with any form of gyrotron facilities. Therefore, once the FTPMF or the superconducting magnet is available, the long-pulse radiation will be obtained. Several experimental results, including the most recent explosion, show the reliability of the proposed system.

Experimental Protection of the Spin Coherence of a Molecular Qubit Exceeding a MillisecondSupported by the National Key R&D Program of China (Grant Nos. 2018YFA0306600 and 2016YFB0501603), the Chinese Academy of Sciences (Grant Nos. GJJSTD20170001, QYZDY-SSW-SLH004, and QYZDB-SSW-SLH005), and Anhui Initiative in Quantum Information Technologies (Grant No. AHY050000). X.R. thanks the Youth Innovation Promotion Association of Chinese Academy of Sciences for

Molecular qubits are promising as they can benefit from tailoring and versatile design of chemistry. It is essential to reduce the decoherence of molecular qubits caused by their interactions with the environment. Herein the dynamical decoupling (DD) technique is utilized to combat such decoherence. The coherence time for a transition-metal complex (PPh4)2[Cu(mnt)2] is prolonged from 6.8 μs to 1.4 ms. The ratio of the coherence time and the length of π/2 pulse, defined as the single qubit figure of merit (Q M), reaches 1.4 × 105, which is 40 times greater than what previously reported for this molecule. Our results show that molecular qubits, with milliseconds coherence time, are promising candidates for quantum information processing.

Development of repetitive pulsed high magnetic field with high repetition rate.

A novel system for generating a repetitive pulsed high magnetic field (RPHMF) with a high repetition rate has been developed at the Wuhan National High Magnetic Field Center. This system consists of multiple capacitor bank modules and a monolithic coil magnet. All the capacitor bank modules are connected in parallel. The RPHMF is achieved by discharging the energy from the capacitor banks to the magnet with a time interval between every two capacitor bank modules. The repetition rate of the magnetic field pulses is controlled by adjusting the time interval. The amplitude and the polarity of the pulses are adjusted by changing the charging voltage of the capacitors and the polarity changing switches. Magnetic fields with a frequency of about 60 Hz and a peak value of about 20 T have been achieved, as well as the magnetic field pulses with increasing or declining peak values. With the diversity of the repeated magnetic field waveforms, the measurement of magnetization of the dispersible SmCo5 nanoparticle permanent magnet has been successfully carried out.

A Current-Injection-Based Approach for Reducing Power Loss of the IGBTs in FTPMF System Driven by Battery-Bank

A linear bypass circuit was developed to realize the high stability flat-top pulsed magnetic field (FTMPF) powered by battery-bank at the Wuhan National High Magnetic Field Center (WHMFC) in 2018. It mainly consists of the insulated-gate bipolar transistors (IGBTs) operating in the active region and a power resistor connected in series with the IGBTs. However, the terminal voltage of the IGBTs is high due to the operation in active region, which limits their through-current capability. In this paper, an approach for injecting current to the power resistor is proposed to reduce the terminal voltage of the IGBTs. Compared with the original technology, the through-current capability of the IGBTs will be enhanced greatly and the regulation scope of the FTPMF system will be expanded with the same number of IGBT. The method is discussed in detail and the number of IGBT is reduced from 35 to 8 in achieving an FTPMF of 40 T.

Upgrade of the Pulsed Magnetic Field System With Flat-Top at the WHMFC

The 65 T pulsed magnetic field with 10 ms flat-top has been achieved at the Wuhan National High Magnetic Field Center (WHMFC). This is an upgrade based on the 41 T with 6 ms flat-top system. Both the magnet and the coupling transformer were newly designed and fabricated. The magnet circuit and the auxiliary circuit were driven by ten and two 1 MJ capacitor bank modules, respectively. At the voltage of 19.1 kV for the magnet circuit and 8.5 kV for the auxiliary circuit, peak field of 64 T with 10 ms flat-top and 0.3% flatness was obtained. The primary specific heat measurement of sample CeRhln5 was successfully performed.

Development of Zylon-Kevlar-Zylon Hybrid Fiber Reinforcement Technology for 100 T Pulsed Magnet at the WHMFC

The reinforcement technology combining Zylon/epoxy composite and Kevlar/epoxy composite has been proposed to design the 100 T Pulsed Magnet at the Wuhan National High Magnetic Field Center (WHMFC). The reinforcement layers in the preliminary 100 T pulsed magnet consisted of Zylon/epoxy composite, which is replaced with Zylon-Kevlar-Zylon hybrid fiber in the layers where the von Mises stress in Zylon/epoxy composite exceeds 3.5 GPa. Kevlar fabric has better wet impregnation properties with epoxy due to its rougher surface than Zylon fiber, which can effectively prevent shear movement of the conductor layer and the reinforcement layer. The simulation shows that the maximum von Mises stress in the Zylon-Kevlar-Zylon hybrid fiber is 3.68 GPa. The measured ultimate tensile strength (UTS) is 4.97 GPa via the explosion test with small pulsed magnet, which shows that the new reinforcement technology meets the stress requirements for 100 T.

A Cryogen Recondensing Cooling System for Scientific Experiment in the Pulsed High Magnetic Field

Traditional cryostat for scientific experiment in the pulsed high magnetic field uses liquid helium as the cooling source. To reduce the running cost and increase the operational efficiency, a cryogen recondensing cooling system based on a GM cryocooler has been developed for a 60 T measurement cell at Wuhan National High Magnetic Field Center. A helium cryostat with a recondensing system provides cooling power for sample holder insert. The bottom of the insert with a vacuum space is immersed in 4 K liquid helium bath, so that the helium gas inside the insert condenses to liquid phase at the neck tube on the top of the insert’s tail. The temperature of the sample can reach 1 K and stay stable at 1.2 K for 2 hours by pumping the helium bath in the insert. The gas evaporated from the helium cryostat is recondensed by a GM cryocooler liquefier, and is circulated in a closed-loop system.

A New Open-Loop Synchronization Method Based on Compensation of Phase Deviation for Pulsed Generator Converter

At Wuhan National High Magnetic Field Center, a 100-MVA/100-MJ pulsed generator and two 67.5-MW converter modules have been installed as power supply for pulsed magnets. Long-pulsed magnetic field with 50-T/100-ms flat-top has been produced and 100-T pulse magnetic field is under commissioning by utilizing this pulsed generator converter. In this high-power occasion, the three-phase voltages undergo serious harmonics, large notches, amplitude variation, rapid frequency fluctuation, and phase jump simultaneously. Synchronization method with fast dynamics, excellent harmonics rejection, and smooth time-domain response is expected, which is difficult for the conventional synchronous reference frame phase-locked loop (SRF-PLL). Therefore, this paper presents a new synchronization method based on compensation of phase deviation. In this method, the phase of three-phase voltages, which is achieved by summing filtered phase deviation and phase of a given $d$ - $q$ rotating reference frame, is open-loop computed directly. The main attributes of the method are the fact that its dynamics is independent of the input vector amplitude, better harmonics rejection/dynamics tradeoff, and smooth time-domain response. Comparative experiments are performed between the proposed synchronization method and SRF-PLL.

Construction and Test of Three-Coil Magnet Power Supply System for a High-Pulsed Magnetic Field

A higher magnetic field can provide more opportunities to reveal new phenomena in a scientific research. Aiming to achieve a higher magnetic field, Wuhan National High Magnetic Field Center (WHMFC), Wuhan, China has designed a a power supply system for 100 T magnetic field under existing power supply conditions in April 2015. The 100 T magnet consisting of three coils is energized by a multipower supply system. The outer coil is energized by a pulsed generator-rectifier connected to battery bank in series. The middle coil and the inner coil are energized by two capacitor bank, respectively. To ensure safety and reliable operation, control sequence as well as protection system for the power system is designed and developed. A test system including the prototype of three-coil magnet is established at WHMFC. A series of tests are carried out on hybrid power supply systems. 73.3 T peak field has been achieved as the highest magnetic field in the test. The test results presented in this paper show the hybrid power supply system is feasible and operable.

Design of a Hybrid Power Supply for a 65 T Quasi-Continuous High Magnetic Field With a Dual-Coil Magnet

A quasi-continuous high magnetic field is a special kind of pulsed magnetic field with a flat-top field longer than 100 ms. This paper proposes a hybrid power supply that consists of a capacitor power supply and a generator–rectifier power supply. A dual-coil magnet is powered to generate a 65 T/100 ms quasi-continuous high magnetic field with a ripple less than 500 ppm at the Wuhan National High Magnetic Field Center, Wuhan, China. A protection circuit consists of an R-C branch and a diode array is discussed for the worst short-circuit fault, which occurs between the first turns of inner coil and outer coil. This type of fault will produce oscillation voltage with a high peak value and high frequency on the rectifiers. A self-adaptive PI controller with the coefficients corrected by a back propagation neural network is adopted to reduce the ripple of a flat-top.

Multishot Analysis of a 65-T Pulsed Magnet Considering Different Failure Modes ofCu-Nb Conductor

Recently, a 65-T pulsed magnet encountered fatigue failure after 2124 shots in total at the Wuhan National High Magnetic Field Center. Multishot simulation is thus carried out by ANSYS according to the working history of the magnet. A constitutive model is applied considering different failure modes of a Cu-Nb conductor so that the mechanical performance and the fatigue life of the magnet can be examined and predicted. It is found that there is a permanent increase in the hoop strain of the conductors, which raises the stress of the reinforcement while reduces that of the conductors. The magnet failure is caused by large axial pressure with the fault origin located at the inner boundary of the sixth layer of the conductors. The 583 shots above 60 T have largely promoted the strain increase in the conductor and induced a rapid reduction in the mechanical performance of the magnet. By comparison, the failure mechanism is different below 60 T, which enables the magnet to operate under more reliable conditions. These results indicate that the practical field of a future pulsed magnet had better not exceed 92% of the maximum value as long as the same conductor is used.

Modified Design of Power Supply System for 100 Tesla Pulsed Magnetic Field

High pulsed magnetic field is becoming widely used in physics, biology, and materials. For better performance of scientific experiments, a power supply system for 100 T magnetic field is designed at Wuhan National High Magnetic Field Center. In the preliminary design, the 100 T magnet consists of three coils in a coaxial structure. Because of the multicoil system with strong-coupling structure, the current of the outer coil will drop when the middle coil is fired. And the current drop will cause adverse effects, which increase the burden of power supply and the stress of magnet. Based on the analysis of the mathematical model for power supply circuit, this paper proposes a modified design for power supply system, which is an auxiliary power supply connected in series with primary power supply of the outer coil for the compensation of the current drop. Corresponding parameters, circuit topology, and control strategy of capacitors are also discussed. The simulation results of the modified design of power supply system are presented in this paper.

Design of an Active Ripple Compensator for the 50-T High-Stability Flat-Top Pulsed Magnetic Field

High-stability flat-top pulsed magnetic field can meet severe demands of field intensity and stability in modern scientific experiments. The higher stability of the flat-top magnetic field would be of more benefit to the observation of experimental phenomena (such as specific heat measurement and NMR experiment). In order to reduce the ripple of the flat-top magnetic field, an active ripple compensator is presented in this paper for the 50-T dual-coil magnet system at the Wuhan National High Magnetic Field Center. The active ripple compensator has the advantages of modularization, flexibility, and low energy consumption. The simulation model including the generator–rectifier power supply is established on the MATLAB/Simulink platform. The ripple of a 50-T magnetic field has been reduced from 1500 to 60 ppm, which verifies the feasibility of the scheme.

Research of Active Regulation for High-Stability Flat-Top Pulsed High Magnetic Field

With large energy storage and flexible control capabilities, pulse generator is an ideal power supply for flat-top magnetic fields. However, characteristics of the pulse generator, such as nonlinearity and time variance, are a limitation on improving flat-top stability. Based on the pulse generator power supply for the flat-top magnetic field at Wuhan National High Magnetic Field Center, this paper proposes an active regulator for the flat-top magnetic field. An optimal control strategy combining selective PI control and repetitive control is proposed to restrain the circulating current between the pulse generator and the active regulator. A 50 T/140 ms flat-top magnetic field with the ripple less than 100 ppm is achieved by applying an active regulator. In order to validate the effectiveness of the optimal control strategy for the high stability flat-top magnetic field with an active regulator, experimental data about the triggering angle of the rectifier are utilized in the simulation model; the result indicates that the circulating current between the pulse generator and the active regulator is suppressed to a great extent.

Development and Performance of 65 T Fast-cooling User Magnet With Long Service Life

Since the accomplishment of the pulsed magnetic field facility at the Wuhan National High Magnetic Field Center in 2013, five 65 T monolithic-coil magnets have been developed and used for scientific research. As the workhorse, the 65 T user magnets show excellent cooling performance and long service life. These user magnets have typical bore size of 21 and 23 mm. For electrical transport measurements, the magnets are energized by a 2 or 3-MJ capacitor bank with crowbar circuit for ∼40–70 ms pulse duration. For magnetization measurements, shorter pulse duration of 11 ms for a field waveform with almost symmetrical rising and falling edges can be obtained with a 1.25-MJ capacitor bank that allows high-voltage reversal. The cooling time between two 60 T pulses ranges from 30 to 50 min depending on the magnets and the field waveform. Up to now, three of these magnets failed after 916, 583, and 947 pulses with the magnetic field above 60 T. The other two have delivered 706 and 56 pulses above 60 T and are still in operation.