100 kA/10kV thyristor stack design for the quench protection system in CRAFT

Thyristors have longer lifetimes,higher reliability,and very high voltage and current ratings and they require less maintenance than other high-power semiconductor devices.As a result,they are particularly suitable for quench protection systems(QPSs),which protect the superconducting magnets in large fusion devices from damage.In this paper,we propose a design for a 100 k A/10 k V thyristor stack supported by both theoretical and simulation-based analyses as well as experimental verification.Due to the ultrahigh electrical performance requirements imposed on the QPS by the Comprehensive Research Facility for Fusion Technology(CRAFT),three main issues must be considered:the voltage-balancing problem caused by multiple thyristors in a series structure,the increased junction temperature problem caused by extremely high currents,and the reverse recovery phenomenon that arises from the thyristor’s physical structure.Hence,a series of detailed theoretical analyses,simulations,and experiments,including a thyristor junction temperature prediction method and reverse recovery process modeling,were carried out to optimize the design.Finally,the reliability and stability of the thyristor stack were verified by a series of prototype experiments.The results confirmed the correctness and accuracy of the proposed thyristor stack design method and also indicated that the proposed thyristor stack can meet the application conditions of a 100 k A QPS in the CRAFT project.

A commutation analytical model for quench protection of the CFETR central solenoid model coil

A central solenoid model coil will be set up to develop and verify the technique for the full-size central solenoid coil of the China Fusion Engineering Test Reactor.In case of quench and failures of superconducting coils,the quench protection(QP)system,which employs fuse-based commutation technology,is designed.This paper presents an analytical model to investigate the commutation process in the QP circuit.The model consists of the QP circuit equations,the breaker arc model,the fuse pre-arc model,and the fuse arc model.The model is employed in the whole commutation process including current transfer from breaker branch to the fuse branch model,then from fuse branch to the discharge resistor branch,and current decrease to zero in the discharge resistor.The experiment result verified the effectiveness of the presented model.The model might be helpful for design of the fuse and optimization of the commutation circuit.

A Study on Main Breaker for Quench Protection of HT-7U Toroidal Superconducting Tokamak

This paper proposes a quench protection project of HT-7U toroidal superconducting tokamak through a forced commutation analysis of DC circuit breaker (DCCB) paralleling fuse. Based on the requirement of quench protection, main parameters are selected. Experimental results demonstrate the validity of this proposed project.

Neutron irradiation influence on high-power thyristor device under fusion environment

Because of their economy and applicability,high-power thyristor devices are widely used in the power supply systems for large fusion devices.When high-dose neutrons produced by deuterium–tritium(D–T)fusion reactions are irradiated on a thyristor device for a long time,the electrical characteristics of the device change,which may eventually cause irreversible damage.In this study,with the thyristor switch of the commutation circuit in the quench protection system(QPS)of a fusion device as the study object,the relationship between the internal physical structure and external electrical parameters of the irradiated thyristor is established.Subsequently,a series of targeted thyristor physical simulations and neutron irradiation experiments are conducted to verify the accuracy of the theoretical analysis.In addition,the effect of irradiated thyristor electrical characteristic changes on the entire QPS is studied by accurate simulation,providing valuable guidelines for the maintenance and renovation of the QPS.

Development of 5 T NbTi Superconducting Magnet with 160 mm Warm Bore for Magnetic Separation

A wide-bore 5 T NbTi superconducting magnet, for magnetic separator, with an operational current of 106 A is designed and fabricated. This magnet with a Ф60 mm roomtemperature bore is installed in a vacuum cryostat and immersed in liquid helium. A two-stage 4 K Gifford-McMahon （GM） cryocooler is used to maintain the cooling shield at 70 K and the condenser at 4 K in order to achieve the zero vaporization loss of liquid helium. The cooling power of the GM cryocooler is 1.5 W. In this paper, the design, heat leakage, stress analysis, quench protection characteristics and preliminary test results are presented.

Study on a Highly Stabilized Power Supply for Hybrid-Magnet Superconducting Outsert

The superconducting outsert of the 40 T hybrid-magnet in High Magnetic Field Laboratory （HMFL） of Chinese Academy of Sciences （CAS） requires a highly stabilized power supply. In this paper, two kinds of power supply design are briefly presented and both advantages and disadvantages are analyzed. In order to overcome the drawbacks of switching power supply, a series regulated active filter is adopted and a new design is proposed which ensures cooperative relationship between the feedback control loops of the switching converter and the series regulated active filter. Besides, unlike the traditional switching power supply, which can generate positive voltage only, this new design can also generate negative voltage which is needed in the quench protection for the superconducting magnet. In order to demonstrate the effectiveness of the methodology, a low-power prototype has been accomplished. The simulation and experiment results show that the power supply achieves high precision under the combined action of two feedback control loops. The peak-to-peak amplitude of the output ripple voltage of the prototype is 0.063%, while the peak-to-peak amplitude of the output ripple current is 120 ppm.

Superconducting multipole wiggler with large magnetic gap for HEPS-TF

A 16-pole superconducting multipole wiggler with a large gap of 68 mm was designed and fabricated to serve as a multipole wiggler for HEPS-TF.The wiggler consists of 16 pairs of NbTi superconducting coils with a period length of 170 mm,and its maximum peak field is 2.6 Tesla.In magnet design,magnet poles were optimized.Furthermore,the Lorentz force on the coils and electromagnetic force between the upper and lower halves were computed and analyzed along with the stored energy and inductance at different currents.To enhance the critical current of the magnet coil,all the pole coils selected for the magnet exhibited excellent performance,and appropriate prestress derived from the coil force analysis was applied to the pole coils during magnet assembly.The entire magnet structure was immersed in 4.2-K liquid helium in the cryostat cooled solely by four two-stage cryocoolers,and the performance test of the superconducting wiggler was appropriately completed.Based on the measured results,the first and second field integrals on the axis of the superconducting wiggler were significantly improved at different field levels after the compensation of the corrector coils.Subsequently,the wiggler was successfully installed in the storage ring of BEPCII operation with beams.

Process Design of Cryogenic Distribution System for CFETR CS Model Coil

The superconducting magnet of Central Solenoid（CS） model coil of China Fusion Engineering Test Reactor（CFETR） is made of Nb_3Sn/Nb Ti cable-in-conduit conductor（CICC）,and operated by forced-flow cooling with a large amount of supercritical helium.The cryogenic circulation pump is analyzed and considered to be effective in achieving the supercritical helium（SHe） circulation for the forced-flow cooled（FFC） CICC magnet.A distributed system will be constructed for cooling the CFETR CS model coil.This paper presents the design of FFC process for the CFETR CS model coil.The equipment configuration,quench protection in the magnet and the process control are presented.

Quench detection and protection system design and analysis of the 7 T superconducting magnet

A NbTi 7 T superconducting magnet with a 338 mm bore system was rebuilt and the first experiment was carried out in Jan. 2009. The balancing-bridge method used for the quench detection system is analyzed and an improvement of the principle of balancing-bridge conformation is given. The inductances of the coils axe calculated to estimate the proportion of the bridge arms. The appropriate parameters axe selected for the balancing-bridge.

The design of the superconducting wiggler power supply system for HEPS-TF

Purpose This paper describes the design of power supply system for superconducting wiggler in the project of high energy photon source-test facility.It includes three sets of high precision DC stabilized current power supplies for superconducting magnet excitation and three sets of quench protection assembly(QPA).A special magnetic load which has a very large time constant is studied in this paper.In this paper,a stable superconducting excitation power supply is developed,which can adjust the response quickly under the condition of very large load time constant.Meanwhile,the energy dissipation circuit of the QPA is integrated into the power supply in an active quenching protection mode to accelerate the energy release of the superconducting magnet and protect the superconducting magnet.Method The main circuit topology,control algorithm,quenching protection circuit and interlocking logic function are designed for the superconducting magnet with large inductance.The upper computer controls the switch of the three main power circuits through the serial port,sets the output current and adjusts the parameters according to the load.Conclusion Through a series of scientific and rigorous experiments,the rationality of superconducting wiggler magnet power supply scheme is demonstrated.The output current stability test and output voltage ripple test of the power supply meet or exceed the requirements of the magnet system.