Mingzhi Guan

Strain-based quench detection for a solenoid superconducting magnet

In this paper, we present a non-electric quench detection method based on the strain gauge measurement of a superconducting solenoid magnet at cryogenic temperature under an intense magnetic field. Unlike the traditional voltage measurement of quench detection, the strain-based detection method utilizes low-temperature strain gauges, which evidently reduce electromagnetic noise and breakdown, to measure the magneto/thermo-mechanical behavior of the superconducting magnet during excitation. The magnet excitation, quench tests and trainings were performed on a prototype 5 T superconducting solenoid magnet. The transient strains and their abrupt changes were compared with the current, magnetic field and temperature signals collected during excitation and quench tests to indicate that the strain gauge measurements can detect the quench feature of the superconducting magnet. The proposed method is expected to be able to detect the quench of a superconducting coil independently or utilized together with other electrical methods. In addition, the axial quench propagation velocity of the solenoid is evaluated by the quench time lags among different localized strains. The propagation velocity is enhanced after repeated quench trainings.

A Parametric Study on Overband Radial Build for a REBCO 800-MHz Insert of a 1.3-GHz LTS/HTS NMR Magnet

A high-resolution 1.3-GHz/54-mm low-temperature superconducting/high-temperature superconducting (HTS) nuclear magnetic resonance magnet (1.3 G) is currently being built at Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology. One of its key components is an 800-MHz HTS insert (H800) comprising three nested coils. Each coil is a stack of doublepancake coils wound with 6-mm-wide 75-μm-thick REBCO tape. For this H800 generating its self-field of 18.6 T and being exposed to a total field as high as 30.5 T, overbanding each pancake coil is necessary to keep the conductor strain at <; 0.6%. Although electromagnetic and mechanical details of the H800 had been considered during its design stage, a parametric study on the overband radial build considering winding tension effect should further confirm the results of our previous analysis. Thus, in this paper, based on Maxwells equations and the equilibrium equations for mechanical deformation, we examine stress levels that the H800 experiences as H800 undergoes winding-energizing sequences during operation at 1.3 GHz. We also discuss the effects of overband radial build and winding tension on conductor stress in each coil. Finally, based on this analysis, we may further optimize the stainless-steel overbanding and winding tension on each H800 coil.

Magneto-Mechanical Coupling Analysis of a Superconducting Solenoid Magnet in Self-Magnetic Field

For high-precision scientific instruments, the accuracy and sensitivity of the magnet system and the quality of field generated strongly depend upon the disturbance of structures. Since the superconducting structural devices with high transport current are often exposed to large Lorentz forces, which lead to the unavoidable deformation inside superconducting coils, the deformation and configuration change will affect the accurate operation of the magnet system and even its stability. For simplicity, the linear theory is commonly utilized for the stress/strain evaluation of the superconducting coils arising from electromagnetic forces. The aim of the present work is to formulate the equations governing the magneto-mechanical characteristics of a superconducting solenoid system. Due to the axisymmetry, 2-D numerical modeling for the superconducting solenoid is performed to calculate the hoop stress/strain and magnetic field. Maxwells equations and the equilibrium equations for mechanical deformation have been simultaneously solved by means of coupled finite element method. The numerical results have good agreement with the experimental observations and show the magneto-mechanical coupling of the solenoid superconducting magnet in the self-magnetic field is remarkable especially for a high field or large transport current in the coils.

Design of a superconducting magnet for CADS

This paper describes a superconducting magnet system for the China Accelerator Driven System (CADS). The magnetic field is provided by one main, two bucking and four racetrack coils. The main coil produces a central field of up to 7 T and the effective length is more than 140 mm, the two bucking coils can shield most of the fringe field, and the four racetrack superconducting coils produce the steering magnetic field. Its leakage field in the cavity zone is about 5 × 10−5 T when the shielding material Niobium and cryogenic permalloy are used as the Meissner shielding and passive shielding respectively. The quench calculations and protection system are also discussed.

Test of an 8.66-T REBCO Insert Coil With Overbanding Radial Build for a 1.3-GHz LTS/HTS NMR Magnet

A 1.3-GHz/54-mm LTS/HTS NMR magnet, assembled with a three-coil (Coils 1-3) 800-MHz HTS insert in a 500-MHz LTS NMR magnet, is under construction. The innermost HTS insert Coil 1 has a stack of 26 no-insulation (NI) double pancake (DP) coils wound of 6-mm-wide and 75-μm-thick REBCO tapes. In order to keep the hoop strains on REBCO tape <;0.6% at an operating current Iop of 250 A and in a field of 30.5 T, we overbanded each pancake in Coil 1 with a 6-mm-wide, 76-μm-thick 304 stainless steel strip: 7-mm-thick radial build for the central 18 pancakes, while 6-mm-thick for the outer 2 × 17 pancakes. In this paper, Coil 1 was successfully tested at 77 K and 4.2 K. In the 77-K test, the measured critical current was 35.7 A, determined by an E-field criterion of 0.1 μV/cm. The center field magnet constant decreased from 34.2 to 29.3 mT/A, when Iop increased from 5 to 40 A. The field distribution at different Iop along the z-axis was measured. The residual field distributions discharged from 10 and 20 A were recorded. In the 4.2-K test, Coil 1 successfully generated a central field of 8.78 T at 255 A. The magnet constant is 34.4 mT/A, which is same as our designed value. The field homogeneity at the coil center within a ±15-mm region is around 1700 ppm. This large error field must be reduced before field shimming is applied.

Strain Effect on Critical Current Degradation in Bi-Based Superconducting Tapes With Different Deformation Modes

A 77-K cryostat system combined with different mechanical loading fixtures has been designed and fabricated to provide various strain modes for Bi-based superconducting tapes. The relations of the corresponding critical current of superconducting tapes and the applied strains are measured. The observations show that the critical current degradation of Bi-2223 superconducting tapes under small strains for different deformation modes is recoverable. While the deformation of tapes increases and exceeds some certain strain values, it is clarified that the critical current and n-value degrade significantly with an irreversible process. The degradation of critical current in Bi-2223 tapes under tension strain is much greater compared to those of bending and torsion strains. To quantitatively predict the degradation dependence upon strains of superconducting tapes, a generalized empirical degradation model based on Ekins exponential model and Weibulls distribution function is developed for capturing the critical current degradation behavior under different deformation modes. The theoretical predictions exhibit quite good agreement with the experimental data for the critical current degradation of Bi-2223 superconducting tapes under axial, bending, and torsion strains, respectively.

Effects of cold-treatment and strain-rate on mechanical properties of NbTi/Cu superconducting composite wires

During design and winding of superconducting magnets at room temperature, a pre-tension under different rate is always applied to improve the mechanical stability of the magnets. However, an inconsistency rises for superconductors usually being sensitive to strain and oversized pre-stress which results in degradation of the superconducting composites’ critical performance at low temperature. The present study focused on the effects of the cold-treatment and strain-rate of tension deformation on mechanical properties of NbTi/Cu superconducting composite wires. The samples were immersed in a liquid nitrogen (LN2) cryostat for the adiabatic cold-treatment, respectively with 18-hour, 20-hour, 22-hour and 24-hour. A universal testing machine was utilized for tension tests of the NbTi/Cu superconducting composite wires at room temperature; a small-scale extensometer was used to measure strain of samples with variable strain-rate. The strength, elongation at fracture and yield strength of pre-cold-treatment NbTi/Cu composite wires were drawn. It was shown that, the mechanical properties of the superconducting wires are linearly dependent on the holding time of cold-treatment at lower tensile strain-rate, while they exhibit notable nonlinear features at higher strain-rate. The cold-treatment in advance and the strain-rate of pre-tension demonstrate remarkable influences on the mechanical property of the superconducting composite wires.

A Criterion of the Strain-Based Quench Decision for a Low-Temperature Superconducting Solenoid

It is important to pursue an efficacious way of detecting the quench in superconducting magnets as soon as possible so that the large magnetic energy stored in the magnets can be discharged duly. By means of strain measurement during the spontaneous quench, we recently proposed a quench-detection method for low-temperature superconducting solenoid magnets. By examining the leap character of strain-rate recorded, we further extended the strain-based quench-detection method. An appropriate criterion for the spontaneous quench was suggested and utilized for two liquid helium-cooled superconducting solenoids with different sizes and coil configurations. It was shown that the proposed method is capable of providing a safe criterion for the quench detection/protection within a short period of time, and it is a simple process comparing with the rate of quench development. The experimental data from the two different solenoids under excitation and quench tests supported the new strain-based detection method.

Stress and Strain Measurements on a 5 T Superconducting Magnet During Coil Excitation

A 5 T superconducting magnet with a warm bore has been successfully fabricated and tested at the Institute of Modern Physics of Chinese Academy of Science (IMPCAS). The strains of the superconducting magnet under cryogenic temperature and intense magnetic field were measured by using low-temperature resistance strain gauges where the compensation methods were introduced for eliminating noise effects of both temperature and magnetic field on the strain measurement. A slow ramp rate of the magnetic field was applied during coil excitation of the superconducting magnet to avoid the temperature risen by eddy current. A wireless strain acquisition system was used for the hoop and axial strain measurements of the magnet. It is clarified that the strains measured in the superconducting magnet give much valuable information characterizing the deformation and the stress state at cryogenic temperature and intense magnetic fields. For the purpose of comparison, the steady-state strains and central magnetic field of the superconducting magnet was valuated by means of a coupled FEM. The simulation predictions and the experimental data show good agreements.

The Influence of Strain Rate on the Tensile Properties of a Nb-Ti/Cu Superconducting Composite Wire Under Variable Cryogenic Temperature

The tensile properties of superconducting wires usually play an essential role in practical applications. The effect of strain rate on the tension responses of commercial superconducting composite wires consisting of niobium-47wt.% Ti filaments in a copper matrix (Nb-Ti/Cu) with a Cu:superconductor ratio of ~4.3, under variable cryogenic temperature, is experimentally investigated in this paper. A variable temperature cryostat system is employed, which provides the cooling environment from room temperature to liquid nitrogen temperature, and a compact-scale cryogenic-type extensometer is utilized to measure the tensile strains on the superconducting wires. Under a range of cryogenic temperatures, the corresponding stress and strains are recorded during the elongation of the wire with a broad range of strain rates (10-1 to 10-4 s-1). The cryogenic mechanical behaviors of the Nb-Ti/Cu composite wires, including the tensile strength and elongation at fracture, as well as the yield stress, are captured experimentally. It is shown that the ultimate tensile strength and the yield strength of the Nb-Ti/Cu superconducting wires increased linearly with cryogenic temperature for the lower strain rates, while notable nonlinear features appear at a higher strain rate. The elongations always increased nonlinearly with decreasing temperature for each tested strain rate, but the elongations decreased with higher strain rate. The Youngs modulus approximately increases linearly with decreasing temperature for each strain rate. Additionally, the effect of strain hardening rate derived from the stress and strain relations for the superconducting wires at different cryogenic temperatures and strain rates is discussed.