Xingzhe Wang

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.

First-principles study on elastic and superconducting properties of Nb3Sn and Nb3Al under hydrostatic pressure

The low temperature superconducting materials, such as Nb 3Sn and Nb 3Al, have similar crystal structures and elastic properties. However, their critical-temperature degradations always show the distinct way under mechanical stresses. In this study, first-principles calculations for the low temperature superconductors based on plane-wave pseudo-potential density functional theory within the generalized gradient approximation are implemented, and the elastic moduli of Nb 3Sn and Nb 3Al and those superconductivities in the presence of hydrostatic pressure are evaluated. The Debye temperatures are obtained by the bulk moduli and shear moduli of superconducting materials. The MacMillan equation is further used to acquire the critical temperatures of Nb 3Sn and Nb 3Al under different hydrostatic pressures. It is found that the elastic constants and bulk moduli of the low temperature superconductors are enhanced by the applied hydrostatic pressure, while the critical temperatures usually are decreased with the pressure. Additionally, the decrease of critical-temperature for Nb 3Sn is more sensitive to the hydrostatic pressure than the one for Nb 3Al. The prediction results show good agreement with the experimental results in the literatures qualitatively.

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.

Pressure induced self-doping and dependence of critical temperature in stoichiometry YBa2Cu3O6.95 predicted by first-principle and BVS calculations

This paper deals with the pressure effect on self-doping and critical temperature in optimum oxygen stoichiometry YBa2Cu3O6.95 of high temperature superconductor (HTS) based on a numerical study combined the first-principle with bond valence sum (BVS) calculations. The microscopic electronic properties and equilibrium ionic position configurations in the superconductor under external pressure are firstly calculated using the first-principle method. The results show that the apex oxygen in the cuprate superconductor shifts towards CuO2 plane due to pressure effect, and the minimum buckling angle of CuO2 plane is correlated with the maximum critical temperature. A BVS formalism is then utilized for evaluating the valences of all ions in the superconductor on the basis of the electronic and ionic properties and the hole concentration in both CuO2 plane and Cu-O chain are deduced. It demonstrates that the pressure-induced charge redistribution leads to a self-doping process of the hole-transfer into CuO2 plan...

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.

Experimental Measurements of the Sensitivity of Fiber-optic Bragg Grating Sensors with a Soft Polymeric Coating under Mechanical Loading, Thermal and Magnetic under Cryogenic Conditions

The strain and temperature sensing performance of fiber-optic Bragg gratings (FBGs) with soft polymeric coating, which can be used to sense internal strain in superconducting coils, are evaluated under variable cryogenic field and magnetic field. The response to a temperature and strain change of coated-soft polymeric FBGs is tested by comparing with those of coated-metal FBGs. The results indicate that the coated-soft polymeric FBGs can freely detect temperature and thermal strain, their accuracy and repeatability are also discussed in detail. At variable magnetic field, the tested results indicate that the cross-coupling effects of FBGs with different matrixes are not negligible to measure electromagnetic strain during fast excitation. The present results are expected to be able to provide basis measurements on the strain of pulsed superconducting magnet/cable (cable-around-conduit conductors, cable-in-conduit conductors), independently or utilized together with other strain measurement methods.