高温超导体磁通跳跃数值模拟

文中基于超导磁通动力学理论,考虑电磁力与热激活对磁通运动的影响,基本物理模型由等效电阻率随超导体温度和磁场变化的磁通扩散方程,以及比热随超导体温度变化的热传导方程组成。在此基础上,用差分法数值求解了这一复杂非线性系统的磁热耦合控制方程,得到了与实验观测结果基本一致的数值模拟结果。结果还表明：外加磁场变化速度、超导体初始温度以及超导尺寸对于磁通跳跃均产生明显的影响。     

交变磁场作用下高温超导体温度变化研究

高温超导体在交变的磁场作用下,由于磁通的运动引起能量损耗,损耗的能量一部分通过超导体表面传递到冷却剂中,另一部分将使得超导体的温度升高。文中用数值的方法研究了外加磁场速度在0.0005T/s—5T/s变化情况下超导体的温度变化;当外磁场的速度由小到大变化时,超导板的状态会发生从稳定→不稳定(磁通跳跃)→稳定的变化;慢变磁场作用下超导体的温度在接近冷却剂温度的温区作微幅的周期性变化,当外加磁场速度比较大时,超导体发生磁通跳跃,温度也呈跳跃性变化,进一步加大外磁场速度,磁通和温度呈准周期的振荡型变化,而且振荡幅值随外磁场速度的增加逐渐减小,最后振荡消失,超导体在更高的温区稳定运行,温度呈周期性变化。     

交流磁场振幅和频率对J_c不均匀超导体谐波的影响

本文采用Jc不均匀超导体模型来研究多晶超导体的磁性质,运用非线性磁通蠕动模型,通过数值模拟得到交流磁化率基波及谐波随温度的变化关系.系统分析了交流磁场振幅和频率改变对Jc不均匀超导体交流磁化率基波及谐波的影响.结果表明,交流磁场振幅和频率的变化对交流磁化率的影响存在显著差异,并归因为交流磁场振幅和频率的改变引起的磁弛豫时间的改变不同所致.上述结果为分析Jc不均匀超导体交流磁化率的实验现象提供了相关依据.     

高温超导体脉冲磁化过程中的磁热不稳定性

该主要研究了高温超导体在脉冲磁场中的磁热稳定性，与传统低温超导体相比，ＨＴＳ所不同的是最不稳定的状态发生在磁通跳跃区，理论上借助Ｅ－ｊ曲线和有限热扩散方程推导了磁通跳跃场与变场速率的关系表达式：Ｂｊ＝Ｆ其规律基本与实验结果相符合，同时从实验中发现磁热不稳定性对永磁体脉冲磁化效果有巨大的影响。     

高温超导圆柱在不同脉冲宽度磁场作用下的温度和磁场分析

多脉冲分段冷却磁化技术是获得强俘获磁场的一种有效方法,近年来受到广泛关注,脉冲磁场的幅值和宽度是影响该方法的两个重要因素。文中通过求解基本的超导电磁场和温度场耦合方程,研究了两种不同脉冲宽度情形下脉冲磁化过程中超导体磁场和温度场的变化。结果表明,宽脉冲作用下超导体在磁化过程中温度较低,分布均匀,而且有更多磁通进入超导体。     

氧缺位对YBa2Cu3O7—x超导体中临界电流密度和磁弛豫行为的影响

用梯化曲线和磁驰豫测量研究了缺氧ＹＢａ２ＣｕＯ７－ｘ超导体中氧缺位对磁通行为的影响。实验表明：在低氧缺位时临界电流密度随氧缺位ｘ的增加稍有增加，且随磁场的增加出现一高起的平面，当氧缺位ｘ进一步增加，临界电流密度下降，且随磁场增加单调下降。零场冷却（ＸＦＣ），长场至固定磁场后磁化强度随时间依半对数关系衷减，磁通蠕动的热激活能随氧缺位ｘ的增加而下降。     

高温超导体中磁通涡旋的量子隧道效应——TIBaCaCuO化合物磁弛豫…

本报道了ＴＩＢＡＣａＣｕＯ超导体（２２１２，２２２３单相，多晶及薄膜材料）中磁弛豫现象的实验结果，从实验上证实了能量运动由热激活到非热激活机制的转变，实验中得到与温度无关的磁弛豫速率与由通量运动量子隧道效应理论计算结果相一致，同时讨论了各向异性和磁场对磁通的量子漂移速率的影响。     

超导薄膜磁-热不稳定与强非线性电磁本构的关联性

常规导体的电磁本构关系一般满足线性欧姆定律,然而超导体的电磁本构关系呈现很强的非线性特征,所以与常规导体相比,超导有截然不同的电磁特性.本文基于超导材料E-J幂次律本构关系,采用快速傅里叶变换方法 (FFT),定量研究了不同环境温度、磁场加载速率以及临界电流密度条件下的超导薄膜磁-热不稳定性与非线性本构的关联性,揭示了强非线性电磁本构是导致超导薄膜磁-热不稳定性(呈现树状、指状磁通崩塌形貌)的重要因素,同时阐明了常规导体观测不到类似的磁-热不稳定现象的原因.另外发现由于超导薄膜抗磁性的增强导致超导薄膜边界磁场迅速增大,较大的磁压极易诱发磁通崩塌,所以超导薄膜内磁通崩塌阈值随幂指数的增加而降低.最后给出了n₀-j_c0和■平面内不同非线性程度下超导薄膜内磁热不稳定状态的分界线.     

通道内空气热磁对流的协同分析

为研究通道内空气热磁对流的流动规律,用数值模拟方法,研究了磁场作用下二维模型水平通道内的流动换热过程,获得了通道内的磁通密度分布和空气温度轴向分布,在此基础上,建立了一维通道内空气热磁对流的数学模型,就温度场和磁场的相对关系对水通道内热磁对流过程的影响进行了数值计算,获得了通道内不同温度场、磁场以及其不同相对位置下的通道空气流量,并讨论了磁极形状对空气流量及其变化的影响。     

GdBCO超导体的顺磁性及低温下磁通分布的动态观测

本文研究了100～300K之间Gd1.8Ba2.4Cu3.4O7-δ(简写为GdBCO)超导体的顺磁性,对GdBCO的磁化强度随磁场和温度的变化分别在100K和0.1T进行了测试,GdBCO的顺磁性遵从朗之万顺磁性理论.在磁光显微镜下,低温12K场冷下,我们观察了从0mT～29mT磁通线进入GdBCO的超导体的过程,以及磁通线在GdBCO超导体中的分布.     

Dynamics of a magnetic flux jump in a composite superconductor

An attempt is made at direct experimental verification of the theory of thermomagnetic instability in composite superconductors under conditions of external magnetic field or transport current variations. The development of thermomagnetic instability in the form of a magnetic flux jump is experimentally studied in a bulk low-temperature composite niobium-tin superconductor. The liquid-helium-cooled sample representing a compressed tape helix (helicoid) is placed in an external magnetic field orthogonal to the turn plane and varying with a constant rate. For the first time, both the magnetic induction inside the sample and its temperature are simultaneously detected in experiments. The sample overheat preceding the magnetic flux jump is measured to be 0.23 + 0.02 K. This value is found to be independent of the rate of the external magnetic field variation and the value of the jump itself and coincides, within the experimental accuracy, with the temperature parameter of electric field buildup involved in the general exponential I-V characteristic of the composite superconductor, which depends on temperature and magnetic induction.     

The influence of measurement and relaxation time on flux jumps in high temperature superconductors

The influence of the magnetization and relaxation time on flux jumps in high temperature superconductors (HTSC) under varying magnetic field is studied using the fundamental electromagnetic field equations and the thermal diffusion equation; temperature variety corresponding to flux jump is also discussed. We find that for a low sweep rate of the applied magnetic field, the measurement and relaxation times can reduce flux jump and to constrain the number of flux jumps, even stabilizing the HTSC, since much heat produced by the motion of magnetic flux can transfer into coolant during the measurement and relaxation times. As high temperature superconductors are subjected to a high sweep rate or a strong pulsed magnetic field, magnetization undergoes from stability or oscillation to jump for different pause times. And the period of temperature oscillation is equal to the measurement and relaxation time.     

Nonlinear dynamics of the critical state in hard superconductors and composites based on them

The nonlinear dynamics of the magnetic flux within a superconducting plate in response to the continuous rise in temperature over the course of the entire process of applying the magnetic field is investigated within the critical-state model. The results of numerical simulations based on a method developed to solve the system of Fourier and Maxwell equations with an unknown internal magnetic flux penetration boundary are compared with the corresponding analytical expressions of the isothermal theory. It is shown that the difference between the isothermal and nonisothermal models increases as the heat transfer coefficient decreases and as the rate of increase in the magnetic field strength and the transverse dimensions of the superconductor increase. The errors appearing in the isothermal approximation are very significant in the case of a thermally insulated, massive conductor. Consequently, the calculated values of the thermal losses occurring during the time period preceding the flux jump in the isothermal approximation can be significantly lower than the corresponding nonisothermal values. Zh. Tekh. Fiz. 67, 29–33 (September 1997)     

The flux jumps in high TcBi1.7Pb0.3Sr2Ca2Cu3Oy bulk superconductor

There was giant flux jumps in high Tc Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O_y bulk superconductor. The relaxation time, τ, decreased with both the increase of magnetic field and the rise of temperature. The maximum τ was about 40 min.. The average-dM/dt increased with both the increase of magnetic field and the rise of temperature. The minimum average-dM/dt was about 4.1×10⁻²G/min.. The flux jump weakened with time. It was dependent on the decrease of gradient of magnetic flux density dn/dx in the sample.     

Effect of heat capacity and conductivity of NbTi normal matrix of a composite superconductor on the stability to magnetic flux jumps

The stability against magnetic flux jumps has experimentally been studied in the external magnetic field for three samples from NbTi composite superconductors, one monofilamentary and two multifilamentary. A comparison between the experiment and theory of thermomagnetic stability of composite superconductors has been carried out. We have determined threshold values of the rates of the external magnetic fields, starting from which heat capacity and conductivity of the normal composite matrix become determining stabilizing factors. For the first time, the increasing dependence of field of first magnetic flux jump on the rate of the rise in the external magnetic field has been experimentally registered in the superconducting wire for MRI. The reason for this effect is the shunting effect of a high pure copper matrix and the low volume fraction of a superconductor in the composite (~10%).     

Thermoelectric phenomena at grain boundaries

Analytical expressions for a temperature jump and electric potential difference that arise when current passes through a grain boundary are derived. The electron flow (current) through the boundary and the current-induced heat flux are assumed to be given. The kinetic equation in the τ approximation for electrons and the Maxwell equation for an electric field are used. The dependence of the temperature jump and potential difference factors on the chemical potential is studied.     

Topological vortices in a two-gap superconductor

Based on the ϕ-mapping theory, we derive a new rigorous equation describing the distribution of the magnetic field for vortices in a two-gap superconductor, of which the so-called modified London equation is just a special case in a one-flavor limit. We explicitly investigate the London penetration depth, the Meissner and mixed states and Josephson effect. A magnetic flux quantization condition for vortices in a two-gap superconductor is also derived, from which it follows that in a two-gap superconductor there exist vortices which carry an arbitrary fraction of magnetic flux quantum. The branch processes during the evolution of the vortices in a two-gap superconductor are discussed.     

Noncollinear orientation of external magnetic field and flux lines penetrating an isotropic hard superconductor

Penetration by Abrikosov flux lines of an isotropic hard superconductor in the critical state induced by changes in the orientation of external magnetic field has been theoretically investigated. The analysis has been based on the microscopic nonlocal model taking into account forces of bulk and surface pinning, alongside magnetic forces of interaction of the row of penetrating vortices with existing flux lines, Meissner currents, and vortex images. New vortices penetrate a superconductor only when the angle through which the field is rotated is larger than a certain critical value. It has been determined that the alignment of entering vortices is essentially different from that of the applied magnetic field. The feasibility of detecting noncollinearity effects is discussed. Zh. éksp. Teor. Fiz. 114, 1804–1816 (November 1998)