超导薄膜磁-热不稳定与强非线性电磁本构的关联性

Dependence of thermomagnetic instability on strong nonlinear E-J models in superconducting films

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n_(0)-j_(c0)和n_(0)-μ_(0)H_(a)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.

The E-J relationship in conventional conductor generally satisfies the linear Ohm’s law.However,the E-J model in superconductors presents strong nonlinear characteristics,which is significantly different from that of the conventional conductor.According to the nonlinear E-J power law of superconducting materials,we quantitatively investigate the relationship between the magnetic-thermal stability and the nonlinear constitutive characteristic of superconducting films at different temperatures,magnetic field ramp rates,and critical current densities by using the fast Fourier transform method(FFT).We find that the strong nonlinear electromagnetic constitutive model plays a crucial role responsible for the onset and morphology(tree-like and finger-like)of the magneto-thermal instability of superconducting thin films.In addtion,the reason why similar magneto-thermal instabilities cannot be observed in conventional conductors is also explained.It can be found that the magnetic field on the border of the superconducting film increases rapidly for a larger creep exponent due to the enhancement of diamagnetism,which results in a large magnetic pressure and easily triggering off flux avalanches.Therefore,the threshold field of flux avalanches in the superconducting film decreases with flux creep exponent increasing.Finally,we present the curves that can clearly divide the n_(0)-j_(c0)plane and n_(0)-μ_(0)H_(a)plane into magneto-thermal stability region and magneto-thermal instability region for superconducting thin film with different levels of nonlinearity.