Fundamental Critical Relations

We consider systems which exhibit typical critical dependence of the specific heat: <artwork name="GPHT31001ei1">) where γ, γ ′ are critical exponents (γ = α for <artwork name="GPHT31001ei2"> for <artwork name="GPHT31001ei3">), as well as, the case when <artwork name="GPHT31001ei4">, uniaxial ferroelectrics; a = 1, liquid He⁴). Starting from the critical behaviour of the specific heat we can exactly find the asymptotic form of the Gibbs (Helmholtz) potential in the vicinity of the critical point for each case separately. We derive in this way many exact critical relations in the limit T → T_C which remain the same for each particular case. They define a new class of universal critical relations independent from the underlying microscopic mechanism and the symmetry of these systems. It, however, means that they are independent from the critical indices, characterizing each particular material. The derived relations are valid for magnetic, ferroelectric and superconducting materials, as well as, for liquid He⁴ and they have very important consequences concerning the mutual relations between critical amplitudes of many thermodynamical quantities near the critical point and therefore can be important and interesting from the experimental and technological point of view.     

The change in symmetry at the i-ii phase transition in dicalcium barium propionate

The symmetry change occurring at the first-order I-II transition at 267 K in dicalcium barium propionate, DBP, has been determined using X-ray diffraction: phase II is orthorhombic, with probable space group Pnma or Pn2₁a. The twinning in phase II is explained and lattice parameters calculated in the temperature range 125 K to 300 K. The behaviour of the spontaneous strain in phase II is compared with that in the similar compound dicalcium barium acrylate.     

Detecting non-Abelian geometric phases with superconducting nanocircuits

We propose a feasible scheme to detect the noncommutative feature of non-Abelian geometric phases (NAGP) with superconducting nanocircuits. The induced NAGP associated with two-fold degenerate states naturally arises in the chosen four-level subsystem interacting with the microwave pulses. We explicitly show the noncommutative effect of the NAGP by considering the difference between the level populations at the end of the composed evolution loops l₂l₁ and the counter-ordered one l₁l₂. The scheme opens the new possibility for detecting the fundamental characteristics of the NAGP with superconducting circuit devices.     

Self-radiation from arrays of niobium Josephson junctions embedded in the open resonator

This paper focuses on self-radiation from arrays of Josephson junctions embedded in a quasi-optical resonator. The mechanism of coupling this radiation to the principal mode of the open resonator is illustrated using experiments and simulations with CST microwave studio software. Comparing the microstrips and dielectric resonators used as the antennas of the series arrays of discrete Josephson junctions, we demonstrate that the dielectric resonator antennas are more effective than microstrips.     

MgB2超导线拉伸性能研究

MgB2 是一种新型超导材料, 它具有较高的临界温度和较低的原料价格. 随着磁约束核聚变实验技术的发展, 未来的托卡马克磁体系统的低场部分磁体有希望使用 MgB2 CICC 导体绕制. CICC 导体的有限元分析需要以超导线的应力-应变函数为基础. 本文中, 来自西部超导公司( WST) 及 Hyper Tech 公司的两种 MgB2 超导线被测试了抗拉强度与弹性模量. 未反应的超导线在室温下的拉伸曲线被用多项式及指数函数拟合. 实验发现西部超导公司的 MgB2 线材在各种条件下的拉升强度都高于 Hyper Tech 线材. 室温下未反应的 MgB2 超导线的应力-应变关系可以用指数函数恰当地拟合, 并且表达式比多项式函数的拟合结果更简单. 超导线的应力-应变函数可以为CICC 导体数学模型的建立提供参考.     

超导扭摆器实验线圈设计与测试

上海同步辐射光源二期工程计划建设一条基于4.05T超导扭摆器的超硬多功能线站。根据用户提出的需求参数,作者开展了超导扭摆器的磁场设计,绕制了两个NbTi螺线管实验线圈,并进行了失超锻炼测试。实验结果表明,实验线圈的临界电流达到了设计要求,所采用的NbTi超导线性能和绕线工艺可以满足下一步的超导扭摆器磁体研制要求。     

Magnetic- and stress-fields of cylindrical non-uniform current density superconducting coils

Superconducting coils with an inhomogeneous current density distribution yield more field for less volume of superconductor. The advantages of these systems are lower costs and an improved cooling. The latter is important for high field magnets. Mechanical stresses induced by the electromagnetic forces are the limiting factor in designing such coils. The inhomogeneous current field is produced by a set of concentric subcoils. The radial and tensile stresses and the radial displacements in the subcoils are calculated. This calculation requires the field-shape in the coils. An exact calculation shows a linear decrease of the field with the radius in each subcoil. Thus the stress-calculations are very simple and one does not need a computer. As an example this method is applied to a 10 T-magnet with three subcoils. In this case the results of our exact calculation differ from the approximation of Kilbet al., Steklyet al. and from the magnetic pressure estimateB ²/2μ ₀ up to a factor of three.     

Microstructural characterization of the 85 K superconducting phase in the Bi−Ca−Sr−Cu−O system

Summary Transmission electron microscopy has been applied to investigate the microscopic features of a superconducting phase present in the system Bi−Sr−Ca−Cu−O. Sintered samples showing a single transition temperature of 85K, which points out the formation of aB-phase, were mechanically and ion-beam thinned. Electron diffraction shows that the Bi₂Sr₂CaCu₂O₈ superconductor possesses a primitive orthorhombic unit cell withPnnn space group. High-resolution electron microscopy reveals that the most common defect corresponds to a (001) lamellar structure caused from frequent twins with (001) composition plane and [001] fourfold axis twin operator. Possible derivative structures, essentially based on the occurrence ofm b/5+n a shifts everyc period, are discussed. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Direct measurement of the AC loss of an ITER relevant coil

In the ENEA Frascati Laboratory, an ITER relevant coil is being tested in pulsed regime. One item of the testing program is the search for possible ramp rate limitations. The range explored (up to 3.2 T/s) covers amply the ITER Central Solenoid operating regime (0.5–1 T/s). An interesting outcome of these tests is that repeated run sets, performed in similar conditions, show an increasing value of the ramp rate that produces a quench. This behaviour is attributed to a continuous decrease of the value of the AC loss constant nτ, due to the increase of the cable transversal resistivity. An explanation on the basis of a ‘classical' training was also considered, but for a number of reasons reported in the paper, it was discarded. Direct measurements of the AC loss on the whole coil confirm that the original value of nτ (140 ms), as measured on a ‘virgin' piece of conductor, has gone below 40 ms. The measurements are made difficult by the presence of a heavy mechanical structure and of a background coil. The techniques applied and the results are reported in detail.     

Influence of the multilayer HTS-cable conductor design on the current distribution

The work presents the basic principles of the multilayer cable conductor design to achieve the maximum current-carrying capacity and the minimum losses in a superconductor and constructive cable elements. The multilayer conductors of two to ten layers are analyzed. The results show that the traditional core design with alternative winding directions from layer to layer is useful only for two-layer conductor. The conductor with more layers must have either the layers wound in one direction but with different pitch lengths or two layer groups wound with different pitch lengths. Only for these cases, the balanced design can be realized and current distribution will be uniform. In such balanced design, the interlayer electrical voltage and as a result, the coupling losses, are absent and interlayer electrical insulation is not needed. The recommendations to achieve the maximum critical current as a function of conductor dimensions are derived.