One hundred years have passed since the discovery of superconductivity. Over this time, the science, especially in the area of physics, relating to superconductivity has improved dramatically. However, no appreciable progress has been made in the education of superconductivity in relation to primary electromagnetism. A superconductor is a unique material in which Ohm's law is not applicable to the flow of electric current. If this physical point is carefully considered, a new perspective on electromagnetic properties emerges. In this series of lectures, electromagnetic phenomena in superconductors are examined from various aspects. The first lecture deals primarily with the magnetic phenomena occurring in the vicinity of superconductors while they are in the Meissner-Ochsenfeld state, showing perfect diamagnetism. It is shown that by introducing this magnetic phenomenon to the present E-B analogy, the style of teaching primary electromagnetism can be dramatically changed. In accordance with this analogy, it would even have been possible to predict the existence of superconductors in the 19th century after the formulation of the Maxwell theory. This kind of discussion reinforces the E-B analogy. The introduction of superconductivity makes it possible to directly derive the magnetic energy through a mechanical action working against the magnetic force. In contrast, the magnetic energy can be derived only after teaching electromagnetic induction in existing textbooks. Other merits of introducing superconductivity are also discussed.
It is observed that the measured critical currents of cable-in-conduit (CIC) conductors for ITER TF coils become lower than expected due to the unbalanced current distribution that is caused by contact resistance between the strands and the Copper (Cu) sleeves in CIC conductor joints. In order to evaluate the contact length, we identify the three-dimensional positions of all strands in the CIC conductor, and then measure the contact number and lengths of strands that appear on the surface of the cable to contact with the Cu sleeves. It is found that some strands do not appear on the surface of the cable, and the contact lengths are widely distributed with a large standard deviation. We develop a numerical code that simulates strand positions in the CIC, and then compare the analyzed contact strand number and contact length with measured ones. It is found that the results are in agreement, and hence the code can be used to evaluate the contact parameters. After varying the twist pitches of the sub-cables, we show that all strands appear on the cable surface and have contact lengths with small standard deviation. It is found that the twist pitches are a key parameter for optimization of the contact resistance.
This study proposes a new indirect method of detecting normal transitions appearing in superconducting coil windings. In this method, multiple sets of pick-up coil pairs arranged outside the cryostat are used in order to electrically measure the occurrence of normal transitions during coil operation. This indirect method has benefits such as high reliability, safety and easy maintenance. The validity of this method was clarified theoretically and experimentally. In this experiment, tests of whether or not normal transitions could be detected were carried out on a Bi-2223 coil immersed in liquid nitrogen. The coil was closely wound into a single-layered, solenoidal coil shape with Ag-sheathed multifilamentary Bi-2223 tapes. The results showed that signs of normal transitions were successfully detected using this method when the winding area of the coil was warmed up locally using nitrogen gas.
We developed a new double-sided patterning (DSP) process using a dilute acid solution (pH=1.65) to fabricate Bi2Sr2CaCu2O8+x (Bi-2212) stacks of intrinsic Josephson junctions. The stacks, which were fabricated from a single crystal of Bi-2212, were surrounded by an acid-treated product. The critical aspect of this process is that the Bi-2212 surrounding the photo-resist pattern was changed to a transparent material, BiOCl, which was connected with the Bi-2212 crystal. As a result, this process provides an easy way to achieve the DSP of the surface of the Bi-2212 crystal. This new DSP process allows for remarkably improved reproducibility in fabricating the Bi-2212 stacks which show good current-voltage characteristics with large hysteresis and multiple branches at T=77 K.
The Japan Atomic Energy Agency (JAEA) and National Institute for Material Science (NIMS) have been collaborating in the development of high-performance (i.e., more than 16 T and 80 kA) Nb3Al cable-in-conduit (CIC) conductor prepared using the rapid-heating, quenching and transformation (RHQT) process, and aiming for the application of this conductor to a demonstration plant. The technical issue relating to application of the RHQT Nb3Al strands to a fusion magnet is stabilization against perturbation. NIMS has developed a technique to attach a copper stabilizer using electroplating, and a sub-scale CIC conductor is developed using this conductor. JAEA performed a stability test of the developed CIC conductor to demonstrate the efficiency of this copper stabilization technique. The initial perturbation was applied via inductive heating, whose energy was calibrated using a calorimetric method. The measured stability margin is sufficiently high compared to that of a similar NbTi CIC conductor previously tested by the authors. In addition, a heat transfer coefficient to define the so-called limiting current is estimated to be about 1 kW/m2K, a sufficiently high value that is almost the same as that of a CIC conductor consisting of normal Cu stabilized strands. From these experimental results, it can be concluded that the copper stabilizer works efficiently from the viewpoint of stability, thus offering a solution to the remaining technical issues relating to the RHQT CIC conductor. On this basis, we can say that the RHQT Nb3Al CIC conductor is the most promising candidate for application to a magnet in the demonstration plant.