Pursuing synergies between plasma physics and high-temperature superconducting magnet development

Plasma Physics

When a resonant magnetic perturbation field is applied to the magnetic confinement field configuration, a structure called a magnetic island appears. In the Large Helical Device (LHD), it has been observed that these magnetic islands spontaneously disappear and/or expand during plasma discharge. Additionally, it has been observed that the magnetic islands remain out of phase with the plasma. Identifying the conditions that govern the dynamics of these magnetic islands is important, as active control of magnetic islands may improve plasma confinement and MHD characteristics. The dynamics of these magnetic islands are closely related to plasma flow and are also correlated with contactless/contact transition phenomena with the divertor. These findings contribute to the understanding of plasma physics based on studies of the confinement magnetic field structure.

High-temperature superconducting magnets

High-temperature superconductivity (HTS) is characterized by its ability to operate at higher temperatures and in higher magnetic fields compared to low-temperature superconductivity. Utilizing HTS in the magnets of magnetic confinement fusion reactors, where high magnetic fields significantly contribute to fusion output, offers a considerable advantage. However, it is also true that there are several challenges that need to be addressed. One of these challenges is the limitation on the shapes of magnets that can be formed using HTS tapes. Unlike conventional string conductors, REBCO HTS tapes have a flat tape shape and exhibit anisotropy in their bending direction. When stacked tapes are impregnated with epoxy resin and then wound, compressive/tensile strain occurs based on the curvature radius. Moreover, there is a significant constraint on bending, especially in the edge direction, due to the limited likelihood. Additionally, there are other issues that need to be resolved, such as the need to suppress current asymmetry caused by differences in the inductance of each tape when AC current is applied, and the generation of circulating currents.

Synergy Effects

Pursuing the synergistic effects of plasma physics and high-temperature superconducting magnet development is a challenging endeavor. There are numerous obstacles to overcome in order to achieve this goal. Given limited resources, it is crucial to minimize additional modifications aimed at defect correction, focusing instead on equipment modifications and enhancements to enhance plasma performance. In essence, designing devices based on the prediction and understanding of actual plasma obtained from plasma experiments is of paramount importance. The actual magnet geometry will deviate from the ideal, and the presence of error magnetic fields resulting from installation errors, interconnections between superconducting wires, and the structure of the current supply system will impact the magnetic field distribution.

These challenges are not confined solely to the realm of high-temperature superconductivity research but constitute significant hurdles that must be surmounted, encompassing the characteristics of the magnetically confined plasma that is actually generated. To realize this innovative concept for future energy supply, efficient research endeavors are necessary.