MASAKI
NISHIURA

The dipole magnetic field-based plasma confinement device was originally proposed by S. Yoshikawa and colleagues as the Princeton Spherator, with plasma experiments reported in 1969 [1]. At that time, the superconducting coil generating the dipole field was supported in vacuum, resulting in significant particle losses due to the support structures. It was optimistically suggested that these losses could be greatly reduced by using a levitated coil without supports. A dipole magnetic field refers to a magnetic field with a north and south pole, like Earth's.

Plasma is a state in which atoms are heated to such high temperatures that their electrons and ions separate. However, due to its high temperature and rapid motion, plasma cannot be contained in ordinary vessels. Thus, researchers devised the idea of magnetic confinement, where charged particles spiral along magnetic field lines. In fusion plasma research, various confinement devices with different magnetic field topologies have been developed, such as the toroidal tokamak, and the dipole configuration is one of them.

Subsequent planetary explorations by Voyager 1 and 2 revealed that many planets in the solar system—including Earth and Jupiter—have dipole magnetic fields. These planets confine plasma within regions called magnetospheres. Despite being influenced by solar wind and magnetic disturbances, these magnetospheres remain stably confined even under high plasma pressure [2–4]. This naturally inspired interest in using dipole magnetic fields for plasma confinement, prompting researchers to look to nature for insights.

Planetary magnetospheres efficiently confine high-pressure plasma within relatively simple dipole magnetic fields. This observation led to the idea of artificially recreating such fields for the development of advanced fusion devices. One pioneer in this field was Hasegawa, who proposed a fusion device using a dipole magnetic field [5]. Theoretical studies by Yoshida and colleagues have shown that fast rotational flows in dipole plasmas play a crucial role in achieving high-beta plasmas—where plasma pressure significantly exceeds magnetic pressure [6–7].

Following these developments, the University of Tokyo built a series of dipole-type devices, starting with Proto-RT, followed by mini-RT and RT-1, which simulates planetary magnetospheres [8]. Similar dipole field plasma devices have since been constructed at institutions such as the Massachusetts Institute of Technology and the Max Planck Institute for Plasma Physics. These experiments have revealed plasma phenomena analogous to those in planetary magnetospheres, contributing to research in fusion and antimatter plasmas.

Fusion devices employing dipole magnetic fields represent an innovative approach inspired by the laws of nature. Through continued research in this area, we aim to accelerate the realization of fusion energy.

Referenced in the University of Tokyo Graduate School of Frontier Sciences publication: 2018, Sōsei Vol. 31 – Frontier Sciences 1 (p. 8)
"Plasma Physics Experiments with the Magnetosphere-Type Plasma Device RT-1: From Laboratory Magnetospheric Plasmas to Fusion Energy Research"

In the planetary magnetosphere-type device RT-1, which employs a unique confinement method distinct from the International Thermonuclear Experimental Reactor (ITER) operated by the ITER Organization and JT-60SA of the National Institutes for Quantum and Radiological Science and Technology (QST), it has been revealed that plasmas exist in different confinement regions depending on whether the electrons are high-temperature or low-temperature.
This peculiar plasma structure, forming a double-layered configuration similar to the Van Allen radiation belts found in the Earth’s magnetosphere, was visualized for the first time through millimeter-wave and X-ray diagnostics.
The self-organization and stability of this plasma are maintained by collective behavior of individual ions and electrons. However, the fundamental reasons why the plasma self-organizes in this way and maintains a stable state remain open questions to be addressed in future research.
Solving these challenges is expected to extract universal physics principles independent of the confinement method, thereby contributing to the development of compact, high-performance fusion reactors and addressing unresolved problems in natural plasma phenomena.

There remain several unresolved issues related to plasma self-organization and stability in planetary magnetosphere-type devices.
Some of these challenges are outlined below.

Plasma is considered the fourth state of matter, following solid, liquid, and gas. Fusion plasma, in particular, is extremely hot, with ions and electrons separated from each other. This high-temperature plasma emits electromagnetic waves across a wide range of wavelengths, from short-wavelength X-rays to infrared. Visible electromagnetic waves have wavelengths ranging from violet to red, while X-rays have even shorter wavelengths and are emitted from electrons.

Interestingly, in fusion plasma, the bright regions visible to the eye are not the hottest areas; rather, they are the cooler peripheral regions of the plasma. The transparent core of the plasma is at an extremely high temperature exceeding 100 million degrees Celsius, which is where fusion reactions occur. Therefore, when measuring plasma, it is necessary to visualize it not only using visible light but also by employing various diagnostic methods. This approach clarifies the plasma’s state and also contributes to the study of astrophysical plasmas, which cannot be fully understood by satellite observations.

In our laboratory, we focus on developing new diagnostic instruments and data analysis techniques. For example, by utilizing coherence imaging spectroscopy based on birefringent crystals, we have been able to obtain high-resolution two-dimensional images that are difficult to achieve with conventional spectrometers using diffraction gratings.

Measurement results using the CIS system show a region of elevated ion temperature (Ti) near the resonance layer around Ωci due to ion heating. This result demonstrates the first successful ion heating in a planetary magnetosphere-type device. By successfully visualizing this with the developed CIS system, we have enabled the experimental verification of ion heating.

The figure shows electromagnetic field simulations of ion cyclotron resonance heating (ICRH) using 2 MHz electromagnetic waves to heat ions, along with the results from heating experiments. It is necessary to propagate the ion cyclotron waves from the strong magnetic field side, and the antenna design and optimization of the electric field distribution were performed using simulation codes. As a result, we demonstrated ion heating for the first time in the world.