Introduction

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Issues as nuclear fusion science

The scope of scientific research in plasma fusion is very wide, ranging from the microscopic scale of atoms and electrons to the macroscopic scale of fluids, solids, plasmas, and actual devices. The collective motion of plasma causes a wide variety of phenomena not only in the plasma itself, but also in the materials in contact with the plasma. The following is a list of issues that the members of this unit are focusing on as an example.

•Global multi-scale turbulence, transport, and distribution formation that affect confinement performance and burnup efficiency

•Nonlinear wave-particle interaction, anisotropy or non-Maxwellian velocity distribution function, energy transport channeling researches expand plasma heating, diagnostic and control schemes for fusion burning plasmas

• Plasma-wall interaction and reactor materials that determine the durability and engineering feasibility of fusion power reactors

In addition to the examples listed here, many issues in fusion science tend to be considered independent at first glance, but by focusing on the hierarchical structure that appears in most of the complex processes and multi-scale phenomena associated with plasma, and by viewing them as common issues, we aim to activate wide collaborations in plasma fusion research.

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Academic characterization (what can be said to be the research)

Natural phenomena spanning a wide range of spatio-temporal scales and parameter domains have been understood by separating and connecting elements in various “hierarchies” in a reductive manner. Examples include the scales of motion of electrons and ions, and particle and fluid models. However, some of the problems that have emerged with the progress of experimental and numerical research are phenomena that cannot be well understood by hierarchical separation. In this unit, we aim to find a breakthrough in solving unsolved problems by reconsidering problems with a focus on the hierarchical nature of natural phenomena.

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Approach (Formulation)

The research of this unit can be broadly categorized into the following four categories. However, they will not work independently, but will collaborate organically and fluidly.

1. Local and global phenomena of turbulence and transport with inherent hierarchical nature
Local and global simulation studies of plasma turbulence and transport phenomena with inherently complex hierarchy will be promoted to explore multi-scale interactions of turbulent eddies driven by multiple microscopic instabilities, meso-scale spontaneous flow formation, scale dependence of global spatial propagation of fluctuations, and geometrical structure dependence of their background magnetic fields.

2. Energy channels of electromagnetic fields and atoms and molecules under anisotropic velocity distributio
In plasmas, electromagnetic fields, particles, and atomic molecules form a hierarchical structure and have complex energy channels between them. The energy transport process can be unraveled by paying attention to the significant deviations from the thermal equilibrium distribution caused by the energy channels. The structure and anisotropy of the velocity distribution function are evaluated from physical models that incorporate high time-resolution measurements and transient responses. Focusing on the phenomena in which large structures appear in the velocity distribution function and the accompanying plasma states and their changes.

3. Hierarchy and multi-physical processes modeling from edge plasmas to plasma facing material
Exploration of modeling across plasma and solid materials from the common viewpoint of hierarchy. Although plasma is controlled in experiment from a macroscopic viewpoint, the experimental result is sometimes discussed with observations from a microscopic viewpoint such as microstructural observations of material surfaces using electron microscopy. The gap between the typical scale of plasma and the typical scale of material surfaces is a key issue to understand the plasma-wall interaction. In addition, this issue causes the difference between microscopic and macroscopic perceptions, and relates the problem of validity between theoretical simulation and real experiments. We will also address the microscopic limits of various statistical descriptions of plasmas, for example the understanding how to connect sheath plasmas to nanoscale surfaces.

4. Physical modeling and universality explored through hierarchy across multiple scales
We will construct physical models that express hierarchical dynamics and search for universality in phenomena that span multiple scales of nature, including plasma. We will also work on methodological development, both theoretical and experimental, and share successful modeling and analytical methods among the units as needed to expand the scope of our research. The successful modeling and analysis methodologies will be shared among the units as needed to expand the scope of our research.

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Academic development

Goals to be achieved in 10 years
We tackle the above issues by re-establishing the various phenomena of fusion plasmas, and by re-formulating the contributions from micro- and macro-models. We will also seek academic collaboration both within and outside the field with new methods and general prescriptions for the hierarchical phenomena obtained in the process.

Anticipated Academic Significance
In this activity, we will pursue the possibility that activities focusing on the hierarchical nature of complex phenomena with fusion plasma* will spill over into activities common to many other sciences, including cosmology, elementary particles, and ecosystems. (* Multiscale turbulence and flow formation/stability, resonance between waves/instability and particle motion, global propagation of fluctuations, plasma-solid interface, velocity space structure and anisotropy of distribution functions, paired plasma properties and mass ratio dependence, etc.)

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Uniqueness, superiority, etc.

Fusion science is a field where phenomena that are difficult to coexist in nature, such as plasma, atoms and molecules, and even solids, can actively coexist and be controlled in a laboratory system. For example, by injecting a strong high-frequency electric field and energetic particles into a plasma, it is possible to freely manipulate the local electric field strength and the spatial distribution of charged particle velocities, enabling efficient prediction by simulation and verification of theoretical models. These features enable precise studies of hierarchical problems. In the National Institute for Fusion Science, units consisting of experts at the forefront of their respective fields have been assembled, and this unit will act as a hub to promote inter-unit collaboration and to demonstrate the advantages of cutting-edge experimental research and theoretical simulation research.