Research Themes
Our research covers a wide range of topics in fusion and plasma science, with plasma wave physics and RF technology as its central pillars. We focus especially on waves and fluctuations, which reflect the essential nature of plasma and often provide highly complex and fascinating research subjects.
In our laboratory, we study plasma phenomena and devices across a broad frequency range, from kHz to MHz and GHz. Many of the instruments required for plasma research must be developed in-house, allowing students to gain extensive knowledge, technical skills, and practical experience. Through collaboration with fellow researchers and strong support from faculty members, students can successfully carry out their research projects.
Research Keywords
●Main supervisor: Idei, ●Main supervisor: Ikezoe S1: Newly launched, S2: Early stage, S3: Advanced stage
- 28 GHz gyrotron, non-inductive plasma current start-up, electron cyclotron heating and current drive ●S3
- 8.56 GHz klystron, electron Bernstein wave heating ●S2
- MHD instabilities, burst phenomena, magnetic fluctuations, EUV-SX diagnostics, AXUV, magnetic probes ●S3
- Edge electromagnetic turbulence, SOL width, fast-sweeping probes, divertor heat flux, calorimeters, Langmuir probes ●S3
- Millimeter-wave interferometer ●S3
- Microwave reflectometer ●S2
- Electron Bernstein wave scattering diagnostics ●S2
- Ion temperature diagnostics, NPA ●S1
- Two-dimensional visible spectroscopy, high-speed camera ●S1
- Oblique electron cyclotron emission ●S3
- Energetic particle probe, velocity distribution diagnostics ●S3
- High-frequency radiation from energetic electrons, whistler waves, wave-particle interactions, high-time-resolution hard X-ray diagnostics, MHz-range wave diagnostics ●S3
- Difference-frequency heating, nonlinear coupling, ion heating ●S1,2
- Innovative control of SOL and divertor transport, RF plugging ●S2
- Detached divertor control, ponderomotive potential ●S1
- EC launcher, high-power millimeter-wave fast switch ●S2
Research Materials
Plasma Diagnostics
In plasmas, electromagnetic wave phenomena such as refraction, reflection, transmission, scattering, and radiation occur. For an overview of the fundamentals and cutting-edge developments in plasma diagnostics using electromagnetic waves, the following lecture article in the Journal of Plasma and Fusion Research may be helpful.
Lecture: Fundamentals and Frontiers of Plasma Diagnostics Using Electromagnetic Waves
Plasma Diagnostics with Electromagnetic Waves: Fundamentals and Frontiers
- Introduction
- Basics of Plasma Diagnostics Using Electromagnetic Waves
- Advanced Diagnostic Technologies and Recent Developments
- Microwave Imaging
- Doppler Reflectometry
- Scattering Diagnostics
- Challenges and New Approaches in ITER Experiments
- Issues and Development Status of Electron Density Measurements
- Problems and New Approaches in Electron Cyclotron Emission Diagnostics
- Introduction of Relativistic Effects in Electromagnetic Wave Diagnostics
- Conclusion
Our laboratory mainly focuses on plasma diagnostics utilizing refraction, reflection, and radiation phenomena. When the plasma to be measured is large, the propagation distance of the waves becomes long, and lower frequency (longer wavelength) electromagnetic waves are used. As a result, the propagating waves spread out, making it difficult to measure local plasma parameters. To address this issue, we are developing phased array antennas and conducting analysis using adaptive array techniques. Remote sensing techniques such as adaptive arrays are also used in fields such as atmospheric and oceanic studies, and disaster information gathering. We are actively engaged in collaborative research across a wide range of disciplines. This is also a joint research project with the Culham Centre for Fusion Energy in the UK.
Plasma generation and heating
In tokamak-type devices, which are considered promising fusion reactors, a plasma current is essential to confine the donut-shaped plasma. The plasma current is generally initiated using an inductive electric field based on transformer principles. However, for steady-state operation in fusion reactors, it is crucial to establish non-inductive current drive methods that do not rely on induction.
The central solenoid coil required for inductive current drive takes up significant space in reactor design. Advanced designs aim to eliminate or minimize this coil, potentially transforming reactor design by using the space for blankets or antennas for advanced RF heating. Therefore, non-inductive current drive for plasma startup is an important technology for solenoid-free reactor concepts.
We have developed a high-power millimeter-wave gyrotron system (250kW, 1s), and since 2021 we have begun installing CW gyrotrons. We have succeeded in generating high-current (~80kA) plasmas and sustaining them in stable configurations.
In high-density plasmas, incident electromagnetic waves are reflected, but using mode conversion, the waves can propagate through the plasma. We are conducting research on plasma heating and current drive via this mode conversion. This interaction between plasma and waves is also relevant to radiation diagnostics. We use ray-tracing analysis for wave propagation and Fokker–Planck analysis for wave absorption and current drive.
References
- Fully Non-Inductive Current Drive Experiments Using 28GHz and 8.2 GHz Electron Cyclotron Waves in QUEST
- ECW/EBW Heating and Current Drive Experiment Results and Prospects for CW Operation in QUEST
- Electron Bernstein Wave Heating and Current Drive Effects in QUEST
- Ray Trace and Fokker-Plank Analyses for Electron Bernstein Wave Heating and Current Drive in QUEST
Plasma Control
Control of fusion plasmas using RF and millimeter waves includes plasma potential (electric field) profile control and stability control. In pursuit of the “burning” plasma—a major milestone in fusion research—the international community is constructing ITER (International Thermonuclear Experimental Reactor).
In burning plasma experiments, detailed control of plasma parameter profiles is essential. In Japan, profile control issues and considerations for the JT-60SA project, developed jointly with Europe to support ITER, are introduced in the following special feature articles from the Journal of Plasma and Fusion Research.
Special Feature: Key Issues in Profile Control Toward Realizing Burning and High-Beta Plasma
The Key Issues in Profile Control of a High-Beta Burning Plasma
- Introduction – Profile Control as a Key to Fusion Power
- Control Objectives and Targets
- Profile Control and Response Characteristics
- Progress and Challenges in Real-Time Control Experiments
- Advances and Challenges in Real-Time Data Processing
- Conclusion – The Future of Profile Control as a Key to Fusion Power
We have previously conducted experiments on plasma potential (electric field) profile control (H. Idei et al., Phys. Rev. Lett. 71, 2220 (1993): Transition of the radial electric field by electron cyclotron heating in the CHS heliotron/torsatron), and in recent years, we have focused on developing high-power RF/millimeter-wave components for beam shaping necessary for control.
Reference Papers
In collaboration with the Japan Atomic Energy Agency, we are developing a “high-speed millimeter-wave waveguide path switch operating at dual frequencies” for the JT-60SA project:
Development of RF and Millimeter-Wave Components
The phased array antennas and fast millimeter-wave waveguide switches introduced in “Plasma Diagnostics” and “Plasma Control” are part of our development of high-frequency and millimeter-wave components. In “Plasma Generation, Heating, and Sustainment” and “Plasma Control,” high-power (MW-level) transmission is required. In such transmission, to reduce arcing (dielectric breakdown) and overheating, it is necessary to lower the power density. Therefore, the components used are intentionally oversized compared to the wavelength.
In oversized components, not only fundamental low-order modes but also various high-order modes may be excited. When multiple modes are excited, mode interference can occur, leading to arcing or overheating. To suppress the excitation of unnecessary high-order modes, it is crucial to shape the beam properly and align it straight through the center of the waveguide.
For beam shaping, phase-correcting mirrors are used. These mirrors perform a reverse process of a “magic mirror” that reflects light to form an image—here, they convert a complex mode interference pattern (“image”) into a clean beam. The mirrors locally correct (control) the wave phase upon reflection.
Since direct measurement of the wave phase in high-power beams is difficult, we reconstruct the phase distribution via simulation. We have experimentally validated this reconstruction method through a collaborative study with MIT (Massachusetts Institute of Technology). In this collaboration, we contributed by achieving high-precision millimeter-wave phase measurement, and we continue our joint research with MIT on waveguide mode content analysis.
3D measurement of millimeter-wave electric field strength and phase (in-device testing in QUEST and low-power test stand)
We are also developing waveguide mode measurement instruments in collaboration with the University of Stuttgart and the Japan Atomic Energy Agency. Recently, we proposed a new concept for a mode analyzer based on the characteristics of Bessel functions, which describe the amplitude distribution of the beam, and we are working on its development.
Reference Papers
- Experimental Verification of Phase Retrieval of Quasi-Optical Millimeter-Wave Beams
- Corrugated Waveguide Mode Content Analysis Using Irradiance Moments
To fully investigate the excitation process of unwanted higher-order modes, it is necessary to develop generators for the primary low-order modes. We have conceived a new mode generator that can theoretically excite nearly 100% mode purity, and have experimentally achieved 98% excitation. The details have been compiled into a doctoral dissertation for a Ph.D. in engineering by an industrial doctoral student.
Reference Paper
Challenge to RF Physics & Technology Towards Fusion Energy
Stars shining in the universe exist in a high-temperature plasma state and emit immense energy into space every day through
nuclear fusion reactions occurring within them.
To realize a "sun on Earth"—expected to be the ultimate energy source of the future—countries around the world are collaborating on the research and development of fusion reactors.
At the Chikushi Campus, we operate
QUEST, the largest spherical tokamak (a type of magnetic confinement device) in Japan.
Our research focuses on plasma heating and control using RF (electromagnetic waves), the development of plasma diagnostic technologies, and fundamental plasma wave physics.
We are also developing high-power millimeter-wave components for use in
ITER and future fusion demonstration reactors.
Collaborative research is being actively conducted with many universities and research institutes both in Japan and overseas.
For more information, please visit the
Center for Advanced Plasma Science and Engineering.
Challenge to Plasma Physics
Plasma is a collection of charged particles (such as electrons and various types of ions) that exhibits a wide range of complex behaviors through electromagnetic interactions.
Due to inhomogeneities and anisotropies, a variety of instabilities can arise, leading to nonlinear phenomena such as self-organization and sudden events.
It is also a subject of study in statistical mathematics as a non-equilibrium system.
The interaction between waves and particles is a universal concept, and we believe that we can uncover the keys to understanding mysterious phenomena occurring throughout the universe through laboratory experiments.
Debye Shielding
Coulomb Collision
Larmor Motion (∇B Drift)