Kyushu University, Research Institute for Applied Mechanics
(Steady-State Plasma Heating Field / Plasma Wave
Science and Engineering)
Interdisciplinary Graduate School of Engineering Sciences
(Department of Advanced Energy Science and
Engineering/Integrative Science and Engineering Advanced Plasma Science and Engineering)
QUEST (Q-Shu University Experiment with Steady-Spherical Tokamak) is the largest spherical tokamak device in Japan, and was constructed in 2008 at the Chikushi Campus (Kasuga City, Fukuoka Prefecture). IIlab is conducting physics and engineering research on plasma and fusion using QUEST. The development of a fusion reactor is the largest international cooperative project since the International Space Station, and requires long-term, continuous research and development. Kyushu University has the Research Institute of Applied Mechanics, where many researchers in the field of plasma fusion are gathered, which is rare among Japanese universities, and is making efforts to build an academic foundation to accelerate the realization of fusion reactors and to train the next generation of researchers. The researchers who will start their research in this field are an exciting generation who may be involved in nuclear burning experiments and DEMO reactor development in ITER and witness the crystallization of mankind's best science and technology (dream energy source).
QUEST (Q-Shu University Experiment with Steady-Spherical Tokamak) is the largest spherical tokamak device in Japan, and was constructed in 2008 at the Chikushi Campus (Kasuga City, Fukuoka Prefecture). IIlab is conducting physics and engineering research on plasma and fusion using QUEST. The researchers who will start their research in this field are an exciting generation who may be involved in nuclear burning experiments and DEMO reactor development in ITER and witness the crystallization of mankind's best science and technology (dream energy source).
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.
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.
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.
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.
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.
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:
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.
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.
The Advanced Plasma Science and Engineering Laboratory (Idei–Ikezoe) collaborates closely with the Laboratory of Fusion Systems Engineering (Hanada–Onchi), the Laboratory of Fusion Plasma Control Engineering (Ido–Kinoshita), and the Laboratory of Advanced Fusion Information Control Engineering (Hasegawa) to promote effective education and research. Student offices are located in the QUEST experimental building, fostering active interaction with researchers from both Japan and abroad. Why not sharpen your problem-solving skills in this excellent environment for education and research?
Faculty Affiliation: Research Institute for Applied Mechanics
Center for Advanced
Plasma Science and Engineering
Division of Steady-State Plasma Heating (Idei)
Division of Plasma Wave Engineering (Ikezoe)
Cooperative Program: Graduate School of Kyushu University
Plasma and Quantum Science
and Engineering Major
Joint Faculty: Department of Basic Engineering and College of Technology Collaboration Program