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

1. Introduction
2. Basics of Plasma Diagnostics Using Electromagnetic Waves
3. Advanced Diagnostic Technologies and Recent Developments
   1. Microwave Imaging
   2. Doppler Reflectometry
   3. Scattering Diagnostics
4. Challenges and New Approaches in ITER Experiments
   1. Issues and Development Status of Electron Density Measurements
   2. Problems and New Approaches in Electron Cyclotron Emission Diagnostics
   3. Introduction of Relativistic Effects in Electromagnetic Wave Diagnostics
5. 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.

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.

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

1. Introduction – Profile Control as a Key to Fusion Power
2. Control Objectives and Targets
3. Profile Control and Response Characteristics
4. Progress and Challenges in Real-Time Control Experiments
5. Advances and Challenges in Real-Time Data Processing
6. 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.

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.