Frontiers of Nuclear Fusion Power Generation

Principles of Tokamak-Type Nuclear Fusion Power Generation

The tokamak is the most extensively researched and developed representative of the magnetic confinement fusion approach, and it has been adopted for the International Thermonuclear Experimental Reactor (ITER).

A toroidal magnetic field is generated by passing current through coils arranged around a toroidal (doughnut-shaped) vacuum vessel called a torus. In addition, a large current flowing through the plasma itself generates a poloidal magnetic field. By combining these two magnetic fields, a powerful helical magnetic cage is formed to confine the ultra-high-temperature plasma.

One of the most critical components in tokamak operation is the divertor. The divertor functions as an "exhaust outlet" that removes helium ash generated by fusion reactions and metallic impurities sputtered from the reactor wall. By controlling the magnetic field, high-temperature plasma is intentionally separated and guided toward the divertor plates, thereby mitigating excessive heat concentration.

Because the divertor is exposed to extreme thermal loads reaching several thousand degrees Celsius - the harshest environment inside the reactor - next-generation fusion reactors employ high-melting-point tungsten as a plasma-facing material, along with advanced cooling structures that circulate coolant water at high speed.

Measurement Methods and Diagnostic Instruments for Tokamak Fusion Reactors

Faraday rotation interferometer systems are used to precisely measure the plasma's poloidal magnetic field and electron density. These systems detect phase shifts and polarization rotation (Faraday effect) in light transmitted through the plasma.

This system uses far-infrared (FIR) lasers that emit submillimeter-wave radiation in the several-hundred-micrometer wavelength range. To optically pump the laser medium gas (such as ¹³CH₃OH high-power and highly stable carbon dioxide (CO2) lasers operating at wavelengths such as 10.6 μm and 9.6 μm are essential.

To accurately reconstruct electron density distributions, heterodyne interferometers using two FIR lasers with different oscillation frequencies are commonly employed.

However, this technology involves severe technical challenges. Since the molecular absorption lines of FIR lasers are extremely narrow, even a frequency fluctuation of only a few MHz in the CO2 laser can significantly reduce output power. Therefore, stabilization of the pump offset frequency through resonator length control using piezoelectric elements is indispensable.

In addition, advanced optical techniques are employed to suppress backtalk (optical feedback) caused by interference between the resonators of the CO2 laser and the FIR laser, such as intentionally introducing the excitation beam at an oblique angle.

Principles of Helical-Type Nuclear Fusion Power Generation

The helical type is another magnetic confinement fusion approach in which Japan is a global leader. A representative example is the Large Helical Device (LHD) operated by the National Institute for Fusion Science (NIFS).

The biggest difference from the tokamak is that "no plasma current is driven through the plasma itself."

In helical systems, complex twisted external coils called helical coils are arranged around the toroidal vacuum vessel, forming a three-dimensional magnetic cage using only externally generated magnetic fields.

Since plasma current is not required, helical systems avoid the catastrophic instability known as disruption, which occurs when plasma current suddenly terminates. This gives them a major advantage for extremely long-duration steady-state operation.

Measurement Methods and Diagnostic Instruments for Helical Fusion Reactors

Heavy Ion Beam Probes (HIBP) are used to investigate the complex plasma interior of helical reactors. These systems inject heavy ion beams into the plasma to precisely measure local plasma potentials and fluctuations.

Because helical devices have highly complex magnetic field structures, the beam trajectory can deviate significantly in the toroidal direction as well. This issue is addressed using octupole electrostatic deflectors.

By controlling the beam trajectory at the injection port and placing additional octupole electrostatic deflectors on the detection side, active trajectory control is achieved to compensate for deviations in the incident angle entering the energy analyzer.

Research is also being conducted into applying this technique to two-dimensional potential distribution measurements by rapidly sweeping the deflector voltage while maintaining a fixed beam energy.

In addition, multi-grid electrodes (multi-grid probes) are used to evaluate plasma-wall interactions in peripheral plasma regions such as divertors and edge plasmas.

By utilizing the plasma sheath potential formed around the probe surface and applying positive voltage to the repeller grid, ion temperature can be calculated from the slope of the ion I-V characteristics.

These systems are indispensable diagnostic tools for detecting high-energy ions escaping confinement and predicting thermal loads.

Power Supplies for Helical Fusion Reactors

Advanced plasma diagnostic systems in helical fusion devices cannot function without high-performance power supply technologies.

The octupole electrostatic deflectors used in HIBP systems require highly responsive and precise high-voltage power supplies capable of correcting complex three-dimensional beam trajectories and rapidly sweeping the beam for two-dimensional profile measurements.

In addition, multi-grid plasma diagnostics require accurately controlled bias voltages ranging from several hundred volts to ± several hundred volts for each grid electrode.

In the intense electromagnetic noise environment near fusion reactors, stable low-noise power supply systems capable of rapidly sweeping repeller-grid voltages are essential for accurately obtaining ion I-V characteristics from extremely small ion currents.

The reliability of these power systems is directly linked to advances in plasma physics research.

Principles of Laser Fusion Power Generation

Laser fusion is an inertial confinement fusion approach that confines plasma using inertia rather than magnetic fields.

A spherical fuel pellet several millimeters in diameter containing deuterium and tritium is irradiated uniformly and simultaneously from all directions by extremely high-power lasers.

The laser energy instantly converts the pellet surface into plasma, causing it to explosively expand outward in a process called ablation. The tremendous reaction force generated by this expansion compresses the internal fuel toward the center at extremely high density. This process is known as implosion.

The central region reaches an ultra-high-temperature and ultra-high-density state, enabling fusion reactions to occur rapidly during the brief moment before the fuel disperses due to inertia.

At the National Ignition Facility (NIF) in the United States, "ignition" (net energy gain exceeding the input laser energy) has been successfully demonstrated, attracting worldwide attention.

Measurement Methods and Diagnostic Instruments for Laser Fusion

Unlike magnetic confinement plasmas, which can be sustained for relatively long durations ranging from seconds to minutes, laser fusion implosions occur within extreme temporal and spatial scales - from nanoseconds (10⁻⁹ seconds) to picoseconds (10⁻¹² seconds), and within spatial dimensions of only several tens of micrometers.

As a result, completely different measurement approaches are required.

Ultra-high-speed imaging instruments such as X-ray streak cameras and X-ray framing cameras are used to capture the symmetry and behavior of the implosion process frame by frame.

In addition, neutron time-of-flight (TOF) measurement systems are used to determine neutron yield and estimate plasma ion temperature from neutron velocity distributions.

These instruments must operate in perfect synchronization with laser firing timing while minimizing timing jitter.