What Are Fusion Reactors?

Fusion reactors are devices designed to release energy by fusing light atomic nuclei, typically isotopes of hydrogen (deuterium and tritium), at temperatures and pressures high enough for the nuclei to overcome their electrostatic repulsion and combine. The fusion of deuterium and tritium produces helium and a high-energy neutron, releasing approximately 17.6 MeV per reaction. Because the fuel is derived from seawater (deuterium) and lithium (for breeding tritium), and because the process produces no long-lived radioactive waste and no carbon emissions, fusion has long been pursued as a near-inexhaustible clean energy source. Reaching sustained net energy gain remains one of the great engineering challenges of the twenty-first century.

Magnetic Confinement: Tokamaks and Stellarators

The dominant approach to fusion is magnetic confinement, which uses strong magnetic fields to hold a hot plasma away from material walls. At the required temperatures (exceeding 100 million degrees Celsius), plasma would destroy any solid container on contact.

The tokamak is the most extensively studied magnetic confinement geometry. It uses a toroidal (donut-shaped) vacuum vessel surrounded by superconducting coils that produce a strong toroidal magnetic field. A transformer-driven plasma current adds a poloidal field component, creating helical field lines that stabilize the plasma. The tokamak concept has been developed through decades of experiments culminating in ITER (International Thermonuclear Experimental Reactor), a 35-nation collaboration under construction in southern France. ITER is designed to produce 500 MW of fusion power from 50 MW of input heating, demonstrating a Q factor of ten. Official engineering documentation and project status are published through the ITER Organization.

Stellarators achieve plasma confinement using only external coils, with no induced plasma current. The complex three-dimensional coil geometries required were historically difficult to engineer, but advances in computer-aided design and high-precision manufacturing have enabled modern stellarators such as Wendelstein 7-X in Germany. Because they do not rely on a plasma current, stellarators avoid certain instabilities (disruptions) that limit tokamak operation and are considered promising for steady-state reactor operation.

Inertial Confinement Fusion

Inertial confinement fusion (ICF) takes a radically different approach: instead of confining plasma for seconds or longer, it compresses a small capsule of deuterium-tritium fuel so rapidly and to such high density that fusion occurs before the plasma can expand and disassemble itself. Energy is delivered to the capsule surface (the ablator) by high-power laser beams or X-rays generated inside a gold hohlraum. The resulting ablation pressure implodes the fuel to densities a thousand times that of liquid water. In 2022, the National Ignition Facility at Lawrence Livermore National Laboratory achieved fusion ignition, producing more energy from fusion than the laser energy delivered to the target, a milestone described in detail in Nature publications on NIF ignition results. The IEEE Nuclear and Plasma Sciences Society covers both ICF and magnetic confinement research.

Plasma Confinement Physics

Achieving and sustaining the plasma conditions needed for fusion requires managing a rich set of instabilities. Magnetohydrodynamic (MHD) modes, disruptions, edge-localized modes (ELMs), and turbulent transport all degrade plasma confinement. Active control systems using real-time magnetic coil adjustments, neutral beam injection for heating and fueling, and radio-frequency wave launchers work together to maintain plasma stability. Superconducting magnet technology, particularly high-temperature superconductors (HTS) such as REBCO tape, is enabling a new generation of compact high-field tokamaks from private ventures like Commonwealth Fusion Systems.

Applications

Fusion reactor technology, once mature, is expected to transform energy and enable related capabilities:

• Baseload electricity generation: Fusion power plants could provide continuous, carbon-free electricity with no intermittency, complementing renewable sources.
• Hydrogen production: Fusion heat could drive thermochemical or electrolytic hydrogen production for industrial decarbonization.
• Neutron sources: Fusion-generated neutrons can breed tritium fuel from lithium blankets and are also valuable for materials testing and nuclear waste transmutation.
• Space propulsion: Compact fusion reactors are studied as high-thrust, high-specific-impulse propulsion systems for deep-space missions.
• Medical isotope production: Fusion neutron fluxes could produce medical radioisotopes currently supplied by aging fission research reactors.
• Materials science: Fusion test facilities expose structural materials to neutron fluences representative of reactor environments for qualification studies.