What Are Tokamaks?

Tokamaks are doughnut-shaped magnetic confinement devices used to confine high-temperature plasma for nuclear fusion research. A tokamak (from the Russian acronym for "toroidal chamber with magnetic coils") confines plasma within a toroidal vacuum vessel using a combination of external magnetic field coils and a current driven through the plasma itself. The resulting helical magnetic field structure prevents the plasma from contacting the vessel walls, enabling plasma temperatures that exceed 100 million degrees Celsius, well above the threshold for deuterium-tritium fusion. Conceived by Soviet physicists Igor Tamm and Andrei Sakharov in the 1950s and first described to international audiences in 1968, tokamaks now represent the most widely studied approach to controlled thermonuclear fusion.

Roughly 60 tokamaks operate worldwide, from compact university research machines with plasma volumes of a few liters to large experimental facilities such as JET in the United Kingdom and KSTAR in South Korea. The international ITER project in Cadarache, France, is under construction with a plasma volume of 840 cubic meters and is designed to demonstrate a fusion gain factor (Q) of 10, meaning ten units of fusion power produced per unit of heating power supplied.

Magnetic Confinement in Tokamaks

Confining fusion plasma requires field configurations strong enough to counteract both the thermal pressure of the plasma and the instabilities that can cause it to drift toward the vessel walls. Tokamaks address this with three overlapping field components: a toroidal field produced by external coils wrapped around the torus, a poloidal field from a central solenoid and poloidal coils, and a field generated by the multi-million-ampere current flowing through the plasma itself. The U.S. Department of Energy's overview of tokamaks explains how these three components combine to produce nested magnetic flux surfaces that confine the plasma. Magnetohydrodynamic stability theory, developed in parallel with experimental work, provides the theoretical basis for the safety factor and the q-profile parameters that operators monitor to avoid disruptive instabilities.

Plasma Control and Stability

Keeping a tokamak plasma stable over the timescales needed for significant energy output requires continuous, real-time control of the magnetic field geometry. Plasma shape, position, and current profile must all be maintained within narrow tolerances, and disruptions, sudden losses of plasma confinement, must be detected and mitigated before they damage the vessel. Research published in Nature on reinforcement learning for plasma control demonstrated that neural-network-based controllers trained with deep reinforcement learning can maintain complex plasma configurations in a real tokamak without human intervention, a development that points toward automated operation of future fusion power plants. Plasma-facing components, particularly the divertor that exhausts heat and particles, represent a separate engineering challenge because they must withstand heat fluxes comparable to those at the surface of the sun.

The Path to Fusion Power

Tokamaks have shown steady progress in plasma performance over five decades, measured by the triple product of density, temperature, and confinement time. JET's 2022 record of 59 megajoules of fusion energy in a single pulse, reported by the IAEA bulletin on magnetic fusion confinement, confirmed the feasibility of significant fusion output from present-day machines. ITER is intended to resolve the remaining physics and engineering questions, paving the way for demonstration power plants. Several private ventures, including Commonwealth Fusion Systems, TAE Technologies, and Tokamak Energy, are pursuing compact tokamak designs using high-field superconducting magnets to accelerate the timeline to commercial fusion.

Applications

Tokamaks have applications in a wide range of fields, including:

• Fusion energy research and demonstration power plant design
• High-temperature plasma physics, transport, and turbulence studies
• Development and testing of superconducting magnet systems
• Neutron source technology for materials testing and medical isotope production
• Training programs for nuclear engineers and plasma physicists at national laboratories