The Lawson criterion can be satisfied in two fundamentally different ways. One is to confine relatively low-density plasma for a comparatively long energy confinement time (n ≈ 10²⁰ m⁻³, τ_E of the order of seconds); the other is to compress the fuel to extremely high density for a very short time. These two approaches form the basis of magnetic and inertial confinement fusion.

Magnetic Confinement

Tokamak.

Magnetic confinement uses magnetic fields to hold hot plasma away from material walls while it is heated to fusion temperatures. Because no material could withstand direct contact with plasma at tens or hundreds of millions of kelvin, magnetic fields provide a contactless means of confinement. Charged particles move in helical paths around magnetic field lines. To prevent losses from the ends of an open device, magnetic mirrors can be used, while closed magnetic configurations avoid open ends altogether.

Magnetic mirrors create stronger magnetic fields at both ends of a confinement region. Under suitable conditions, charged particles approaching these regions are reflected back towards the centre. However, the mirror effect works only for particles within a certain range of velocities and pitch angles, so some particles escape and magnetic-mirror systems are inherently leaky.

Closed magnetic surfaces avoid the open ends of a magnetic mirror. A toroidal magnetic field can be produced by toroidal field coils, allowing charged particles to circulate around the torus. Tokamaks and stellarators are based on this general idea. However, a simple toroidal magnetic field alone does not provide adequate confinement. Because the toroidal magnetic field is stronger on the inner side of the torus than on the outer side, charged particles experience drifts that would gradually move them out of the plasma. A helical twist of the magnetic field makes particles sample both the inner and outer regions of the torus, helping to compensate these drifts and improve confinement.

Stellarator.

Stellarators create the required three-dimensional helical magnetic field entirely with external magnetic coils, often of complex shapes. Because they do not rely on a large transformer-driven plasma current, they can in principle operate continuously.

Tokamaks combine the toroidal magnetic field from external coils with a poloidal field generated largely by an electric current flowing through the plasma. In conventional operation, this plasma current is induced transformer-like by the central solenoid. Because inductive current drive is inherently pulsed, additional non-inductive current-drive methods are required for steady-state tokamak operation.

Spheromaks and field-reversed configurations (FRCs) are compact toroidal plasma configurations in which much of the confining magnetic field is produced by currents within the plasma itself. They offer relatively simple geometries but face significant challenges in plasma stability and confinement. Both concepts have been investigated as possible approaches to fusion energy.

In a Z-pinch, a strong electric current flows through the plasma and generates an azimuthal magnetic field. The resulting magnetic force compresses, or “pinches”, the plasma towards the axis.

Z-pinch.

Inertial confinement fusion at the National Ignition Facility.

Inertial Confinement

Inertial confinement fusion compresses a small amount of fuel to extremely high density and temperature for a very short time. The fuel is not held by an external confinement field; instead, its own inertia keeps it together long enough for fusion reactions to occur before the target disassembles. A small spherical capsule containing cryogenic D–T fuel is compressed symmetrically from all sides until a central hot spot reaches conditions for ignition.

Laser-driven inertial fusion can use either direct or indirect drive. In direct drive, laser beams illuminate the fuel capsule directly; ablation of its outer surface drives the capsule inward. In indirect drive, laser beams enter a hohlraum, where they heat the inner walls and generate X-rays that symmetrically compress the capsule.

Alternative Types of Confinement

Fusor.

A fusor is a type of inertial electrostatic confinement where ions are accelerated by electric potential toward the centre of the device where they can fuse.

A thermonuclear weapon also uses inertial compression, but it is obviously unsuitable for controlled energy production. In such a device, energy from a fission explosion is used to compress and heat the fusion fuel.

Gravitational confinement powers fusion in stars, whose enormous masses create the pressure and temperature required in their cores. It is not practical for terrestrial fusion devices.

Muon-catalysed fusion is an experimentally demonstrated low-temperature fusion process. If an electron in a hydrogen isotope is replaced by a muon, which is about 207 times more massive than an electron, the resulting muonic molecule is much smaller and its nuclei can approach closely enough for fusion to occur. However, producing muons requires far more energy than can presently be recovered from the fusion reactions they catalyse, so the process is not a practical energy source.

Bubble fusion, or sonofusion, was proposed as fusion occurring inside rapidly collapsing bubbles produced by acoustic cavitation in a liquid. Claims of fusion under these conditions have not been reproducibly demonstrated.