The story of nuclear fusion starts in 1920 when Eddington proposed that this is the source of energy in the sun. From 1950, there have been numerous devices constructed to test concepts of fusion reactors. Most of these devices have used Magnetic Confinement Fusion (MCF), with a  few using the technique of Inertial Confinement Fusion (ICF). The most successful operational devices have been MCF machines called Tokamak, a concept which originated in the Soviet Union. These were: the Princeton TFR tokamak from 1993 and the European JET tokamak from 1997. Since then, the mainstream approach to fusion, worldwide, has been with Tokamaks.

In 2007, construction started for the international ITER tokamak, with expectations to have the first Deuterium-Tritium operations from 2035. After ITER will come actual demonstration reactors capable of producing electricity.


Tokamak Fusion Reactors

The confinement of the hot plasma in a Tokamaks is achieved with complex magnetic field which in recent devices are produced by superconducting coils.

The magnetic fields induce currents in the plasma which cause ohmic heating and additional heating is provided by external devices such as neutral beam injectors and electromagnetic wave antennae.

The illustration below shows the principal components of a tokamak fusion reactor, followed by a schematic indicating component functionality outwards from the plasma.

The system contains the following vital components:

  • Superconducting Magnets - Toroidal; Poloidal and Solenoidal.
  • Vacuum Vessel
  • First Wall (FW) - absorbing part of the heat from neutrons
  • Blanket - breeding tritium and absorbing rest of heat
  • Divertor - exiting plasma at the end of confinement
  • Shield - absorbing remaining neutrons to project magnet coils
  • RF Antenna - providing heating for plasma
  • Cryostat - helium cooling to maintain superconductivity

The enormous flux of neutrons from the Deuterium-Tritium reactions in the hot plasma pass through the plasma facing first wall and are stopped in the "Blanket" in which coolant is circulated to drive a turbine for electricity generation. The blanket also has the essential task to breed tritium fuel which is one of the most difficult developments necessary for a successful commercial fusion reactor. Beyond, the shield must reduce as much as possible the neutron flux escaping the blanket to avoid radiation damage and activation in the magnet coils outside the vacuum vessel.

An unavoidable feature of fusion reactors using the D-T reaction, is that the inner components will rapidly become radioactive due to Neutron Activation  and suffer Radiation Damage from the enormous neutron flux emitted from the fusion reactions in the plasma. The plasma facing components such as the First Wall and the Divertor must be replaced because of the radiation damage and doing this will require complicated remote handling.

The image below indicates the complexity of the components required for such a remote handling system

Surrounding all these components is a massive concrete "Bioshield" to contain any escaped radioactivity and toxic material from any accidents. Beyond the bioshield it the complex of building with diverse functions.

The historical progression of Tokamaks has been towards larger and larger machines with the objective of making more stable plasmas and ultimately a power station with competitive electricity output. The figure below illustrates this progression.


ITER: in construction 

The ITER project is the latest in the long series of increasingly large magnetic containment devices. Construction started in 2007 and was 77% complete by the end of March 2022. The ITER machine is described in detail on the project official website: Machine .

The objectives of ITER are 

  • Produce 500 MW of thermal fusion power
  • Demonstrate the integrated operation of technologies for a fusion power plant
  • Achieve a deuterium-tritium plasma in which the reaction is sustained through internal heating
  •  Test tritium breeding
  •  Demonstrate the safety characteristics of a fusion device

ITER will never send electricity to the grid because there is no system to make the heat from the plasma work a turbine.

The ITER tokamak will have a toroid radius of 6 meters and weigh about 25000 tons.


DEMO: next step

The objective of the next phase of fusion R&D, is to construct prototype fusion power plants with the extraction of energy from the fusion reactor to generate electricity.

While ITER is an international collaboration project, the next stage to build an actual prototype fusion reactor will consist of separate machines in different countries. Most use the word "DEMO" in their description but have different designs. The blog: There Will Be No International DEMO Reactor That Follows ITER describes the international situation for DEMO projects and the figure below shows their various schedules. Many optimistically predict DEMO operations by 2050 and commercial power plants after 2060.

The different DEMO projects have different design feature. The EU DEMO design is for a radius of 9 meters and 2 GW of fusion power, making a reactor volume about three times that of ITER.