Sadly, after forty years of groundbreaking science, it was announced in 2023 that the Joint European Torus in Oxfordshire, England, would be decommissioned. JET is an experimental nuclear fusion reactor, the goal of which was to help advance the dream of producing clean, efficient energy that doesn't impact the environment.

Traditional thermal nuclear power stations take their energy from fission reactions, where unstable uranium nuclei split into smaller nuclei with a release of energy. On the other hand, the purpose of the JET tokamak, a donut shaped magnetic plasma chamber originally designed in the Soviet Union in the 1960s, was to harness the power of nuclear fusion, which is what powers stars like the Sun.

Fusion is the joining together of two light nuclei to produce a heavier nucleus with, again, the release of binding energy; the energy which is equivalent to the mass defect (or the difference between the mass of the new nucleus and the sum of its constituent parts).

But why try to design a technologically difficult and potentially expensive fusion power source when the world already has many operational fission plants? Well, apart from the obvious advantages of both types of nuclear energy production (the rare combination with reliability and a lack of greenhouse gas emissions), energy from nuclear fission does come with some dangers and potential cost to the environment; the waste products of fission reactors are highly radioactive materials and there is the risk of an uncontrolled runaway chain reaction, leading to a meltdown or catastrophic failure. With a fusion plasma chamber, there is no chance of a runaway reaction, there are no inherently nasty by-products from the process and just one kilogram of fusion fuel is equivalent to ten million kilograms of fossil fuel in terms of energy production: E = mc² in action!

So, how does a tokamak work? To artificially achieve the fusion of light nuclei (usually a combination of deuterium and tritium), the fuel gas needs to be heated to a temperature of a hundred million degrees, thus producing a plasma with sufficient energy that the positively charged nuclei can overcome their mutual electrostatic repulsion and get close enough that the strong nuclear force takes over. The main energy source that heats the plasma comes from a central coil which acts as the primary winding in a transformer. A current is induced in the resistant gas, whose moving charged particles act as the secondary winding. One of the main inherent problems with maintaining such an energetic environment is preventing contamination and cooling of the plasma. The ionized gas, which has a density a million times less than air, is kept away from the internal surface of the chamber by a powerful toroidal magnetic “bottle”, which forces the charged particles to travel in a spiral.

Along with the application of microwaves and the high-speed injection of neutral hydrogen atoms, the confining magnetic field also contributes to heating the plasma by compressing it. Like a fission reactor, the useful energy is extracted from the kinetic energy of the escaping neutrons which are produced alongside the helium nuclei formed in the fusion reactions.

The JET tokamak has produced some very promising results over its decades of use. However, we are a long way off getting a commercially useful amount of energy out of a fusion reactor (i.e. more than the vast amount of energy required to get the fuel gas up to operating temperature) and then confining it. In its latter years, JET has been used experimentally to determine the best operating materials and conditions for its follow-up experiment, called the International Thermonuclear Experimental Reactor (ITER), which is to come online in France in several years.

The fate of JET appears to be sealed. The decommissioning process will take several years and, whilst no radioactive matter is formed in the fusion process, unstable nuclei may well have been formed by neutrons combining with elements in the fabric of the experiment. Also, tritium is itself radioactive. However, at the time of writing, a body of the physics community has requested that the facility be retained, at least until ITER is up and running, so right now it's difficult to predict what the outcome for this remarkable experimental facility will be.

Images

• JET: By Image sourced from the EFDA-JET public relations page. High resolution at [1]. Copyright is held by EFDA-JET., Fair use, https://en.wikipedia.org/w/index.php?curid=12421973
• Fusion reaction: By Rfassbind - Own work., Public Domain, https://commons.wikimedia.org/w/index.php?curid=32503440
• ITER location and participating regions: By Rfassbind - Own work., Public Domain, https://commons.wikimedia.org/w/index.php?curid=32503440