The Tokamak Fusion Reactor

Following the advancements achieved recently by the JET laboratory in Oxfordshire, humanity stands one step closer to commercialising nuclear fusion power. JET employs the magnetic confinement technique to support fusion, achieved by a device called a tokamak. The principles of tokamak reactors are outlined in this article.

Why Fusion is Hard

Nuclear fusion involves the formation of a single, larger nucleus from two smaller nuclei. An extra neutron is emitted as a byproduct of this process and carries large amounts of energy. A typical fusion reaction is displayed in Figure 1.

Figure 1: Two nuclei merge to form a single, larger nucleus in a fusion reaction. An extra neutron, that carries large amounts of energy, is emitted in the process.

The energy carried away by the neutron in a fusion reaction is one million times larger than typical chemical reaction energies. The production potential for fusion is consequently immense.

So, what’s the catch? It turns out to be incredibly difficult to make fusion happen in the first place. Nuclei are positively charged, meaning they repel one another. We need to input a huge amount of energy to induce fusion, meaning reactions will only occur under extreme conditions, such as temperatures exceeding 10 million °C.

Why Fusion is a Great Energy Source

Fusion research is difficult and requires astronomical levels of funding. How do we justify investing billions of pounds into reactor experiments each year?

There are a number of reasons fusion is an excellent energy production method:

  • Energy output and efficiencey of a fusion reaction provides huge production potential.
  • Fusion fuel can be chemically extracted from seawater with relative ease.
  • Only byproduct of fusion is Helium which, being inert and unreactive, doesn’t contribute to atmospheric degradation.
  • Removes the possibility of meltdown that plauges nuclear fission reactors.

Magnetic Confinement Fusion

Nuclei within a fusion reactor are required to possess high energies but also need to remain in the reactor long enough to provide a steady power output. Unfortunately, particles with high energies are not awfully inclined to stay in one place.

Charged particles such as nuclei experience a force within magnetic fields. Magnetic confinement fusion (MCF) confines nuclei within a reactor using this effect. The geometry of the field defines the reactor. A pair of simplistic field models are presented in Figure 2.

Figure 2: The theta and z pinch field geometries confine particles magnetically. These simplistic models are plagued by stability issues and end losses. Something more advanced is required.

Within a theta-pinch, field lines propagate through the reactor, causing nuclear gyration. Alternatively, field lines can be wrapped around the circumference of the device, causing nuclei to stream through the reactor within a z-pinch.

Whilst achieving confinement to some level, these simple models are plagued by stability issues and end losses. It turns out we require a combination of the two field geometries to achieve confinement for long periods.

Tokamak: Design and Function

The tokamak utilises a pair of magnetic fields in order to maintain confinement. The layout of a typical tokamak reactor is shown in Figure 3.

The toroidal field coils produce the toroidal field that circles the device. Comparable to the theta-pinch, field lines stream through the device, causing nuclei to circle around their centre.

In addition, the central solenoid creates the poloidal field which circles the device cross-section. Similarly to the z-pinch, the field lines cause nuclei to stream through the device. The toroidal geometry of the reactor also prevents end losses.

Other Methods and the Future

MCF is currently the most researched method of achieving fusion. Other methodologies include inertial confinement fusion (ICF), wherein tiny samples of nuclei are compressed and ignited using high-intensity lasers. ICF presently provides the fiercest competition to teams developing tokamak reactors.

The recent results published by JET are important as they motivate upgrades to a larger scale project, ITER. ITER is planning to begin experiments in 2025, and will likely shatter the record for confinement time, taking further steps towards a world powered by fusion.

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