The tokamak is today’s most advanced and best investigated fusion device design. It is a torus-shaped vacuum chamber surrounded by magnetic coils, which create a toroidal magnetic field.
A second set of coils is centred on the axis, or pole of the torus (the hole in the donut). This poloidal magnetic field adds a vertical component to the magnetic field, which has the effect of giving the magnetic field throughout the vessel a twist. This circulates the particles that have drifted towards the outside of the ring back into the centre, preventing the plasma from escaping.
A drawback of the tokamak concept is that it has to operate in pulsed mode. This is because the plasma current is induced by an increasing current in the poloidal coils; once the current reaches its maximum value, then the induction, and consequently the pulse will cease. Furthermore, a tokamak needs very strong toroidal fields and the strong currents flowing through the magnetic coils generate a lot of heat. At JET, for instance, much of the energy used for an experiment is spent on cooling the coils. A fusion power plant based on the tokamak design will only operate efficiently if it employs superconducting magnet coils. Today, two tokamak experiments utilise superconductors, Tore Supra in France and KSTAR in Korea.
A spherical tokamak works in the same way as conventional tokamaks, but it holds the plasma in tighter magnetic fields, shaping it more like a cored apple than a donut. Compared to conventional tokamaks, they achieve higher plasma pressure for a given magnetic field. Because the power output is correlated with the plasma pressure, a spherical tokamak yields more power. It does not require superconducting magnets, and can be smaller in size than conventional tokamaks. However, its capacity to be upscaled may be limited because the narrow core restricts the size of the magnetic coils and amount of cooling possible.
Culham Centre for Fusion Energy (CCFE) in the UK operates the spherical tokamak MAST.
Reverse Field Pinch devices
Reversed Field Pinch (RFP) devices also derive from the tokamak concept, but their toroidal magnetic field reverses direction as one moves further from the torus’ centre. RFPs yield the same power density as tokamaks with smaller toroidal fields. RFPs could operate without superconducting magnets. Moreover, experiments show that an RFP can generate enough heat from the plasma current to omit external heating systems. However, RFPs are more susceptible to non-linear effects and turbulence. Large efforts are being made to develop methods that allow stable plasma operations. For instance, the RFP experiment in Padua, Italy, investigates the use of active feedback systems to solve this issue.