Serviceable fusion power is a near-ideal source of effectively limitless clean energy—its high energy density, safety, and sustainability distinguish fusion from any other energy source today. Despite this, the funds required for fusion research and its highly specialized conditions of operation hinder its feasibility in the market (Cowley, 2016). However, as global energy use continues to rise, experts must look towards efficient and environmentally responsible energy sources, making the demand to harness fusion power greater than ever (Smith and Cowley, 2010).
The fusion reaction most commonly used as an energy source on Earth requires two isotopes of hydrogen fusing to form helium and one neutron. The two isotopes are deuterium (D = 2H) and tritium (T = 3H) and the reaction is shown in Equation 1. Initiating this reaction requires both highly dense gas and temperatures over 100 million degrees Celsius to enable the isotopes to overcome mutual electrostatic repulsion and allow the strong nuclear force to act and bind the two isotopes (Smith and Cowley, 2010).
However, due to tritium being unstable with a 12.32 year half-life, fusion reactors generally breed tritium using lithium and the neutron from Equation 1 in the reaction shown in Equation 2 (Cowley, 2016).
These two equations illustrate that for fusion to be successful, immense temperatures and pressures must be created to increase proton kinetic energy for sufficient probability of fusion reactions. At these conditions, matter only exists in plasma, a highly demanding state for efficient energy production (Freidberg, Mangiarotti and Minervini, 2015). In response to this, Russian physicists devised a donut-shaped magnetic confinement device named the tokamak to confine and competently control plasma for energy production (Freidberg, Mangiarotti and Minervini, 2015).
The fusion reactions within the tokamak take place within the torus, a toroidal-shaped steel vacuum chamber surrounded by two configurations of magnetic coils: poloidal and toroidal. Poloidal rings are circular rings around the perimeter of the torus and toroidal rings are larger circular rings that enclose the torus. As a consequence of our fundamental understandings of electromagnetism, we know that the poloidal and toroidal coils each create a high-intensity electric current, which then creates two perpendicular, induced magnetic fields that can safely contain the electrically charged plasma ions shown in Figure 1. As the current moves around the tokamak, a heating effect is produced, which energizes and pressurizes ions to generate collisions (El-zmeter, et al., 2017).
Figure 1: Schematic of a toroidal vacuum chamber. The torus (blue donut) and the electrically charged plasma (gray donut) are depicted. The poloidal (circular white dashed arrow) and toroidal (curved white dashed arrow) magnetic fields are shown. Due to the foundational understanding that electric currents produce perpendicular magnetic fields, these arrows are perpendicular to the physical field coils of a tokamak. Accordingly, the resultant magnetic field is helical (twisting black arrow labelled magnetic field line). Helical movement allows for long-term confinement because in a parallel field, closely orbiting particles can easily collide and fuse (Hazeltine and Prager, 2002).
The tokamak reaction is initiated by deuterium and tritium being fed into the torus at immense atmospheric pressures and temperatures (World Nuclear Association, 2021). These conditions drive the isotopes to fuse together, similar to the environment in the Sun (World Nuclear Association, 2021). The neutrons produced from the fusion reaction (Equation 1) break away from the isotopes and collide with the encompassing structure called the blanket (Buzhinskij and Sements, 1999). The blanket is typically lined with lithium, a heat-resistive element, to allow the chamber to withstand the extreme heat and allow tritium to be bred as shown in Equation 2 (El-zmeter, et al., 2017; Buzhinskij and Sements, 1999). Tokamak reactors generate about 1.5 GW of electricity, enough for more than one million people in a conventional setting (Smith and Cowley, 2010). A simplified scheme of a tokamak is shown in Figure 2.
Figure 2: Schematic of a tokamak. The toroidal field coils (blue rings) and the poloidal field coils (green column) produce perpendicular magnetic fields (blue arrow and circular green arrow, respectively). The outer poloidal field coils (gray rings) shape the plasma positioning. To produce the plasma current, a transformer action in the inner poloidal field coils (green column) is driven and then sustained by the toroidal and poloidal field coils. Deuterium and tritium are fed into the chamber (pink donut) and heated to force their fusion. Although not depicted, the blanket surrounds the reaction chamber and is the location of tritium breeding. The energy released to the blanket is extracted using a cooling circuit and then used to create steam to drive turbines for distribution of electricity to the grid (De Tommasi, 2018).
Despite its high energy density and capacity to produce no greenhouse gases or radioactive waste, the future of fusion remains unclear. The tokamak layout may be proven, but it is incredibly difficult to make this design economically viable. However, with further research and innovation, we can work around these complications and potentially use fusion as a launching pad towards an endless supply of clean energy.
Works Cited:
Buzhinskij, O.I. and Sements, Yu.M., 1999. Thick boron carbide coatings for protection of tokamak first wall and divertor. Fusion Engineering and Design, 45(4), pp.343–360. https://doi.org/10.1016/S0920-3796(99)00007-1.
Cowley, S.C., 2016. The quest for fusion power. Nature Physics, 12(5), pp.384–386. https://doi.org/10.1038/nphys3719.
De Tommasi, G., 2018. Plasma Magnetic Control in Tokamak Devices. Journal of Fusion Energy, 38, pp.406–436. https://doi.org/10.1007/s10894-018-0162-5.
El-zmeter, N., Schmiga, B., Boyd-Weetman, B. and Murphy, A., 2017. Analysis of Tokamak fusion device parameters affecting the efficiency of Tokamak operation. PAM Review Energy Science & Technology, 4, pp.87–102. https://doi.org/10.5130/pamr.v4i0.1444.
Freidberg, J., Mangiarotti, F. and Minervini, J., 2015. Designing a tokamak fusion reactor—How does plasma physics fit in? Physics of Plasmas, 22(7), pp.1–17. https://doi.org/10.1063/1.4923266.
Hazeltine, R.D. and Prager, S.C., 2002. New Physics in Fusion Plasma Confinement. Physics Today, 55(7), pp.30–36. https://doi.org/10.1063/1.1506748.
Smith, C.L. and Cowley, S., 2010. The path to fusion power. Philosophical transactions. Series A, Mathematical, physical, and engineering sciences, 368(1914), pp.1091–1108. https://doi.org/10.1098/rsta.2009.0216.
World Nuclear Association, 2021. Nuclear Fusion Power. [online] Nuclear Fusion Power. Available at: <https://world-nuclear.org/information-library/current-and-future-generation/nuclear-fusion-power.aspx> [Accessed 18 Nov. 2021].