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| Fig. 1: Basic components of the toroidal tokamak. [3] (Source: Wikimedia Commons) |
For several decades, humanity has been on a quest for a clean, sustainable energy source, promising to harness the same processes that power stars namely, nuclear fusion. At a high level, in a fusion process, two lighter atomic nuclei (often Deuterium and Tritium) combine to form a heavier nucleus (e.g. the Helium-4 nucleus) in a reaction that produces neutrons and releases a tremendous amount of energy. This energy can be harnessed using devices called "fusion reactors". As of 2024, while no fusion reactor has reached net positive power, Magnetic Confinement Fusion (MCF) offers an interesting possibility of a technique that may work, achieving high values up to Q = 0.67 in the past. [1] This report will introduce this technique, give an example of a practical design based on MCF, talk about its limitations and look forward to promising avenues of research for building the first net power fusion reactor.
The process of fusing two lighter nuclei is governed by the strong nuclear force and Coulombic repulsion, with the former overcoming the latter at extremely high temperatures and pressures, enabling the nuclei to fuse. These temperatures and pressures range at around 100 million degrees Celsius and hundreds of billions of Pascals respectively this temperature range ensures that the kinetic energy of the nuclei is sufficient to come into close proximity such that the strong nuclear force overcomes their electrostatic repulsion. Operating with such extremely hot plasma, any materials would vaporize upon contact. So, Magnetic Confinement Fusion aims to use magnetic fields, which guide and contain the ionized gasses of Deuterium and Tritium (in the form of hot plasma) to prevent contact with the walls of the nuclear reactor.
Another condition for MCF to occur is called the Lawson criterion. The criterion specifies the minimum required value for the product of the ionized plasmas electron density (ne), the plasma temperature T, and the energy confinement time (eT) that leads to net energy output.
Utilizing the physics of charged particle motion, a popular MCF-based design for a reactor is called a Tokamak, a Russian acronym for 'toroidal chamber with magnetic coils'. Composed of several thick walls of carbon composites and tungsten, Tokamaks achieve extremely high temperatures by employing ohmic heating, nuclear beam injections to ionize the plasma using high-energy atoms, and radiofrequency heating. A toroidal magnetic field is generated by superconducting coils wrapped around the torus, confining the plasma within the toroidal chamber the field lines run parallel to the direction of the plasma current, helping to minimize radial losses of charged particles. To capture the kinetic energy of neutrons (which carries over 80% of the reactions total energy), Tokamaks leverage a lithium or lithium-bearing blanket surrounding the plasma chamber, surrounded by coolants and allows for tritium breeding via the following reactions:
| 6Li + n | → | 4He + 3T |
| 7Li + n | → | 4He + 3T + n |
There are numerous ambitious projects using Tokamaks to build net power nuclear reactors today, such as the International Thermonuclear Experimental Reactor (ITER), Joint European Torus (JET), ST40 at Tokamak Energy Ltd in Oxford, UK, and the China Fusion Engineering Test Reactor (CFETR), aiming to achieve Q-values (the ratio of fusion power output to input power) of greater than 1, sometimes even as high as aiming for Q ≥ 10. Today, JET holds the record for producing 69 MJ of energy output over a 5-second period. [2]
Tokamaks and designs employing Magnetic Confinement Fusion still face significant challenges in the goal of reaching net power, i.e. a Q > 1 value. Tokamaks face various instability modes, such as kink, ballooning, and tearing modes, which can lead to plasma disruption; neutron irradiation can affect the material durability of the first-wall and divertor materials of the lithium blanket, or even cause material activation, making reactor components radioactive and brittle (via neutron embrittlement).
A number of core technical challenges need to be solved in order to reach the goal of producing a sustainable, net power fusion reactor. From material and structural challenges to efficient energy capture (e.g. via innovations in supercritical CO2 cycles), magnetic confinement fusion today remains a challenging yet the most promising and ambitious goal to produce a clean, sustainable energy source for humanity's future.
© Kushal Thaman. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. L. Watkins et al., "Physics of High Performance JET Plasmas in DT," Nucl. Fusion 39, 1227 (1999).
[2] E. Gibney, "Fusion Reactors Smashes Energy Record," Nature 602, 371 (2022).
[3] S. Li et al., "Optimal Tracking for a Divergent-Type Parabolic PDF System in Current Profile Control, Abstr. Appl. Anal. 2014, 940945 (2014).
