Authors: Neel Gonugunta, Ikshika Pant, and Jake Weiner
Mentor: Dr. Kay Song (DPhil, University of Oxford). Dr. Song is currently a post-doctoral researcher in the Department of Engineering Science at the University of Oxford with research specialization in Nuclear Physics.
Abstract
With the increasing costs of fuel extraction and growing energy demand of mankind, an alternative and high-density source of power is crucially needed. The variable nature of renewable energy sources like solar and wind energy require the supplementation of an alternate energy source that can provide continuous and dispatchable baseload power. Nuclear fusion is a promising candidate to meet these needs. Currently, there are two main methods of implementation: magnetic confinement fusion (MCF), including tokamaks and stellarators, and inertial confinement fusion (ICF). Tokamak reactor designs are currently the front runner, with projects like JET and ITER established in the hopes of soon developing a commercially viable energy distribution project. Stellarators are devices that are relatively new to the field of nuclear fusion research but hold great promise if actualized. They offer greater plasma stability and design flexibility, though with the drawback of being more technically complex and costly. ICF devices, such as NIF, hold great potential but the issue of sustained fusion reactions remain a great hinderance. Projects like ITER and NIF hold great promise for the commercial viability of nuclear reactors, but they are yet to achieve a net-positive energy output at a frequency to make power generation sustainable. While the commercialization of nuclear energy is likely a few decades away, it shows potential for a clean, abundant, and sustainable energy source in the next few decades.
Abbreviations and Symbols
COILOPT++: Coil Optimization Code using Spline Representation
HELIAS (HELical-axis Advanced Stellarator)
HSX: Helically Symmetric Experiment
HTS: High-Temperature Superconductors
ICF: Inertial confinement fusion
MCF: Magnetic confinement fusion
NIF: National Ignition Facility
PPPL: Princeton Plasma Physics Laboratory
ReBCO: Rare-earth barium copper oxide
STELLOPT: Stellarator Optimization
Introduction
The pursuit of fusion energy is driven by the critical global need to reduce the costs of fuel extraction and diversify away from reliance on politically unstable regions for fuel provision through the development of alternative energy sources. Even though they are crucial parts of many modern energy portfolios, renewable energy sources like solar and wind power suffer from the issue of being unable to provide continuous baseload energy output (Hasan, et al., 2013). They are influenced by many external factors, making them difficult to fully rely on without extensive storage solutions (e,g, Bañares-Alcántara et al., 2015). Their variable nature necessitates the development of alternate energy sources that can provide continuous and dispatchable power output (Hamacher et al., 2013). Nuclear fusion energy offers a dispatchable solution. Fusion poses an advantage over existing nuclear fission reactors, which has considerable drawbacks such as the production of vast amounts of radioactive isotopes with long half-lives (Brodén et al., 1998). There are many safety and environmental issues with the handling and disposal of this waste (De Vincente et al., 2022), which poses significant issues for its widespread continued usage. Thus, opting for fusion - which releases a significant quantity of energy and generates a notably smaller amount of long-term radioactive waste - is a more environmentally compatible choice that can provide the energy solution to reduce air pollution without compromising power output for commercial and industrial usage (De Vincente et al., 2022). Moreover, the fuel required for nuclear fusion can be mined in effectively limitless quantities from seawater, and a byproduct of the fusion reaction is valuable helium gas, which has limited terrestrial supplies and is expected to become increasingly scarce.
The process of nuclear fusion combines light atomic nuclei to form heavier nuclei. When they fuse, the resultant nucleus has a lower mass than the sum of the original nuclei (Smith & Cowley, 2010). This mass difference (m) is converted into energy (E) according to Einstein's mass-energy equivalence principle E=mc^2, where c is the speed of light (Ongena, 2016). The fusion reaction with the least energy barrier that is achievable on Earth is the fusion of deuterium and tritium, both isotopes of hydrogen (Kirk, 2016). When the reactant nuclei are subjected to extremely high temperatures (millions of degrees Celsius), they gain sufficient kinetic energy to overcome the repulsive electromagnetic forces between their respective nuclei, since they are both positively charged (Smith & Cowley, 2010). When deuterium and tritium nuclei decrease in separation, eventually the strong nuclear force takes over to fuse the nuclei together, into a helium nucleus and a neutron, releasing significant amounts of energy (Ongena, 2016):
D + T → He (3.5 MeV) + n (14.1 MeV)
For this process to occur, however, there are a few prerequisites, also known as the Lawson criterion or triple product rule (Petkow et al., 2012). Firstly, the temperature (T) must be high enough to provide kinetic energy to the nuclei, in order for them to overcome their electrostatic repulsion to each other. Secondly, the pressure, or density (n), in the reactor needs to be high to increase the likelihood of collisions between the nuclei. Lastly, the plasma needs to be confined for an adequate amount of time to allow the nuclei to fuse, i.e. long confinement time (τ) (Abu-Shawareb et al., 2022).
There are two major branches of nuclear fusion energy research: magnetic confinement fusion (MCF) and inertial confinement fusion (ICF). Magnetic confinement fusion makes use of magnetic fields to trap plasma within the confined space of a reactor to prevent it from coming into physical contact with the walls, which would cool the plasma and halt fusion reactions, or cause the reactor surface to melt (Ongena, 2016). Devices such as tokamaks and stellarators rely on the principle of MCF (Huang & Li, 2018). The other branch, ICF, achieved through the use of powerful lasers to compress and heat a small spherical pellet of fuel. The inertia of the plasma confines it long enough for the process to produce significant reactions (Betti et al., 2016).
The goal of advancing research on nuclear fusion is not only to utilize a renewable energy resource, but also redefine how we use energy as a whole. Achieving net power output from nuclear fusion means generating more energy from the fusion reactions than the energy invested in creating and sustaining the plasma conditions necessary for fusion (De Vincente et al., 2022). This is crucial for fusion to become a viable and practical energy source.
This review paper aims to present in greater detail each proposed concept to achieve fusion on Earth - tokamaks, stellarators, and inertial confinement, analyzing the current technology, benefits and limitations of each method, and the road ahead for fusion research.
Literature Review
Magnetic Confinement - Tokamaks
Figure 1. The typical tokamak configuration showing the toroidal (navy) and poloidal (green) field lines. The plasma is confined inside the vacuum vessel with a D-shaped cross-section by these magnetic fields (Li et al., 2014), CC BY 4.0.
Tokamaks operate under the principle of magnetic confinement fusion. A tokamak is an axially symmetric system, in the shape of a torus, in which high-temperature plasma is contained to sustain nuclear reactions (Huang & Li, 2018). The typical set of components of a tokamak is shown in Figure 1 (Ongena, 2016). A set of magnetic coils is wound around the torus, generating a toroidal field, along the azimuthal direction of the torus. The vertical field coils around the outside of the torus creates a perpendicular magnetic field along the poloidal direction. The resultant magnetic field confines the particles in the plasma, so it is kept stable and away from the walls of the container (Huang & Li, 2018).
There are several advantages that tokamaks have, regarding their engineering, operation, and research. Tokamaks are technically simple to engineer and their underlying operational principle is relatively straightforward, compared to alternate nuclear fusion methods like ICF (Betti et al., 2010). The relatively simple design allows for more advancements in its engineering. Moreover, tokamaks are able to sustain fusion reactions over longer durations, with the record currently held at over 1000 seconds (Zheng et al., 2022). The ability to confine the plasma and sustain the nuclear fusion process for extended periods is crucial for commercial power generation to provide a consistent energy supply without many power outages. Additionally, there is a deeper understanding of the operation and logistics of tokamaks, given that they have been a primary focus of fusion research since the 1950s (El-Guebaly, 2010). The operational, material, and engineering challenges specific to tokamaks are better comprehended and adhered to by scientists, as extensive experimental data and theoretical insights have been accumulated, which guide current and future designs (Meschini et al. 2023).
Tokamaks have a few current limitations which may challenge their widespread application and usage. These devices must handle extreme conditions, such as high temperatures (up to 150 million °C), particle fluxes, and intense neutron bombardment to carry out nuclear fusion processes (Knaster et al., 2016). These conditions are harsh on materials used in the vacuum vessel and breeding blanket (Wirth et al., 2011), which can lead to severe degradation over time. These damages can lead to more maintenance or replacement needs, increasing costs and hinder long-term sustainability. Accounting for these factors is a drawback for the implementation of tokamaks as commercial fusion reactors (Suri et al., 2010). Thus, finding and developing materials that can withstand such conditions while maintaining functionality is a significant challenge for tokamaks. Additionally, maintaining a stable plasma for long periods in tokamaks is difficult (Betti et al., 2010), as the generally achieved limit is currently between 5-10 minutes (Ding et al., 2024). The very first improvement of the stability of tokamaks was accredited to the tokamak T-3, an improvement of the Russian tokamak TM-2, by altering total current of the system, shot duration, and safety factors (Mirnov, 2019). Although there have been significant improvements, these instabilities still pose risks to the efficiency of the fusion process. Moreover, current tokamak designs consume a significant amount of energy to initiate and sustain the fusion process, particularly for heating the plasma and powering the magnetic coils (Hora, 2017). The break-even point has not yet attained by recent developments, thus, there is still more energy being input than output (Zheng et al., 2022). Hence, this method has not yet reached practical levels for commercial energy production.
The current state of nuclear fusion research has brought about many national and international collaboration projects. ITER is an international-collaboration megaproject that aims to create energy through magnetic confinement fusion in a tokamak reactor, while demonstrating its feasibility as a large-scale energy source (Geng, 2022). Its primary objective is to achieve sustained nuclear fusion reactions where the energy output from the fusion process exceeds the energy input required to sustain the plasma and operate the tokamak (Shimada et al., 2005). The projected triple product value for ITER is ~5 x 1021 keV s m-3, 10 times the value achieved by its predecessor, JET (Meschini, 2023) . Moreover, the ITER central solenoid will be the largest and most integrated superconducting magnet system ever built, producing a field of 13 tesla (Aymar et al, 2002), which is equivalent to 280,000 times the Earth’s magnetic field. Apart from ITER, there are several tokamaks in operation around the world, including JET, ASDEX Upgrade, JT-60SA, DIII-D, EAST, KSTAR, and T-15 (Meschini, 2023).
As an extension and succession of ITER, the DEMO project aims to bridge the gap between the experimental nature of ITER and a commercially viable power source of nuclear energy (Federici 2014). DEMO aims to produce electricity from fusion reactions consistently and economically, to be distributed among the population. It is expected to be larger and more powerful than ITER, with the capacity to generate several hundred megawatts of electricity (Federici et al., 2019). DEMO is currently in the conceptual and early design phases, and its construction and operation are expected to follow if ITER's demonstration of sustained fusion reactions is successful. It will potentially finish by 2050, depending on technological readiness and funding (Federici, 2014).
Magnetic Confinement - Stellarators
Figure 2. Stellarator design of the Wendelstein 7-X (Max-Planck-Institut für Plasmaphysik, 2010). Note the nonplanar coil system for magnetic confinement (blue), and the magnetic field line (green) on the plasma surface (yellow). CC By 3.0.
Stellarators are magnetic confinement fusion devices distinguished by their twisted, non-axisymmetric magnetic field configurations. The principal design feature of a stellarator is its external magnetic coil system, which generates the requisite confining magnetic fields independently of a toroidal plasma current (Gates et al., 2017). These coils are intricately shaped in three dimensions, often taking the form of a helix or interwoven loops, and are arranged around the toroidal vacuum vessel. A combination of planar and nonplanar coils ensures the plasma flows in a consistent, cyclic manner through the torus (Clery, 2016), as seen in 1 of Klinger et al. (2019). Unlike tokamaks, stellarators' coils generate twisted magnetic fields that confine the plasma without necessitating a toroidal plasma current (Gates et al., 2017). This external control mitigates instabilities, as the magnetic fields can be precisely tailored to optimize plasma confinement and stability (Beidler et al., 2002). Therefore, stellarators offer more stability because there is no plasma current susceptible to disruption and affect the plasma path (Privat et al., 2022).
Stellarators primarily use high-temperature superconducting (HTS) tapes for their magnetic coils (Xu et al., 2024). HTS materials permit higher current densities and stronger magnetic fields compared to conventional superconductors. Rare-earth barium copper oxide (ReBCO) is a common material used in creating HTS because it can carry high current density over a wide range of temperatures (4.2K-77K), magnetic fields, and help generate a dipole field of 20T (Prestemon et al., 2020). Compared to conventional superconducting pure metals such as niobium, high temperature superconductors such as ReBCO are typically made of ceramic compounds such as cuprates rather than metal alloys and can be cooled by liquid nitrogen rather than liquid helium (Nishijima et al., 2018). This increases the flexibility of cooling systems for high temperature superconductors, making operation cheaper and easier to handle.
Stellarators offer several significant advantages in the field of nuclear fusion, particularly in terms of stability, steady-state operation, and design flexibility. One of the key benefits of stellarators is their inherent stability, as they inherently avoid the plasma instabilities and disruptions common in tokamaks due to the lack of a toroidal plasma current (Beidler et al., 2001). In tokamaks, the toroidal current can cause kink nodes, where magnetic field lines becoming overly twisted, or sawteeth, which are periodic oscillations in plasma temperature and density, creating plasma instabilities that can lead to loss of plasma confinement (Helander et al., 2012). As a result of the lack of toroidal current, stellarators are more stable than tokamaks and less susceptible to kink nodes and sawteeth (Xu, 2016).
Additionally, stellarators can operate continuously without the need for the pulsed operation cycles of the tokamak (Beidler et al., 2001). Though the theoretical limit of stellarator operation is still being researched, the Wendelstein 7-X stellarator built by the Max Planck Institute for Plasma Physics is projected to reach 30 minutes of continuous operation (Wolf et al., 2019). Stellarators can achieve steady-state operation due to their ability to maintain plasma confinement using external magnetic fields alone, without requiring an induced plasma current (Xu, 2016). This contrasts with tokamaks, which rely on a toroidal plasma current for confinement, necessitating pulsed operation. This continuous operation of stellarators is beneficial for maintaining a stable plasma environment and simplifies reactor design and operation for future use (Kerekeš et al., 2023). Additionally, complex three-dimensional magnetic fields of stellarators allow for greater flexibility in plasma shaping and optimization, leading to improved confinement properties and potential reactor designs that can be tailored to specific needs (Privat et al., 2022).
However, stellarators pose unique challenges in their design and construction, primarily due to the intricate shaping required for their magnetic coils (Xu, 2016). This complexity arises from the necessity to precisely manufacture three-dimensional magnetic fields that are crucial for confining plasma. Achieving this precision increases both design and manufacturing complexities (Lu et al., 2022). Key challenges include maintaining optimal distances between the plasma and coils to maximize magnetic confinement while minimizing radiation and heat damage. Additionally, ensuring the structural integrity of the coils under intense magnetic fields and precisely assembling and positioning them are critical aspects (Raffray et al., 2007).
Moreover, stellarators are particularly susceptible to neoclassical transport losses, which refer to particle and energy losses caused by collisions and drifts within the plasma (Helander et al., 2012). The complex, non-axisymmetric magnetic field geometry of stellarators exacerbates these losses by inducing larger and more irregular particle orbits (Xu, 2016). Consequently, this increased transport reduces the efficiency of confinement and energy retention, presenting a significant drawback compared to more symmetric configurations like those found in tokamaks (Helander et al., 2012).
In terms of size and cost, stellarators tend to be larger and more expensive to construct than tokamaks. The intricate coil configurations and the need for advanced materials contribute significantly to this higher cost (Xu et al., 2024). For instance, as of 2023, the Wendelstein 7-X stellarator project has incurred costs totalling 1.06 billion euros, whereas smaller experimental tokamaks range from tens to hundreds of millions of dollars. Though notably, the ITER tokamak has already cost US$23.5 billion (Moynihan & Bortz, 2023). Furthermore, the maintenance of stellarators presents additional challenges due to their complex coil arrangements and tight spatial constraints. These factors can complicate maintenance and repair procedures, potentially leading to increased operational costs and longer downtimes (Gates et al., 2017).
Recent advancements in stellarator design include the development of computational optimization techniques such as the Coil Optimization Code using Spline Representation (COILOPT++) code (Brown et al., 2015). This code uses spline representations for coils, simplifying the design and improving maintainability. Additionally, turbulence optimization methods have demonstrated significant reductions in turbulent heat loss, enhancing plasma confinement efficiency (Gates et al., 2017).
While there are no universally standardized protocols for stellarator design and operation, optimization frameworks like the Stellarator Optimization (STELLOPT) suite of codes are widely used in the development and refinement of stellarator configurations (Gates et al., 2017). Existing stellarator reactors include the Wendelstein 7-X in Germany (Warmer et al., 2024), the Helically Symmetric Experiment (HSX) in the US (Gerard et al., 2023), and the Large Helical Device in Japan (Dhard et al., 2024).
The current state of research on stellarators is focused on further refining optimization algorithms for coil design, improving fast particle confinement, and integrating divertor design into the overall optimization framework (Gates et al., 2017). Experimental validation of theoretical predictions, especially regarding turbulence reduction and fast particle confinement, remains a significant area of ongoing investigation (Warmer et al., 2024). This is becoming an ever more important problem as the gap between the limits of theoretical plasma physics waits for more robust computing power to simulate such experiments. Privat et al. (2022) used computational models such as COILOPT to propose new, optimal shapes for stellarators with an emphasis on magnetohydrodynamic stability.
Another notable development is permanent magnets as part of MUSE. MUSE is a new type of stellarator constructed by the Princeton Plasma Physics Laboratory (PPPL) (Qian et al., 2023). This stellarator uses permanent magnets instead of complex electromagnets, offering a simpler and more cost-effective design. MUSE showcases significant advancements, particularly in demonstrating quasisymmetry, which enhances plasma confinement (Qian et al., 2022). The device is largely made from commercially available parts and features innovative use of 3D printing. Researchers aim to further investigate MUSE’s capabilities and its potential to contribute to future fusion power plants (Qian et al., 2023).
The future of stellarator research involves developing more irradiation-resistant structural materials, improving computational models for turbulence and particle confinement, and exploring new coil shapes that allow for greater plasma-coil separation (Privat et al., 2022). This could reduce engineering constraints and improve the feasibility of stellarator reactors.
Inertial Confinement
Inertial confinement fusion (ICF) relies on driving the fuel component together and relying on their inertia to fuse the atoms release energy (Betti & Hurricane, 2016). The process of ICF is illustrated in Gordinier et al. (2003). ICF comes in several different forms, mainly with and without the use of lasers to directly accelerate the fuel (Tikhonchuk, 2020). For the purposes of this paper, laser-induced will be the main ICF scheme discussed. In these reactions, lasers are used to heat a capsule that contains the fuel, which is kept in a cryogenic state as either solid or liquid (Betti & Hurricane, 2016). The lasers heat the material, causing compression of the fuel, to the point of high enough pressure and temperature for nuclear fusion to occur (Craxton et al. 2015). There are two types of laser-induced ICF: direct drive and indirect drive. Direct drive is when the lasers are directly irradiating a spherical capsule that is usually made of a polymer that vaporizes during the reaction (Craxton et al. 2015), whereas indirect drive is when the lasers heat a cylindrical capsule known as a hohlraum that releases X-rays that irradiate the spherical capsule containing the DT fuel (Lindl, 1995). In the case of indirect drive ICF reactions, the hohlraum used is made of a high-Z material, such as gold or uranium (Lindl, 1995).
ICF allows for extremely high pressures, which can decrease the needed temperature to achieve fusion (Tikhonchuk, 2020). This method also requires less energy input to initial nuclear fusion compared to MCF. ICF reactions have a ‘hot spot’ that is based on a principle much like the Lawson criterion (Betti & Hurricane, 2016). The ‘hot spot’ is a small space that is brought to a much lower temperature and much higher pressure, which is needed for fusion. This hot spot decreases the needed energy for the reaction to occur because it is far more energy efficient to increase the pressure than it is to increase the temperature (Tikhonchuk, 2020).
The mass of the fuel in a single ICF capsule for a reaction is very limited-only a few milligrams. The capsule itself is very small (only a few millimeters), so the lasers must be very accurate to hit the target (Tikhonchuk, 2020). ICF reactions also require extremely high reaction temperatures of requiring many KeV to occur (Betti & Hurricane, 2016). This high temperature can be supplemented in part by high pressure (the requirement of reaching such high temperatures can be reduced by reaching the high pressures of even exceeding a Gbar level during ICF), both of which are achieved in an ICF reaction (Betti & Hurricane, 2016). However, this presents its own problems, including the confinement of the high temperatures and radiation produced within the reaction (Tikhonchuk, 2020). In addition, ICF reactions, on the whole, still do not reach or exceed breakeven. These reactions lose energy from incomplete fuel burn, a lack of heating and compression efficiency, and energy conversion efficiency (Tikhonchuk, 2020). Some of these losses can be decreased via the use of a magnetic field (Betti & Hurricane, 2016).
Recently, the use of indirect drive has been able to achieve ignition, with over two times the energy consumed being produced in February 2024 (Abu-Shawareb et al., 2022). Due to these recent advancements, as well as previous occurrences of ignition occurring in the Lawrence Livermore National Laboratory (LLNL) using a hohlraum, current research in the inertial confinement field is more toward indirect drive. ICF research is still a very active field, from improvements in the capsule or hohlraum (McClarren et al., 2021) to even the supports for the capsule (Kritcher et al., 2022). One key recent advancement has been a change in the capsule shape. With the recent success of using a hohlraum, there has been talk of using a new heavier metal in the ablator, which allows for better directed heating of the fuel pellet (Lin et al. 2022).
Discussion
In general, the development of nuclear fusion appears still far away from widespread application in society. It has remained at a research stage for over 50 years, and there are currently no commercial reactors in operation (Tikhonchuk, 2020). Nuclear fusion is also expensive. The estimated cost of only a single DEMO fusion reactor is approximately US$20 billion (Lopes Cardozo et al., 2016).
MCF and ICF have many of the same drawbacks/difficulties, but differ in what is being attempted for the future, as well as in design. They differ also in the timeline of potential implementation and technological maturity. Specifically, there appears to be no set timeline for a commercial ICF reactor. If MCF were to follow its predecessors (in energy generation) in advancement rate, there would be an attempted three DEMO reactors made in the 2050s, 10 first generation plants in the 2060s, and 100 second generation plants in the 2070s (Lopes Cardozo et al., 2016).
However, MCF has seen no net energy gain as of yet, while ICF has achieved up to slightly over double the energy put in than out (Abu-Shawareb et al., 2024). Fusion is pursued because fusion reactors can produce power on demand, with no practical limits to the fraction of the world’s demand for energy that fusion can supply (Lopes Cardozo et al., 2016). However, even with no practical limits on the energy produced in the future through the use of nuclear fusion, as of now, in general more energy is used to sustain the reaction than is produced as output (Tikhonchuk, 2020).
ICF uses less space and can be more cost efficient than MCF if future research is done to optimize this (Hora, 2007). NIF reached a net power output of 3.15 MJ from a 2.05 MJ input on December 5th, 2022 (Abu-Shawareb et al., 2024). Further experiments in 2023 have increased laser energy, improved capsule surface imperfections, and reduced drive asymmetry to increase power output and make ICF more cost efficient (Casey et al., 2023). ICF also achieves the highest plasma density out of the two methods, but the miniscule confinement time and complexity of achieving the Lawson Criterion for nuclear fusion limits the viability of ICF as a commercial fusion reactor (Abu-Shawareb et al., 2022). ICF also requires lower temperatures than MCF to achieve fusion (Betti et al., 2010). The lower temperatures of ICF reduce the triple product of the Lawson criterion, making it more difficult to achieve nuclear fusion due to its reliance on maximizing plasma density. However, lower operating temperatures reduce the need for materials with extreme temperature resistance, potentially saving on development and operational costs.
Amongst the MCF methods, more research has been carried out on tokamak reactors and thus they are more favorable for commercial energy supply in the near future (Wang, 2023). Nuclear fusion reactions require a supply of tritium to be used as a component of the fuel needed for the reaction. Tritium is not easily obtainable, with the main source being from the byproducts of nuclear fission reactors (Pearson et al., 2018). Due to this, tritium must be ‘bred’ within the fusion reactor. Tritium breeding is not easy to do. Tritium breeding involves neutrons launched from the reaction to strike a lithium containing blanket to produce tritium. However, the tritium breeding rate is slow, with a conversion rate, at most, of one neutron only producing one tritium atom (Rubel, 2019). Tokamaks such as ITER and DEMO are also projected to generate tritium self-sufficiently through tritium breeding, promoting tokamaks as a more sustainable fusion reactor option (Someya et al., 2018). Whilst stellarators also have potential to breed tritium in configurations such as the HELical-axis Advanced Stellarator (HELIAS), it is more difficult to implement a breeding blanket compared to that of tokamaks such as DEMO due to the intricate geometry, large size, weight, and precise spacing of the coils of stellarators (Sosa & Palermo, 2023). Future developments in stellarator-specific tritium breeding blankets such as the work done by Sosa and Palermo (2023) could lead to more energy efficient stellarators. But for the near future, tokamaks appear to be more favorable for tritium breeding.
However, the requirement of tokamaks to be pulsed and their inherent discontinuous operation may hinder its potential to be a long-term energy supplier (Huang & Li, 2018). Stellarators avoid this issue due to the lack of a toroidal current, but their exceedingly complex design means that the physics and computational technology must advance far enough to develop a commercially viable stellarator (Beidler et al., 2002). With the rapid advancements in computational technology though, stellarators may become the preferred magnetic confinement option in the distant future (Gates et al., 2017).
One issue with nuclear fusion, both magnetic and inertial confinement, is the decommissioning of the reactors. The tritium breeding process causes the inner pieces of the reactor to become radioactive. These radioactive parts need to be stored for a cooling period, or buried near the surface, which are all solutions that demand further research (De Vincente et al., 2022).
Conclusion - Nuclear Fusion Research
Achieving nuclear fusion on Earth involves two main potential approaches: inertial confinement fusion and magnetic confinement fusion. Inertial confinement relies on high-energy lasers to compress fusion fuel, offering advantages such as rapid energy release and potential for smaller reactors. However, it faces specific challenges such as the need for precise symmetry in compression, which is difficult to achieve consistently, and the short duration of fusion reactions, reducing its triple product. NIF is currently researching ways to maximize energy output for ICF after recently reaching a net power output.
On the other hand, magnetic confinement fusion employs magnetic fields to confine plasma, with tokamaks and stellarators as prominent designs. Tokamaks like ITER aim for continuous operation and have shown progress in achieving sustained plasma temperatures and confinement times. However, they encounter issues such as plasma instabilities due to magnetic field irregularities and turbulence. Stellarators, while potentially offering better confinement stability through complex magnetic field configurations, face challenges related to their intricate design and computational demands. Research with the Wendelstein 7-X stellarator is ongoing and aims to address these issues.
Currently, magnetic confinement fusion, particularly through tokamaks like ITER, appears more promising due to its design simplicity relative to inertial confinement schemes and stellarators, potential for sustained energy production, and multitude of research and technological advancements. Overall though, the main key challenge to implementation for both magnetic and inertial confinement approaches remains achieving sustained plasma stability under the extreme conditions necessary for efficient energy production.
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