After many decades of very slow development, fusion power has recently begun attracting billions of dollars' worth of private investment. Dozens of companies are now seeking to commercialize fusion power, exploring a wide range of approaches and making very optimistic promises about future deployments. Nevertheless, enormous challenges continue to limit the progress of fusion power. Nuclear-fusion power could theoretically offer limitless clean energy without many of the drawbacks that existing sources of clean energy have. Even if fusion power cannot compete economically against other sources of clean energy, it could eventually become a valuable source of clean energy for applications that other technologies do not serve well.

Nuclear-fusion reactors produce power by fusing atomic nuclei to produce heavier nuclei, releasing energy in the process. Achieving nuclear fusion requires enormous temperatures and pressures to overcome the repulsive forces that prevent nuclei from fusing. Although humanity has been capable of producing uncontrolled nuclear fusion since 1951, producing sustained nuclear fusion in a controlled fashion has proved exceptionally technically challenging. Progress has been very slow and very expensive. For example, the International Thermonuclear Experimental Reactor (ITER), the world's largest experimental fusion reactor, has been under development since 2006 and will not achieve fusion until at least 2035, according to the project's schedule. The US Department of Energy has estimated that ITER's costs will exceed $65 billion by 2025.

By the time ITER achieves fusion, private companies may already be using nuclear fusion to supply electrical power commercially with a total development cost that may be an order of magnitude lower than ITER's. Breakthroughs in artificial intelligence and materials science have helped make new reactor designs—or novel refinements of old designs—feasible. Some notable emerging reactor designs follow:

• Compact tokamaks. Tokamaks are fusion reactors that use toroidal magnets to confine superheated plasma in which fusion reactions take place. Like most research reactors, ITER is a tokamak design and uses liquid-helium-cooled superconducting magnetic coils. Tokamak Energy and Commonwealth Fusion Systems are examples of private fusion companies that are using high-temperature superconducting materials to produce small, spherical tokamak reactors. These reactors could have better magnetic stability than is possible with conventional designs and eventually use magnet-cooling systems that are less costly and complex.
• Pulsed magnetic reactors. Helion Energy attracted substantial private investment to develop a fusion reactor that uses electromagnets to fire plasma pulses at each other from opposite ends of a linear reaction chamber. When the pulses meet in the center of the chamber, electromagnets compress the plasma, releasing energy. The design extracts electrical energy from the plasma through use of direct magnetic coupling rather than through use of a heat exchanger.
• Field-reversed-configuration reactors. Like Helion Energy, TAE Technologies (formerly Tri Alpha Energy) has developed a fusion reactor that confines plasma within a linear reaction chamber. TAE Technologies' design uses boron injectors to impart angular momentum into the plasma to improve its stability. TAE Technologies hopes eventually to achieve boron-hydrogen fusion. Doing so will require temperatures vastly higher than those the company has been able to achieve so far but will have many benefits, including abundant fuel availability and an absence of damaging neutron radiation.
• Magnetized-target-fusion reactors. General Fusion has pioneered a fusion-reactor design that confines a torus of fusion plasma within a rapidly rotating mass of liquid lithium, surrounded by an array of steam-powered pistons. The pistons periodically compress the magnetic lithium, which in turn compresses the plasma, initiating fusion reactions. The lithium mass absorbs the heat and neutron radiation from the fusion reactions, facilitating heat transfer for electricity production and breeding tritium fuel for sustaining the fusion reaction.

As banks tighten capital availability in an effort to contain inflation, investment in private fusion power could diminish rapidly. Relevant research could nevertheless continue through a combination of existing private funding and ongoing public funding. Despite projections from private companies, severe technical challenges make fusion power's becoming commercially viable before 2035 extremely unlikely. However, the future is uncertain, and changing conditions could trigger alternative outcomes. Some examples of potential events that could transform the future of commercial fusion power follow:

• Achievement of self-sustaining reactions for long periods. Despite decades of research, fusion-power experiments still cannot produce more energy than they consume on an ongoing basis. The first example of fusion ignition (net energy production from fusion) took place in November 2021 and used methods that are wholly impractical for producing sustained fusion reactions. Researchers' achieving fusion ignition in a manner that allows for ongoing extraction of electrical power would not necessarily mean that fusion power would be anywhere close to becoming a commercial reality. But such a feat could nevertheless stimulate a massive surge in new investment into fusion power.
• Emergence of viable solutions to fusion's neutron-radiation problem. A long-standing problem with fusion power is the production of large quantities of high-energy neutrons. The neutron radiation quickly degrades the reactor material, ultimately transforming it into radioactive waste. Methods to deal with the neutron problem include using fusion fuels that produce no neutron radiation and using advanced materials that can absorb radiation without damage. Other methods aim to leverage neutron radiation for a useful purpose such as aiding in the conversion of the fusion energy into electrical power or producing tritium fuel to sustain the fusion reaction. However, no method has yet proved to be a workable solution to the neutron-radiation problem.
• Major improvements in efficiency of reactor auxiliary systems. Current fusion-reactor designs have enormous parasitic losses—that is, they require massive amounts of energy to operate. In addition to expending considerable energy to ignite the fusion reaction, reactors must expend energy to run the systems that sustain their operation. Innovations in areas such as high-temperature superconductors and direct-coupled fusion-energy extraction could significantly reduce parasitic losses.
• Increased competition from other sources of clean energy. Fusion power is not the world's only source of clean energy. Solar and wind power are already vastly cheaper and overwhelmingly likely to remain so. Advanced forms of nuclear-fission power could emerge as a serious competitor to fusion power by 2030. Fusion power could become redundant long before it becomes practical.