The stars, including our Sun, are celestial powerhouses fueled by the process of nuclear fusion. In the core of the Sun, immense gravitational pressure and temperatures create conditions favourable for hydrogen nuclei to collide, combine, and form helium. This fusion process liberates an immense amount of energy in the form of light and heat, sustaining the Sun’s radiance and providing life-giving energy to our planet.
If we could replicate this process on Earth, we would have access to a virtually limitless source of clean, safe, and abundant energy. However, achieving controlled fusion on Earth has been a long and challenging journey.
The first step is to create and sustain a plasma, a superheated state of matter, within a controlled environment. By reaching temperatures of hundreds of millions of degrees Celsius, plasma can overcome the mutual repulsion of atomic nuclei, allowing them to come close enough for the strong nuclear force to take effect, causing fusion to occur.
How Fusion Works – The Atomic Dance of Fire
Nuclear fusion occurs when two light atomic nuclei collide and fuse, forming a heavier nucleus and releasing energy. This is the opposite of nuclear fission, where a heavy nucleus splits into smaller parts. In stars like our sun, the immense gravitational pressure and heat in the core create conditions for hydrogen nuclei to overcome their electrostatic repulsion and fuse into helium. This fusion process powers the sun’s radiance.
The most promising fusion reactions for energy production on Earth involve isotopes of hydrogen, deuterium and tritium. When deuterium and tritium nuclei fuse, they form a helium nucleus and a high-energy neutron. This reaction releases 17.6 MeV of energy, mostly in the form of kinetic energy of the neutron:
D + T → 4He + n (17.6 MeV)
To initiate fusion reactions, the fuel must be heated to form an ionized plasma, where nuclei can move freely. Plasma temperatures over 150 million °C are needed to overcome the electrostatic forces and bring nuclei close enough together for the strong nuclear force to fuse them. The plasma must also be confined at suitable density and temperature long enough for sufficient fusion reactions to occur.
If controlled fusion can be achieved, it offers potential benefits including:
- Virtually limitless fuel available from seawater (deuterium) and lithium
- No greenhouse gas emissions during operation
- Less radioactive waste than nuclear fission
- No risk of meltdown or runaway reactions
- High energy density
However, bottlenecks remain in confining the plasma, breeding tritium fuel, and engineering materials that can withstand the harsh conditions inside fusion reactors. Ongoing research aims to overcome these hurdles.
Approaches to Plasma Confinement
Achieving a sustained fusion reaction requires confining a plasma at extremely high temperatures and pressures. Plasma particles and energy must be contained long enough for sufficient fusion reactions to occur. Different confinement concepts have been developed, with magnetic confinement, inertial confinement, and magnetized target fusion being the primary approaches pursued today:
Magnetic Confinement Fusion (MCF)
Magnetic confinement utilizes powerful magnetic fields generated by superconducting coils to contain the plasma within a vacuum chamber. The magnetic field lines inhibit cross-field leakage of particles and heat, confining the charged plasma particles into a defined region.
The most developed MCF devices are tokamaks, which use a toroidal chamber and poloidal magnetic coils to create a helical field geometry. Both the toroidal and poloidal fields are required for plasma stability. The plasma current induces its own poloidal field, acting as the secondary winding in a transformer. However, the pulsed nature of the central solenoid necessitates development of steady-state current drive techniques for continuous tokamak operation.
Stellarators also rely on external magnetic coils but in a more complex 3D geometry that provides an intrinsic twist to the field lines. This removes the need to drive current in the plasma for confinement, enabling steady-state operation. However, the non-planar coils are more difficult to manufacture. Wendelstein 7-X and HSX are optimizing stellarator configurations for stability and confinement.
Spherical tokamaks, with their compact shape and high-field HTS magnets, offer a cost-effective route to fusion energy. They operate at low aspect ratio to increase plasma pressure and bootstrap current fraction. Spherical tokamaks face engineering challenges such as tighter space for a breeding blanket. START, MAST, and the ST40 are pioneering this approach.
Inertial Confinement Fusion (ICF)
In contrast to MCF, inertial confinement relies on the inertia of the fuel mass to provide a transient confinement through rapid compression heating. Powerful laser or particle beams ablate the outer surface of a fuel target, generating a rocket-like implosion that compresses the fuel to extraordinary densities. Ignition can be achieved even with confinement times around 10 picoseconds.
The symmetry of the implosion is critical, as any deviations from sphericity will reduce the compression effectiveness. Major inertial confinement research is conducted at the National Ignition Facility (NIF) in the US, which uses 192 high-power lasers focused onto a BB-sized fuel pellet. In 2022, NIF exceeded fusion energy breakeven for the first time.
Magnetized Target Fusion (MTF)
MTF combines elements of magnetic confinement and inertial compression to reach fusion conditions in a medium-density, magnetized plasma. Pre-imposed magnetic fields slow the expansion of a target plasma which is rapidly compressed via laser or mechanical implosion. This aims to achieve ignition at more modest plasma pressures than conventional MCF and ICF.
For example, General Fusion uses synchronized pistons to radially compress a field-reversed configuration plasma injected into a wall chamber. MTF benefits from a simpler driver system than lasers or tokamak magnets. But seamlessly integrating the magnetic and inertial components remains challenging. Overall, MTF seeks to find an optimal intermediate path to fusion between the extremes of long-pulse MCF and nanosecond ICF implosions.
Major Fusion Reactor Projects
Significant progress towards fusion energy is being made through major experimental devices and reactor development projects around the world. Both the public and private sectors are contributing unique capabilities and resources to advancing fusion technology.
Public Sector Projects
Large government-funded fusion programs have produced valuable scientific knowledge and plasma confinement breakthroughs over the past decades. While foundational, direct energy production has not been the primary aim of these public research projects. However, they are now laying the groundwork for future fusion power generation.
ITER – One of the most ambitious international scientific collaborations ever undertaken, ITER aims to be the first fusion device to produce net energy gain. Under construction in France and funded by seven partner countries, ITER will use a large superconducting tokamak to generate 500 MW of fusion power from 50 MW of heating input. It seeks to demonstrate the feasibility of magnetic confinement fusion at a scale approaching that of a power plant.
NIF – The National Ignition Facility located at Lawrence Livermore National Laboratory in California focuses on achieving inertial confinement fusion ignition in the laboratory. NIF’s 192 high-power laser beams compress small fusion targets to extraordinary densities and temperatures to initiate nuclear fusion burn. In 2022, NIF exceeded the break-even point with a megajoule-scale fusion yield for the first time.
W7-X – This optimized stellarator in Greifswald, Germany successfully contained a hydrogen plasma for up to 30 minutes, demonstrating the potential of stellarators for continuous operation. By using a complex three-dimensional magnetic coil geometry, stellarators can achieve good plasma confinement without running current through the plasma as in tokamaks.
ET – The EU roadmap has proposed a compact tokamak called the European Tokamak as a next-step device to achieve deuterium-tritium plasma operation and test components for fusion power plants. ET would leverage the knowledge gained from operating JET in the UK.
Private Fusion Ventures
In parallel to government programs, startups and privately-funded companies are rapidly proliferating in the fusion space. Attracted by the potential market for fusion energy, these ventures are exploring diverse confinement schemes and enabling technologies. Notable private players include:
CFS – This MIT spinout is developing compact, high-field tokamak reactors by leveraging advanced superconducting magnets. Their ARC power plant concept would produce 190 MWe. They are building a net energy gain prototype called SPARC to demonstrate their HTS confined fusion approach.
TAE – With two decades of research into beam-driven field-reversed configuration plasmas, TAE Technologies seeks to harness the simplicity and low costs of this approach. Their latest device, Norman, aims to create a pathway to economical carbon-free baseload power generation.
Helion – Based in the US, Helion is developing magneto-inertial fusion systems involving the compression of compact field-reversed configuration plasmas. They believe their pulsed direct energy conversion approach based on decoy nuclei can lead to simpler, lower cost fusion plants.
General Fusion – This Canadian MTF startup applies mechanical piston implosion to compress plasmas for fusion energy production. They aim to build a demonstration fusion plant by 2030. Acoustic waves shape the liquid interface lining their compression chamber.
Tokamak Energy – A UK company developing a compact, high-field spherical tokamak using HTS magnets. Their ST40 prototype aims to achieve 100 million degree plasma temperatures in a device just 3 meters wide. They target grid electricity in the 2030s.
Critical Challenges on the Path to Fusion
While the feasibility of fusion has been proven scientifically for decades, tremendous obstacles remain to be overcome before commercial fusion reactors come to fruition. Both physics and engineering challenges must be tackled to develop practical, economical, and safe fusion power plants. Some key issues still facing fusion development include:
Plasma Stability – Microinstabilities and turbulence within the plasma can enhance the transport of particles and energy out of the core, degrading confinement. Disruptions triggered by macroscopic instabilities can completely terminate the plasma. Advanced control systems, optimized coil configurations, flow shear stabilization, and other methods are needed to improve plasma stability.
Fusion Materials – The extreme conditions inside a fusion reactor place stringent demands on structural materials used for the vacuum vessel, blanket, divertor, and magnets. They must withstand high temperatures, intense neutron irradiation, plasma exposure leading to erosion, sputtering and blistering, thermomechanical stresses from cyclic duty, and embrittlement over long service lifetimes. Developing radiation-resistant materials compatible with the fusion environment remains a major materials science challenge.
Tritium Breeding – D-T fusion reactors must breed their own tritium fuel from lithium in the blanket to be self-sufficient. Uncertainties in nuclear data, modeling assumptions, and blanket physics introduce uncertainties in tritium breeding ratio predictions as high as 5-15%. Advanced breeding blanket designs with neutron multipliers are needed to reliably exceed the minimum TBR of 1.0 for tritium sustainability, while also extracting high-grade heat and shielding the vacuum vessel and magnets.
Heat Exhaust – Handling the enormous heat flux escaping the plasma to the divertor and first wall is a major challenge for compact, high-power density tokamaks. Average fluxes can exceed 10 MW/m2 in the divertor. Novel solutions such as vapor shielding, rotating liquid metals, and advanced divertor configurations are required to spread and dissipate this power below material limits.
Magnet Technology – All magnetically confined reactors rely on precise, stable, high-field superconducting magnets to confine and shape the plasma. HTS magent innovation is still required to improve performance, reliability, quench tolerance, and durability under fusion conditions. The high cost of large device construction motivates compact HTS designs. Qualifying industrial HTS production is also key.
Fusion Engineering – Designing an integrated power plant also demands expertise across numerous disciplines – tritium fuel processing, high heat flux handling, hybrid fission-fusion systems, safety and licensing, remote maintenance, balance of plant, and more. Substantial engineering work remains to translate conceptual designs into practical, reliable fusion power systems.
Economics – Fusion energy ultimately must produce electricity at competitive prices to gain widespread adoption. Reducing the capital costs of fusion plants through increased engineering efficiency, economies of series production, and design simplification remains imperative. Improving Tokamak modeling tools and expanding private investment can aid cost optimization.
Overcoming these challenges will require substantial resources, coordinated efforts across public and private entities, and further revolutionary advances in fusion science and technology. But the societal and economic benefits of safe, clean, and abundant fusion energy continue to provide strong motivation.
Outlook for the Fusion Age
After decades of steady progress, the realization of practical fusion energy now appears increasingly within reach. Burning plasma experiments like ITER will soon demonstrate the integral performance of reactor-scale fusion systems and provide vital data to validate modeling tools. As component technologies like high-field magnets continue maturing, private companies are positioning themselves to capitalize on the fusion opportunity.
With expanded resources and cross-sector collaboration, fusion could progress from electricity grid pilot plants in the 2030s to wide-scale adoption by mid-century. Moving even faster, if new physics concepts or breakthrough innovations emerge, is not out of the question.
Fusion promises to transform energy production and enable sustainable growth for human civilization and prosperity. While monumental challenges remain, we have never been closer to harnessing the power of the stars. The clean energy abundance of the fusion age now lies tantalizingly before us. With ongoing research building on today’s progress, its realisation may soon shift from aspiration to reality.