Magnetic confinement fusion

Fusion reactions for reactors usually combine light atomic nuclei of deuterium and tritium to form an alpha particle (helium-4 nucleus) and a neutron, where the energy is released in the form of the kinetic energy of the reaction products. In order to overcome the electrostatic repulsion between the nuclei, the fuel must have a temperature of hundreds of millions of kelvin, at which the fuel is fully ionized and becomes a plasma. In addition, the plasma must be at a sufficient density, and the energy must remain in the reacting region for a sufficient time, as specified by the Lawson criterion (triple product). The high temperature of a fusion plasma precludes the use of material vessels for direct containment. Magnetic confinement fusion attempts to use the physics of charged particle motion to contain the plasma particles by applying strong magnetic fields.

Tokamaks and stellarators are the two leading MCF device candidates as of today. Investigation of using various magnetic configurations to confine fusion plasma began in the 1950s. Early simple mirror and toroidal machines showed disappointing results of low confinement. After the declassification of fusion research by the United States, United Kingdom and Soviet Union in 1958, a breakthrough on toroidal devices was reported by the Kurchatov Institute. Its tokamak demonstrated a temperature over around 10 million kelvin and milliseconds of confinement time, and was confirmed by a visiting British team. Since then, tokamaks became the dominant line of research globally with JET, TFTR and JT-60 being constructed and operated. The ITER tokamak experiment under construction, which aims to demonstrate scientific breakeven, will be the world's largest MCF device.

Experiments with deuterium-tritium plasmas in TFTR created 1.6 GJ fusion energy during 1993-1996. The peak fusion power was 10.3 MW from 3.7 x 10¹⁸ reactions per second, and peak fusion energy created in one discharge was 7.6 MJ. Subsequent experiments in JET achieved a peak fusion power of 16 MW (5.8 x 10¹⁸ /s) during DTE1 , and a discharge producing 69 MJ during the recent DTE3 (consuming 0.2 mgm of D and T).

The current record of fusion power generated by MCF devices is held by JET. In 2021, JET sustained a gain factor of Q = 0.33 for 5 seconds and produced 59 megajoules of energy. The record for the gain factor was 0.67 achieved in 1997, and the gain in the plasma core Q_core for that discharge is calculated to be 1.3, using the TRANSP integrated modeling code.

While early stellarators of low confinement in the 1950s were overshadowed by the initial success of tokamaks, interests in stellarators re-emerged owing to their inherent capability for steady-state and disruption-free operation, distinct from tokamaks. The world's largest stellarator experiment, Wendelstein 7-X, began operation in 2015.

One of the challenges of MCF research is the development and extrapolation of plasma scenarios to power plant conditions, where good fusion performance and energy confinement must be maintained. Potential solutions to other problems such as divertor power exhaust, mitigation of transients (disruptions, runaway electrons, edge-localized modes), handling of neutron flux, tritium breeding and the physics of burning plasmas are being actively studied. Development of new technologies in plasma diagnostics, real-time control, plasma-facing materials, high-power microwave sources, vacuum engineering, cryogenics and superconducting magnets are essential in MCF research.

Types

Magnetic mirrors

A major area of research in the early years of fusion energy research was the magnetic mirror. Most early mirror devices attempted to confine plasma near the focus of a non-planar magnetic field generated in a solenoid, with the field strength increased at either end of the tube. In order to escape the confinement area, nuclei had to enter a small annular area near each magnet. It was known that nuclei would escape through this area, but by adding and heating fuel continually it was believed this could be overcome.

In 1954, Edward Teller gave a talk in which he outlined a theoretical problem that suggested the plasma would also quickly escape sideways through the confinement fields. This would occur in any machine with convex magnetic fields, which existed in the centre of the mirror area. Existing machines were having other problems and it was not obvious whether this was occurring. In 1961, a Soviet team conclusively demonstrated this flute instability was indeed occurring, and when a US team stated they were not seeing this issue, the Soviets examined their experiment and noted this was due to a simple instrumentation error.

The Soviet team also introduced a potential solution, in the form of "Ioffe bars". These bent the plasma into a new shape that was concave at all points, avoiding the problem Teller had pointed out. This demonstrated a clear improvement in confinement. A UK team then introduced a simpler arrangement of these magnets they called the "tennis ball", which was taken up in the US as the "baseball". Several baseball series machines were tested and showed much-improved performance. However, theoretical calculations showed that the maximum amount of energy they could produce would be about the same as the energy needed to run the magnets. As a power-producing machine, the mirror appeared to be a dead end.

In the 1970s, a solution was developed. By placing a baseball coil at either end of a large solenoid, the entire assembly could hold a much larger volume of plasma, and thus produce more energy. Plans began to build a large device of this "tandem mirror" design, which became the Mirror Fusion Test Facility (MFTF). Having never tried this layout before, a smaller machine, the Tandem Mirror Experiment (TMX) was built to test this layout. TMX demonstrated a new series of problems that suggested MFTF would not reach its performance goals, and during construction MFTF was modified to MFTF-B. However, due to budget cuts, one day after the construction of MFTF was completed it was mothballed. Mirrors have seen little development since that time.

Toroidal machines

Z-pinch

The first real effort to build a controlled fusion reactor used the pinch effect in a toroidal container. A large transformer wrapping the container was used to induce a current in the plasma inside. This current creates a magnetic field that squeezes the plasma into a thin ring, thus "pinching" it. The combination of Joule heating by the current and adiabatic heating as it pinches raises the temperature of the plasma to the required range in the tens of millions of degrees Kelvin.

First built in the UK in 1948, and followed by a series of increasingly large and powerful machines in the UK and US, all early machines proved subject to powerful instabilities in the plasma. Notable among them was the kink instability, which caused the pinched ring to thrash about and hit the walls of the container long before it reached the required temperatures. The concept was so simple, however, that herculean effort was expended to address these issues.

This led to the "stabilized pinch" concept, which added external magnets to "give the plasma a backbone" while it compressed. The largest such machine was the UK's ZETA reactor, completed in 1957, which appeared to successfully produce fusion. Only a few months after its public announcement in January 1958, these claims had to be retracted when it was discovered the neutrons being seen were created by new instabilities in the plasma mass. Further studies showed any such design would be beset with similar problems, and research using the z-pinch approach largely ended.

Stellarators

An early attempt to build a magnetic confinement system was the stellarator, introduced by Lyman Spitzer in 1951. Essentially, the stellarator consists of a torus that has been cut in half and then attached back together with straight "crossover" sections to form a figure-8. This has the effect of propagating the nuclei from the inside to outside as it orbits the device, thereby cancelling out the drift across the axis, at least if the nuclei orbit fast enough.

Not long after the construction of the earliest figure-8 machines, it was noted that the same effect could be achieved in a completely circular arrangement by adding a second set of helically-wound magnets on either side. This arrangement generated a field that extended only part way into the plasma, which proved to have the significant advantage of adding "shear", which suppressed turbulence in the plasma. However, as larger devices were built on this model, it was seen that plasma was escaping from the system much more rapidly than expected, much more rapidly than could be replaced.

By the mid-1960s it appeared the stellarator approach was a dead end. In addition to the fuel loss problems, it was also calculated that a power-producing machine based on this system would be enormous, the better part of a thousand feet (300 meters) long. When the tokamak showed exceptional performance in 1968, interest in the stellarator vanished in the USA, whose latest design at Princeton University, the Model C, was converted to the Symmetric Tokamak in 1970. Stellarator research continued in Germany, the Soviet Union, and Japan.

Stellarators have seen renewed interest since the turn of the millennium as they avoid several problems subsequently found in the tokamak. Newer models have been built, but these remain about two generations behind the latest tokamak designs.

While stellarators, today, are less widely used and built than tokamaks, they are still attractive for their steady-state operation and reduced risk of plasma disruptions, as compared to tokamaks. However, their main disadvantage is the higher complexity in building the systems as well as having lower energy confinement when compared to similar tokamaks. However, more recent studies have found a new family of stellarator magnetic-field designs that try to discount these setbacks. With advanced optimization techniques, researchers have found ways to better confine the particles to reach the energy confinement closer to tokamaks. Additionally, these new designs maintain the steady, stable benefits of stellarators. The results build upon the Wendelstein 7-X experiment which demonstrated similar qualities with a stellarator. Increasing research and design allow researchers to refine magnetic geometries that were previously too complex, which may lead to a new generation of stellarator reactors to compete directly with the industry dominant tokamaks.

Tokamaks

In the late 1950s, Soviet researchers noticed that the kink instability would be strongly suppressed if the twists in the path were strong enough that a particle traveled around the circumference of the inside of the chamber more rapidly than around the chamber's length. This would require the pinch current to be reduced and the external stabilizing magnets to be made much stronger.

In 1968 Soviet research on the T-3 tokamak was presented at the 3rd IAEA international fusion energy conference, with results that far outstripped existing efforts from any competing machine. Since then, the majority of effort in magnetic confinement has been based on the tokamak principle. In the tokamak, a current is periodically driven through the plasma itself, creating a field "around" the torus that combines with the toroidal field to produce a winding field, in some ways similar to that in a modern stellarator (in that nuclei move from the inside to the outside of the device as they flow around it).

In 1991, START was built at Culham, UK, as the first purpose-built spherical tokamak. This was essentially a spheromak with an inserted central rod. START produced impressive results, with β values at approximately 40% - three times that produced by standard tokamaks at the time. The concept has been scaled up to higher plasma currents and larger sizes, with the experiments NSTX (US), MAST (UK) and Globus-M (Russia) currently running. Spherical tokamaks have improved stability properties compared to conventional tokamaks and as such the area is receiving considerable experimental attention. However, spherical tokamaks to date have been at low toroidal field and as such are impractical for fusion neutron devices.

A recent bibliometric and patent trend analysis studied the global development and research of tokamak technology in the decade prior to 2024. The authors highlight China, South Korea, and Japan as the top contributors in the previous decade, focusing on superconducting magnets in high temperatures. Another emerging area of research in the latter half of this study's dates is artificial intelligence and machine learning control. Researchers have utilized AI and machine learning resources to predict and control plasma stability. Ultimately, the industry seems to be heading towards the commercial realm, considering the study's findings on patent trends, suggesting potential industrialization with tokamak system fusion technologies. Collectively, the research points towards a more practical, steady operation in future reactors.

In tokamak reactors, operational regimes (or modes in which a system operates) are a critical focus in their design. High-confinement mode (H-mode) has led to great success in reaching high energy gain; however, it is always accompanied by edge-localized modes (ELMs), which have the potential to damage plasma-facing components. One alternative regime, I-mode, offers improved confinement without the ELMs and instabilities on the edges. However, I-mode had only been found in tiny, short pulses. In 2023, researchers operating the Experimental Advanced Superconducting Tokamak (EAST) achieved what they called "Super I-mode." Super I-mode both improved confinement, as well as the duration of confinement to over 1000 seconds: one of the longest improved-confinement discharges ever observed in a tokamak system. Super I-mode also was found to maintain all of the benefits of I-mode while extending the duration of the pulses, making a massive step towards steady operation that would be needed in a power plant. These findings may be the first step in overcoming the limitations of H-mode and the ELMs that accompany H-mode.

Compact toroids

Compact toroids, such as the field-reversed configuration or the spheromak, attempt to combine the improved confinement of closed magnetic surface configurations with the simplicity of machines with no central core. An early experiment of this type in the 1970s was Trisops, which formed two theta pinch (θ-pinch) rings and fired them toward each other.

Although tokamaks and stellarators dominate the magnetic confinement fusion field of research, other configurations are also being researched to explore possible alternatives. Field-reversed configurations (FRCs) are one example of compact toroidal devices that do not require large external toroidal coils. "Norm," a recently proposed configuration, seemed to address many stability issues that have limited FRC success. Relative to previous approaches with FRCs, Norm may enable one hundred times more power at a fraction of the cost. FRCs also operate with alternative fuel sources such as proton-boron-11, compared to deuterium tritium used in tokamak systems. While Norm, and other FRCs like it are in early stages of research and the claim has yet to be proven, it is a step towards smaller, simpler, and more cost-effective reactors that could compete with the industry dominating tokamaks and stellarators.

Other

Some more novel configurations produced in toroidal machines are the reversed field pinch and the Levitated Dipole Experiment.

The US Navy has also claimed a "Plasma Compression Fusion Device" capable of TW power levels in a 2018 US patent filing:

"It is a feature of the present invention to provide a plasma compression fusion device that can produce power in the gigawatt to terawatt range (and higher), with input power in the kilowatt to megawatt range."

However, the patent has since been abandoned.