Andreas Holm

Fusion: producing clean, emission-free, virtually unlimited energy for the world

Fusion is a nuclear process where two light nuclei are brought within such proximity of one another that the strong force binds them together, fusing them into a heavier element. For fusion applications on earth, the hydrogen isotopes deuterium (one proton and one neutron) and tritium (one proton and two neutrons) will undergo fusion reactions producing a helium atom and a neutron. The products carry a tremendous amount of kinetic energy based on Einstein’s famous mass-equivalence formula E=mc². The stable helium nucleus carries 20% of the energy and is used to sustain the fusion reaction, and the neutron carries the remainder of the energy isotropically to the walls. The neutrons heat the walls which in turn are cooled by water, generating steam that drives a turbine which, in turn, produces electricity.

Immensely high temperatures required

The positively charged nuclei are subject to the repulsive Coulomb force, resulting in a Coulomb barrier that needs to be overcome for fusion reactions to occur. This is achieved by supplying sufficient thermal energy, i.e. heating the fuel, also known as thermonuclear fusion. At sufficient temperatures the quantum mechanical tunneling probability through the Coulomb barrier is sufficiently high for fusion reactions to occur. The temperatures required for fusion reactions are of the order of 100s of millions of degrees. Consequently, the fuel becomes a fully ionized gas, known as a plasma.

In addition to sufficient temperatures the fuel densities need to be high for fusion reactions to occur with sufficiently high frequency. In order for the fusion reaction to produce net energy the energy of the plasma needs to be confined for sufficiently long to overcome Bremsstrahlung and transport losses. The figure of merit for fusion break-even (net energy production) and ignition (no auxillary heating required) is called the fusion triple product: it is the product of the temperature, density, and confinement time. Technical considerations, such as fuel dilution and impurity radiation, also need to be considered and considerably decrease the parameter space for break-even and ignition.

Key facts

The fusion equilibrium is inherently unstable, without risk for runaway reactions
The reaction produces helium and neutrons, no high-level radioactive waste
The fuel isotope deuterium is present all seawater, and virtually unlimited
Tritum has a half-life of 12 years, and is bred from the fusion neutrons and lithium in the reactor

An engineering challenge: vacuum vessels, cryopumps, and gigantic supercooled coils

Due to the high plasma temperatures, the plasma cannot be contained by any material. Instead, the plasma must be suspended in a vacuum. The plasma consists of free charges which gyrate around magnetic field lines according to the Lorentz force. Thus, the plasma can be confined in a vacuum vessel using magnetic field lines, known as magnetic confinement fusion. Since the plasma is not confined in the direction parallel to the field lines, a toroidal magnetic topology is utilized to remove plasma end-losses. The most common fusion device is the tokamak, where the plasma confinement is improved by inducing a current in the plasma, generating a poloidal magnetic field component, resulting in a net helical field.

Divertor and scrape-off layer physics

It is common for tokamaks to operate with a diverted configuration: the magnetic topology is diverted to a region of the device especially designed to sustain large heat loads by external magnetic coils. This separates the burning plasma core from the plasma-surface interaction, and allows for extraction of the helium ash from the core by pumping. Additionally, the divertor volume can be used for power dissipation and separates impurities created by the plasma-wall interaction from reaching the burning plasma core. Due to the strong magnetic fields, the plasma interacting with the material surface, known as the scrape-off layer, is very narrow, leading in concentrated power loads in the divertor. These power loads must be reduced to maintain the structural integrity of the device

Key facts

There are few materials that can sustain the conditions in fusion reactors and are suitable for use
Presently devices are designed to operate with tungsten and beryllium wall components
Considerable biological shielding is required due to the highly energetic neutrons of the fusion reactions
The next step in this undertaking is the completion and operation of ITER in Cadarache, France
ITER is "big science" in the true meaning of the world:
- Weights 23,000 metric tonnes
- Nearly 30m tall
- The tokamak building is 73m tall and located 13m under ground
- 35 countries share the cost
- Depicted above: note the people for scale.
- More information at https://www.iter.org/mach

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