Nuclear fusion is the process by which at least two atomic nuclei combine to form at least one heavier nuclei, as well as other subatomic particles such as neutrons. In stars such as the Sun, this process releases vast amounts of energy (the Sun has an energy output of 3.846×10^26 watts), and has the potential to be used on Earth as a relatively source of energy. Unfortunately, as of now, no man-made fusion reactor has released more energy than has been put in, with the result that currently they are not able to be used as a source of
energy.
At room temperature and atmospheric pressure, it is impossible to achieve nuclear fusion, as the force pushing the two nuclei together must be greater than the electrostatic force between the two positive nuclei pushing them apart. In addition, as the nuclei move closer together, the force between them increases rapidly, as the electrostatic force between two charged objects is inversely proportional to the square of the distance, such that when the nuclei are extremely close, a vast force is needed to keep them from moving away from each other.
The fusion materials also need to be heated to a very high temperature, both to increase the pressure, and to convert the gas into a plasma, where the atoms become ionised, losing their electrons. The resulting matter can conduct electricity, and due to the moving charges, produces magnetic fields. This also means that when the ions are moved close to each other, there is no additional repulsion between the electrons.
When the atoms are brought close enough together, one of the nuclei moves through the Coulomb barrier (electrostatic field surrounding the nucleus) by quantum tunnelling, and the nuclei fuse. Without quantum tunnelling, the temperature needed to cause fusion would be so great(in the order of 10^9 to 10^10 kelvin) that building a fusion reactor on Earth would be impractical, and not release enough energy to justify its cost and energy consumption. In fusion reactions where the mass of the products is less than that of the reactants, energy is released overall. For example, deuterium(hydrogen-2) and tritium(hydrogen-3) can be fused to form helium-4 and a neutron, releasing energy overall of 17.6MeV.
While a fusion reaction can be achieved with a relatively small laser, there is little value in doing this, as for nuclear fusion to be a source of power, more energy must be released than put in. Such a reaction would have to satisfy the Lawson Criterion (see below). Using this equation, we can see that at an optimum temperature of 20keV, the product of the particle density of the plasma(n) and the confinement time(t) must be greater than 10^20(m^-3)s.
Inertial confinement is a promising method for nuclear fusion. First, a laser is used on a DT(deuterium-tritium pellet). The laser ablates the outer layers of the pellet, causing an equal and opposite force on the rest of the pellet, which in turn compresses it to a volume roughly 1/1000th of the original pellet core, while heating it, producing extremely high temperatures and pressures in the pellet core. For the reaction to release energy, the product of the compressed pellet density and the radius of the pellet must be greater than 3g/cm^2.
Unfortunately, to achieve such conditions, a laser with a power output of roughly 500TW must be used. As of today, only one such laser is in existence, at the National Ignition Facility in Livermore, California, which has a power output high enough to meet this condition. Despite this, even after roughly $4bn of funding, the NIF has been unable to produce a fusion reaction that released energy overall, making inertial confinement a less viable option.
Colliding beam fusion is where at least two beams of plasma are accelerated to very high speeds and collide with each other, leading to many collisions between ions moving in the beams. One advantage of this method is that lower temperatures are needed than in inertial confinement, meaning that less energy is used in heating. However, even more energy is required to confine the beams to a viable density for fusion to occur. This value was calculated by Marshall Rosenbluth to be in excess of the energy that it was possible to generate from such a method. This estimate was made in the 1950s, however, and technological advances have made colliding beam fusion more plausible. Another difficulty is that the beams are moving at such fast speeds that the nuclei are not close to each other for a long enough time for much fusion to occur, again limiting viability.
The ITER(International Thermonuclear Experimental Reactor) is a tokamak-style reactor that uses magnetic fields to concentrate plasma into a torus(ring shape). One tokamak reactor, JET(the Joint European Torus) achieved the record for the highest energy output of a fusion device, outputting 16MW of energy. Unfortunately, it required 24MW in heating and therefore was not viable. Tokamak reactors are plagued with a number of engineering problems surrounding the containment of the plasma. The original problem is that if a solenoid (coil of wire) is used to contain the field, by being stretched to form a ring around the tokamak, the field is lower on the outside, disrupting the orbit of the ions, and over time derailing them, causing them to strike the outside wall of the tokamak.
While this problem has been rectified, new problems have emerged, such as the problem of "banana orbits", where low-energy nuclei are deflected outward once they move too close to the edge, and collide with high-energy nuclei, forcing them into the outer wall of the tokamak, and resulting in fuel loss. However, the loss of fuel from this process is estimated to not be enough to prevent fusion from producing energy overall, and therefore the tokamak is still a viable and promising energy source.
Supplying fuel for fusion is not a significant problem that might hinder nuclear fusion. Deuterium accounts for roughly 1/6420 of the hydrogen atoms in water and can be refined from seawater in bulk for relatively low costs. Large refinement plants for heavy water(water but with deuterium instead of hydrogen-1) already exist in India and Canada, and production could be increased worldwide. Tritium is harder to obtain and currently is produced mostly in nuclear reactors where stray neutrons are absorbed by lithium-6 atoms surrounding the reactor, converting them into helium-4 and tritium. This could be rectified by fusing helium-3 with deuterium instead. While helium-3 is not abundant on Earth, it is believed that large sources of it exist on the Moon, and so if a permanent base on the Moon was built, helium-3 could be mined and sent back to Earth for use in fusion.
Overall, nuclear fusion is a viable and comparatively clean source of energy, as if a reactor was built which used a minimal amount of energy to initiate fusion, fuel density would be very high, and no radioactive waste is generated in fusion reactors. However, significant challenges still exist, and as a result, fusion is not currently able to be used as an energy source.
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