Nuclear fusion energy is a reaction in which hydrogen atoms fuse at high temperatures and pressures to become helium, releasing enormous amounts of energy in the process, and it is attracting attention as a clean energy source that can solve global warming and energy shortages. There are magnetic confinement and inertial confinement methods, and scientists are currently overcoming many challenges to commercialize them.
Nuclear fusion is a reaction in which hydrogen atoms fuse at high temperatures and pressures to become helium atoms, releasing energy, and is the source of solar energy that powers more than 7 billion humans and countless other living things. The isotopes of hydrogen that fuel the fusion reaction release a lot of energy during the fusion process, which is governed by Einstein’s mass-energy equivalence principle. The mass of one helium atom after the reaction is about 0.7% smaller than the mass of four hydrogen atoms before the reaction, and this difference is called the “missing mass”. The missing mass is converted into energy during the fusion process. Power generation using this fusion reaction is about five times more efficient than nuclear fission, and can produce twice the power of a nuclear reactor with only 500 grams of fuel. It also produces less radioactive waste and greenhouse gas emissions than other power plants, making it a cleaner source of energy. What’s more, there is enough fuel in the oceans and under the earth’s surface to last humanity 15 million years, so scientists have been trying to control nuclear fusion reactions and use them as an energy source since the mid-20th century.
But for fusion reactions to occur, they require temperatures and pressures high enough to allow hydrogen nuclei to overcome electromagnetic forces and combine into helium atoms. At the center of a star like the Sun, the star’s own gravity solves this problem, but on Earth, special methods are needed to create this environment. Two methods that scientists have devised to solve this problem are magnetic confinement fusion and inertial confinement fusion. In this article, we will introduce the principles and characteristics of these two methods.
Magnetic confinement fusion, as the name suggests, uses a magnetic field to confine the plasma. It started with long linear devices, but the problem of energy loss at both ends led to the development of donut-shaped torus devices. Early torus devices used only a toroidal coil as a means of controlling the plasma, but this caused the plasma to drift inside the torus. To solve this problem, an additional magnetic field was applied to the plasma inside the torus to twist the plasma flow into a pretzel-like shape. The tokamak, invented by Tom and Sakharov of Russia in the early 1950s, and the stellarator, proposed by Lyman Spitzer of the United States, are two examples of such technologies. The tokamak uses electromagnetic induction to create an additional magnetic field indirectly by running a current through the plasma, while the stellarator generates a magnetic field directly by adding a helical coil, a pretzel-like twisted conductor, to the outside of the torus. Although tokamaks have difficulty maintaining stable and controllable plasma currents for long periods of time, they have been studied since the mid-20th century due to their simple construction. Stellarators, on the other hand, despite the ease of controlling and maintaining the current, suffered a long downturn until the 1990s due to the complexity of their structure. Today, however, thanks to technological advances, both tokamaks and stellarators are being actively researched around the world, and more complex devices are being built around the world.
Inertial confinement fusion involves rapidly compressing and heating fuel to reach fusion conditions and then burning it before it escapes. It is also known as “laser fusion” because it requires precise targeting with a powerful laser, and has been under research since the 1960s, primarily in the United States, France, and the United Kingdom. Inertial confinement occurs when a laser is directed at a small plastic bead called a pellet, and when the laser is focused on the pellet, the fuel inside the pellet reaches fusion conditions, causing it to rapidly undergo a fusion reaction and release energy. However, due to technical limitations, the energy generated by the fusion reaction is much smaller than the energy used by the laser, making it less feasible than magnetic confinement fusion. Laser fusion is divided into indirect and direct methods depending on how the laser is focused onto the pellets. The indirect method uses a cylindrical metal container (hohlraum) to focus the energy onto the pellet. When the laser is focused on the metal cylinder, the metal emits powerful X-rays and the temperature at the center of the cylinder rises to 40 million K in a hundred millionth of a second. This causes the pellet at the center of the cylinder to explode instantaneously, and the reaction compresses the fuel inside the pellet to an ultra-high density, resulting in a nuclear fusion reaction. The problem with this approach is that it is very similar to the principle of a hydrogen bomb. In fact, some researchers claim that it violates the Comprehensive Nuclear-Test-Ban Treaty and the Nuclear Non-Proliferation Treaty. Therefore, some countries, such as Japan, which has restrictions on the development of nuclear weapons-related technologies, are developing a direct laser focused on the pellet to melt and expand the shell instead of the indirect method.
To summarize, magnetic confinement fusion uses a magnetic field to confine the plasma to meet the conditions for fusion, and is divided into tokamak and stellarator depending on how the additional magnetic field is created. Inertial confinement (laser) fusion, on the other hand, aims to focus energy momentarily to trigger a fusion reaction before the fuel disperses, and is divided into direct and indirect methods depending on how the laser is irradiated. Both of these methods began as weapons, like fission, but research into nuclear fusion as an energy source began in 1961 through an international collaboration organized by the International Atomic Energy Agency. In 1998, seven countries collaborated to build the International Thermonuclear Experimental Reactor (ITER) in Kadarache, France. Despite these decades of work by scientists, the commercialization of nuclear fusion power remains a challenge. At present, it is estimated that humanity will use nuclear fusion as an energy source around 2050, a huge and complex project that will require many countries to invest manpower and resources over a long period of time. However, nuclear fusion is the ultimate solution to global warming and energy scarcity, two of the biggest problems facing humanity in the 21st century, and it requires talented people to be found and nurtured and long-term policy support.
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