Controlled Thermonuclear Fusion

Thermonuclear fusion is a method to attain nuclear fusion by using extremely high temperatures. There are two types of thermonuclear fusion: uncontrolled, wherein the resulting energy is released in an uncontrolled manner, for example, in thermonuclear weapons ("hydrogen bombs") and in most stars. The second form is controlled, wherein the fusion reactions happen in an environment allowing some or all of the energy released to be harnessed for constructive purposes. 

Temperature requirements

Temperature is a system consisting of the average kinetic energy of particles. After the sufficient temperature is reached, according to the Lawson criterion, the energy of accidental collisions occurring within the plasma is large enough to overcome the Coulomb barrier which might lead the particles to fuse together.

There are two effects that lower the actual temperature required. Some nuclei at the sufficient temperature would actually have much higher energy than 0.1 MeV, while others would be much lower. For most of the fusion reactions, nuclei in the high-energy tail of the velocity distribution matters. The second effect is quantum tunneling. The nuclei do not actually possess enough energy to overcome the Coulomb barrier fully. If they have the approximate required energy, they can tunnel through the remaining barrier. 

Confinement

The key problem in achieving thermonuclear fusion is the confinement of the hot plasma. , the plasma cannot be in direct contact with any solid material in high temperature and therefore, has to be located in a vacuum. At high pressures, the plasma tends to expand and some force is required to act against it. This force can be the gravitation in stars, magnetic forces in magnetic confinement fusion reactors, or inertia.

Gravitational confinement

Gravitational force is capable of confining the fuel well enough to satisfy the Lawson criterion. The mass needed is massive that gravitational confinement can be found only in stars. In stars which satisfy the mass required, after the supply of hydrogen gets over in their cores, their cores (or a shell around the core) start fusing helium to carbon. In the heaviest stars (at least 8–11 solar masses), the process is continued until some of their energy is made by fusing lighter elements to iron. Iron has one of the highest binding energies. Thus reactions producing heavier elements are endothermic in nature. Therefore significant amounts of heavier elements are formed only in supernova explosions.

All the elements that are heavier than iron have some potential energy to release. The heavier elements can produce energy during the process of being split again back toward the size of the iron, in the process of nuclear fission happening at the end of element production. The energy which is released during nuclear fission is stored energy, probably stored even billions of years before, during stellar nucleosynthesis. 

Magnetic confinement

Electrically charged particles follow magnetic field lines. This is applicable to fuel ions. A strong magnetic field can, therefore, trap the fusion fuel. The toroidal geometries of tokamaks, stellarators and open-ended mirror confinement systems are magnetic configurations that can be used.

The other ways to do this are;

• Inertial Confinement – rapid pulse is dispensed to achieve optimal conditions

• Electrostatic Confinement – the electrostatic field is used to confine ions

Why is this not practically possible?

Chain reactions are almost impossible to occur. Hence it’s much easier to control and stop them than fission reactions. So logically, it is better to tap into this source rather than using fission reactions. 

Unfortunately, the possibility of harnessing this energy in the near future is very less. It might not be possible for at least two decades from now. A rather disturbing thought that now prevails is, in case this is not a foreseeable and assuring future, the number of resources spent on research could have been used for other renewable sources of energy.