Basic fusion physics

The characteristic of stars, such as our sun, is that their gravity keeps the nuclei present on them so close and hot that a fusion process is triggered, producing a huge amount of energy. On earth, the potential advantages of energy by controlled nuclear fusion are manifold:

• Limitless energy production, available all over the world, not subject to local or seasonal variations;
• No greenhouse gas emissions;
• No accidents such as melt-downs or explosions, due to an intrinsically safe physical process;
• No production of radioactive waste with long half-lives; and
• No or only a modest proliferation risk.

Energy obtained from fusion and fission reactions is based on differences in the nuclear binding energy. The mass of the products of a fusion reaction is smaller than the mass of its reactants. The difference or "missing mass" is converted into energy in accordance with Einstein’s equation E=mc². Because c is very large, a small amount of missing mass turns into a large amount of energy.

The main fuels used in nuclear fusion devises are deuterium and tritium, both heavy isotopes of hydrogen. The Deuterium (D) – Tritium (T) reaction has the largest cross section (in other words, the probability of a reaction to take place) and also the largest Q-value (the released energy of a reaction) of all varieties of fusion reactions. It produces an alpha particle (or Helium-4 nucleus) and a neutron, and releases 17.6 megaelectron volt (MeV) of energy in the form of kinetic energy of the products (3.5 MeV to alpha particle and 14.1 MeV to neutron). 

Three main conditions are necessary for a controlled thermonuclear fusion:

1. The temperature must be hot enough to allow the ions of deuterium and tritium to have enough kinetic energy to overcome the Coulomb barrier and fuse together.
2. The ions must be confined with a high ion density to achieve a suitable fusion reaction rate.  
3. The ions must be held together in close proximity at high temperature with a confinement time long enough to avoid cooling.

Nowadays, there are two main approaches for fusion energy research:

• Magnetic confinement fusion

Magnetic confinement fusion is based on the fact that ions and electrons cannot easily travel across a magnetic field. Therefore, hot plasma can be confined by strong magnetic fields.

• Inertial confinement fusion

This approach is based on maximizing density by the rapid compression and heating of a small solid DT pellet through the use of lasers or particle beams.

Because of extremely high temperatures (T ~ 10 kiloelectron volt [keV]), matter transition to plasma state occurs. Plasma is in fact called “the fourth state of matter” along with solids, liquids and gases. It consists of a fully ionized or partially ionized gas, containing ions, electrons and neutral atoms. At present, thermonuclear fusion is the main area of research in plasma physics.