Growing energy requirements around the world will place a strain on our current energy sources. Affordable and plentiful energy is essential towards maintaining healthy industrial societies, as well as raising the standard of living within developing countries. Fusion energy could provide the energy to meet these requirements, having potential benefits including:

a very abundant supply of energy world-wide
an environmentally cleaner source of energy (no air pollution and little if any high level nuclear waste), as well as an alternative to fossil fuels and fission reactors
no creation of material for weapons
research and development in fusion could create technological spin-offs (superconducting magnets, high-power lasers, high speed computing, etc.)
help economic growth as a reliable electricity supply
no chance of runaway reactions leading to accidents

Fusion is the Sun's energy source, joining light atomic nuclei to form heavier atoms like helium. Here on Earth, future fusion plants will imitate the Sun, fusing deuterium and tritium atoms at temperatures over 100 million degrees K, releasing energy for a variety of uses, including electricity. The fuel for this fusion is found in water, and can therefore provide energy for the world for billions of years. Progress in fusion research indicates fusion to be a pratical energy source some time in the 21st century.

The Basics - Conditions for Controlled Thermonuclear Fusion

To cause fusion here on Earth, the atoms to be fused must be in the form of a plasma. To achieve this new state of matter, a gas is heated, causing the atoms to move very rapidly. At a high enough temperature, the electrons become separated from the nuclei, thus creating a cloud of charged particles, or ions. This cloud of equal amounts of positively charged nuclei and negatively charged electrons is called a plasma. The Sun, stars, lightning, and the gas in neon signs are all plasmas. Even higher temperatures are needed to cause the nuclei to collide and fuse. Such a condition where the thermal energy (how hot it is) of nuclei is high enough to fuse despite their repulsion is called thermonuclear.

1. Temperature. Simply put, the hotter the plasma, the more fusion occurs. This is because the nuclei will have enough energy to overcome their electromagnetic repulsion and fuse. Actually, it is possible to go too fast, causing the nuclei to zoom by each other, not staying together long enough to fuse. For the Deuterium-Tritium fusion reaction described below to work, the temperature must reach around at least 100 million K.
2. Density. The more dense the plasma, the higher the probability of collision.
3. Containment. The plasma must also be confined in order for fusion to take place. No material can contain this hot plasma; it would either damage the material or the material would cool the plasma down, so an alternative is needed. The Sun uses its huge gravitational force to squeeze the particles together. However, here on Earth, using gravity is not feasible. Instead, this confinement process is achieved through one of two methods: magnetic confinement or inertial confinement (more on these two later).
4. Confinement Time. The longer the "energy confinement time", the more likely the plasma will sustain the high temperature and cause the fusion reactions to become self-sustaining. This confinement time is simply how long it takes the energy of a confined plasma to "leak out".

And the not so Basic - The Lawson Product

Also known as the Lawson number or condition, this slightly more advanced concept is a quantitative measure that fusion scientists use to measure their progress towards achieving fusion at a practical level. Technically, it is the plasma density multiplied by the energy confinement time given a certain temperature. When this number is large enough, the fusion reactions release the same amount of energy that was used to start the reactions, also known as breakeven. At this point the Lawson product has reached a special value, known as the Lawson criterion. The reactor can achieve ignition (the point at which the reactor releases enough energy to sustain itself, despite the loss of heat through radiation and conduction) when it exceeds the Lawson criterion. The condition at which

n*t > 10²⁰ sec/m³ (Where n is the particle density and t is the confinement time)

is the Lawson criterion for the deuterium-tritium reaction at around 100 million K. What this also tells us is that the plasma can either consist of a lot of particles for a short period of time (inertial confinement) or few particles confined for a long period of time (magnetic confinement). Before we take a closer look at the two types of confinement, however, lets finally take a look at the frequently mentioned and most promising reaction for fusion on Earth in the near future.

Let's Review: