Fusion occurs naturally throughout the universe inside of stars. Enormous clouds of Hydrogen gas start to collapse due to the force of gravity, causing them to heat up as they do so. Eventually, once the cloud gets dense enough and hot enough, fusion reactions will occur, heating the cloud up further and igniting even more fusion reactions. This is how a star is born.

Temperatures found at the cores of normal main sequence stars are typically 15 million degrees. This number is lower than the 20 KeV mentioned earlier, so the probability of a reaction is thus much lower. The star makes up for this by utilizing a huge reaction volume, a very high density, and an eternally long confinement time. Obviously, gravity only acts on very large scales, so the construction of a gravitationally confined fusion reactor is out of the question, short of creating a whole new star.

NASA image of the sun

This is currently the most popular form of fusion, having been researched for the past 50 years. Magnetic confinement schemes use huge toroidal (doughnut-shaped) chambers, surrounded by electromagnetic coils to trap and confine a plasma. The plasma is heated using high power radio frequency generators, producing what is known as a Maxwellian temperature distribution. The average kinetic energy of the plasma is typically 10-20 KeV, but there are some particles going much faster, and some going much slower, as seen in the graph at right. Since the more energetic particles are the ones more likely to fuse, you lose some efficiency with the lower energy particles. Heating the plasma to a higher energy can help mitigate this, but that takes even more input energy.

The most popular magnetic confinement reactor is the Tokamak (pictured at right, from general atomics website), a Russian reactor design from decades ago. Many people believe that this is currently the most promising fusion design that is out there; once you build one big enough, the fusion energy lost from the plasma will be minimal and the reaction will self-sustain. Unfortunately, none have managed to do that yet, and building a Tokamak reactor costs billions of dollars, making it hard for anybody but the biggest organizations to conduct experiments. None has run for a period of longer than a few seconds, which is unacceptable for power production and extraction. Better cooling systems will need to be devised. How big will one have to be? How long can it run before neutrons emitted in the fusion reactions compromise the structure? These are all questions that will need to be addressed if the concept becomes power-ready.

Inertial Confinement Fusion

This form of confinement involves compressing pellets of fusion fuel to ultrahigh densities using converging laser or particle beams. The beams impinge upon the pellet, ablating material on the surface, thus creating an inward implosive force. The fusion fuel burns, creating a high energy pulse of neutrons and charged particles. This procedure requires astronomical amounts of energy input, using some of the world's most powerful lasers. Couple that with the fact that it is extremely hard to efficiently extract energy from a pulsed system, and inertial confinement fusion begins to look less viable altogether. These problems must be solved before ICF, or any pulsed fusion scheme, can be considered as a future power source.

This isn't really a form of controlled fusion, but it is worth mentioning for historical reasons. The only breakeven fusion reaction achieved on earth has been in the form of hydrogen bombs. A few kilograms of fusion fuel is compressed by the x-ray flux from an atomic bomb, causing fusion reactions to ignite and self-sustain briefly until the energy release blows the system apart. There are very few, if any constructive use for huge hydrogen bombs, and it is not in our best interest to proliferate them.

Ivy Mike, the first Hydrogen bomb

An amateur IEC device

This is a relatively small scale method of doing fusion developed by television pioneer Philo T. Farnsworth many years ago. It uses electrostatic fields and potentials to accelerate and collide ions of fusion fuel in a spherical geometry. An IEC device is quite easy to make, and can even be done by a determined individual for roughly $1000. It is unlikely that an IEC device will break even unless some drastic and novel design changes are made. At the present time, they are useful for demonstrating the concept of fusion, and as a low-cost neutron source for materials identification, well logging, and medical isotope production. If one ever achieves breakeven in the future, it would most likely be used as a rocket engine or small scale power source, not a huge centralized power station like the ones common in the world today. Professional IEC research takes place at institutions like MIT, U of Illinois, U of Wisconsin, Los Alamos, NASA, and at several locations in Japan. 

Cold Fusion, or "Low Energy Nuclear Reactions" (LENR) has been the subject of much controversy over the past few years. While there are many "types" of cold fusion experiments out there, the best known one involves running electricity through Deuterium Oxide (Heavy Water) in a process known as electrolysis. Using a Palladium cathode in the electrolysis system, one can expect to "bury" large amounts of Deuterium in the crystal structure of the cathode. Some believe that this can lead to exothermic fusion-like reactions.

Neutrons aren't usually detected in cold fusion systems, but many experimenters have observed excess energy in the form of heat. This can mean one of several things. Some believe this phenomenon could be due to result of a previously unknown process, while skeptics believe it's simply the result of experimental error or improper techniques.

A cold fusion electrolysis cell

By far the easiest way to produce fusion reactions is simply to slam a beam of Deuterium atoms into a stationary, solid target of either Deuterium or Tritium. This method is useful for producing neutrons, but clearly not for electrical power.

Although we have many methods of obtaining fusion reactions, ranging from simple to complex, the underlying principle remains the same. All you need to do is find a way to make Deuterium nuclei get close enough to one another so they fuse. Go onto the next section to see an easy way to create fusion reactions in your own home laboratory!