Nuclear Fusion is constantly present in our solar system. In the core of the sun, Hydrogen is converted to Helium providing enough energy to sustain life on earth. This process occurs at temperatures of 10-15 million degrees Celsius. Scientists are now trying different methods here on Earth to make fusion the next large-scale energy source. The most suitable reaction occurs between the nuclei of the two isotopes of Hydrogen - Deuterium and Tritium.
Scientist have already begun to test reactions involving just Deuterium or Deuterium and Helium (3 He). The basic equation for D-T is Deuterium + Tritium = Helium-4 + neutron. The amount of material fuel needed for fusion is rather low. For example 10 grams of Deuterium and 15 g of Tritium would produce enough fuel for the lifetime electricity needs of an average person in an industrialized country.
The problems with fusion have been numerous. One of the problems is attaining the correct temperature for fusion to take place without using more energy then you produce. Fusion reactions occur at a sufficient rate only at very high temperatures. Over 100 million degrees Celsius is needed for the Deuterium-Tritium reaction, while other reactions require even higher temperatures. The density of fuel ions must be sufficiently large for fusion reactions to take place at the required rate. The fusion power generated is reduced if the fuel is diluted by impurity atoms released from surrounding material surfaces or by the accumulation of Helium "ash" from the fusion reaction.
As fuel ions are burnt in the fusion process they must be replaced by new fuel and the Helium ash must be removed. To fuel nuclear fusion reactions, scientists heat a gas until its components separate in a process appropriately named dissociation. Once the gas's particles are separated into a mass of charged particles, ions, and electrons, plasma is formed. Plasma, characterize by very hot temperatures, is the actual fuel of fusion reactions as well as the fourth state of matter. Because plasma is commonly found floating in space or in stars, it is the most common state of matter. On earth, you can see plasma in flames, aurora, fluorescent lighting, neon signs, and lightning.
Even with the hot plasma formed from heating gas until dissociation, fusion reactions aren't likely to occur because of the repulsion between similar or like charges. To overcome the repulsive force, more energy is added into the particles by heating the plasma to even greater temperatures. In order to get the plasma warm enough so that fusion can occur several different heating techniques are used. Ohmic heating, which drives a current through to plasma to create heat, is used at the beginning to heat the plasma to 20 million degrees. After 20 million degrees, Ohmic heating becomes un effective and then microwaves, radiofrequences, compression, and neutral-beam injection are used. Resonance (sound waves) created by microwaves and radiofrequences are used to give energy to ion particles, while compression of the magnetic fields increases the pressure, which in turn increases the temperature.
Neutral beams of Deuterium and Tritium, accelerated by a potential of 140, 000 volts or more, are shot through the magnetic fields where they ionize and add energy to the plasma. The last method is self-heating. Here the product of fusion, helium-4, stays in the plasma and gives its energy to other particles in the fusion that are slower and colder. In theory, the temperature could become warm enough that the energy in the helium alone would be enough to heat the plasma without the need of outside sources.
This is called ignition and it's currently being researched. Once the temperature meets the requirement around 100 million to 200 million degrees, nuclear reactions occur when particles collide together and overcome the repulsive force to become a larger particle. This temperature requirement is well above the 15 million-degree estimate of the sun's temperature. Since the temperature of plasma is far above the melting point of any solid known to exist, the confinement of plasma has been a difficult task. In space, the hot plasma is confined by gravity that compresses the plasma into reacting.
On earth, scientists have to use magnetic confinement or inertial confinement. Inertial confinement uses the property of matter that causes it to resist any change in its motion so that lasers or ion beams can bombard a glass microspheres containing a small amount of hydrogen which compresses the fuel to densities 1000 times and creates a few fusion reactions. Magnetic confinement uses strong magnetic fields arranged differently in several different structures to contain the plasma. Scientists now are working on several different fusion techniques. The question of which one will work first is left unanswered.