How does fusion occur?
Deuterium and tritium (isotopes of hydrogen) are the only elements required for fusion to occur. Deuterium can be easily harvested from seawater, a practically infinite resource. Tritium rarely occurs in nature, but can be obtained by allowing the free neutron produced by a fusion reaction to react with lithium, liberally available throughout the natural world.
A fusion reaction requires a high-density, high temperature environment, and scientists have focused on creating such an environment within plasma. Plasma is a gas that is so hot that it is ionized; the electrons separate from the nuclei and both types of particles float around in a "soup".
Currently, scientists are focusing on synthesizing fusion in a machine called a tokamak. A tokamak is a doughnut-shaped (torus) chamber that uses magnetic coils to confine plasma into such a shape.
Making and maintaining the plasma is perhaps the most difficult part of the fusion process. No material on earth can withstand a temperature of 90,000,000 degrees Celsius, so large magnet coils are used to suspend the plasma away from the cooler chamber walls. The Alcator C-Mod tokamak at the Plasma Science and Fusion Center at MIT is able to create a magnetic field 100,000 times stronger than that of the Earth. Under such high-density and high-temperature conditions, Alcator C-Mod can create up to 100 trillion fusion reactions per second.
There are two ways to "confine" the plasma on Earth, which means maintaining it at such a high temperature and pressure.
Magnetic Confinement:
Large magnet coils suspend the charged plasma away from the walls of the chamber.
Inertial Confinement:
Powerful particle beams compress and heat the plasma to the conditions necessary for fusion to occur.
Presently, scientists are focusing on magnetic confinement as the most efficient method for applications to clean energy.