Fusion is the process that powers the sun and the stars. In one type of this reaction, two atoms of hydrogen combine together, or fuse, to form an atom of helium. In the process some of the mass of the hydrogen is converted into energy. The easiest fusion reaction to make happen combines deuterium (or “heavy hydrogen”) with tritium (or “heavy-heavy hydrogen”) to make helium and a neutron. Deuterium is plentifully available in ordinary water. Tritium can be produced by combining the fusion neutron with the abundant light metal lithium. Thus fusion has the potential to be an inexhaustible source of energy.

Hydrogen gas is typically heated to very high temperatures (100 million degrees or more) to give the atoms sufficient energy to fuse. In the process the gas becomes ionized, forming a plasma. If this plasma is held together (i.e. confined) long enough, then the sheer number of fusion reactions may produce more energy than what's required to heat the gas, generating excess energy that can be used for other applications. The sun and stars do this with gravity. More practical approaches on earth are magnetic confinement, where a strong magnetic field holds the ionized atoms together while they are heated by microwaves or other energy sources, and inertial confinement, where a tiny pellet of frozen hydrogen is compressed and heated by intense radiation, such as a laser beam, so quickly that fusion occurs before the atoms can fly apart.

Who cares? Scientists have sought to make fusion work on earth for over 50 years. If we are successful, we will have an energy source that is inexhaustible. One out of every 6500 atoms of hydrogen in ordinary water is deuterium, giving a gallon of water the energy content of 300 gallons of gasoline. In addition, fusion would be environmentally friendly, producing no combustion products or greenhouse gases. While fusion is a nuclear process, the products of the fusion reaction (helium and a neutron) are not intrinsically radioactive. Short-lived radioactivity may result from interactions of the fusion products with the reactor walls, but with proper design a fusion power plant would be passively safe, and would produce no long-lived radioactive waste. Design studies show that electricity from fusion should eventually be about the same cost as present day sources.

We’re getting close! While fusion sounds simple, the details are difficult and exacting. Heating, compressing and confining hydrogen plasmas at 100 million degrees is a significant challenge. A lot of science and engineering had to be learned to get fusion to where we are today. Both magnetic and inertial fusion programs expect to build their next experiments which will produce more energy than they consume within the next 15 years. If all goes well, commercial application should be possible by the middle of the 21st century, providing humankind a safe, clean, inexhaustible energy source for the future.