Photo Credit: Lawrence Livermore National Laboratory
There are two broad categories of fusion reactions; exothermic and endothermic. When atoms lighter than iron fuse together, energy is released. Conversely when atoms heavier than iron fuse, they consume energy. In order forthe nuclei to fuse, they must come into close proximity so that the strong nuclear force can overpower the electrostatic force trying to keep the nucleiapart; this is called the Coulomb Barrier.
Due to the extreme temperatures needed for fusion to occur (in the range of millions of degrees Kelvin), no known material can withstand the aggressive conditions. Therefore confinement of the fusion process is achieved by one of three methods: gravitational, magnetic, and inertial. Gravitational confinement is what stars use to contain the reaction. It is not practical for terrestrial purposes since mankind has yet to learn how to manipulate gravity. Magnetic confinement can be used if the fuel has an electrical charge. In this case, a strong magnetic field (which does not get destroyed by high temperatures and pressures) can be used to both contain and compress the fuel as needed. Inertial confinement uses explosive compression (via high energy particle beams, lasers, fission bombs etc.) of thefuel in order to initiate and contain the process.
The useful type of fusion for energy generation purposes is the exothermic variety. In particular, the Deuterium-Tritium fusion process has been identified as the most suitable fusion fuel cycle to run in ICF (Inertial Confinement Fusion) reactors. The idea behind ICF is to use high energy lasers to explosively compress a target fuel pellet. During this compression, the shockwaves increase the pressure and temperature of the fuel to ignition conditions. Ignition occurs when the fusion reaction becomes self sustaining; which means no external power input isrequired to maintain the fuel temperature. Ignition conditions are governed by the Lawson Criterion.
If fusion energy ever does work, it will solve the global energy problems for the foreseeable future as proven in this report. Some major challenge to overcome if ICF does end up generating power include reducing the time to correctly position the target fuel pellet, reducing the cost of manufacturing a pellet to commercially viable levels, and finding the enormous financial support needed to construct the fusion power plants.