Migma - Conventional Fusion

Conventional Fusion

Fusion takes place when atoms come into close proximity and the nuclear strong force pulls their nuclei together. Counteracting this process is the fact that the nuclei are all positively charged, and thus repel each other due to the electrostatic force. In order for fusion to occur, the nuclei must have enough energy to overcome this coulomb barrier. The barrier is lowered for atoms with less positive charge, those with the fewest number of protons, and the strong force is increased with additional nucleons, the total number of protons and neutrons. This means that a combination of deuterium and tritium has the lowest coulomb barrier, at about 100 keV (see requirements for fusion).

When the fuel is heated to high energies the electrons disassociate from the nuclei, which are left as ions in a gas-like plasma. Any particles in a gas are distributed across a wide range of energies in a spectrum known as the Maxwell-Boltzmann distribution. At any given temperature the majority of the particles are at lower energies, with a "long tail" containing smaller numbers of particles at much higher energies. So while 100 KeV represents a temperature of over one billion degrees, in order to produce fusion events the fuel does not have to be heated to this temperature as a whole. Even at a much lower temperature, the rate of fusion may be high enough to provide useful power output as long as it is confined for some period of time. Increased density also increases the rate, as the energy from the reactions will heat the surrounding fuel and potentially incite fusion in it as well. The combination of temperature, density and confinement time is known as the Lawson criterion.

Two primary approaches have developed to attack the fusion energy problem. In the inertial confinement approach the fuel is quickly squeezed to extremely high densities, increasing the internal temperature in the process. There is no attempt to maintain these conditions for any period of time, the fuel explodes outward as soon as the force is released. The confinement time is on the order of nanoseconds, so the temperatures and density have to be very high in order to any appreciable amount of the fuel to undergo fusion. This approach has been successful in producing fusion reactions, but to date the devices that can provide the compression, typically lasers, require more energy than the reactions produce.

In the more widely studied magnetic confinement approach, the plasma, which is electrically charged, is confined with magnetic fields. The fuel is slowly heated until some of the fuel in the tail of the temperature distribution starts undergoing fusion. At the temperatures and densities that are possible using magnets the fusion process is fairly slow, so this approach requires long confinement times on the order of tens of seconds, or even minutes. Confining a gas at millions of degrees for this short of time scale has proven difficult, although modern experimental machines are approaching the conditions needed for net power production.