# Practical realization of fusion

Aside from requiring high temperatures (100-200 million degrees for the most promising reaction, burning deuterium and tritium), the product of the number density and the time the very energetic particles can be contained needs to exceed a large number of the order of 10**20s/m**3 (the exact number depends on many factors; see the paper on "Burn criteria" in the list of downloadable publications for more details). This necessary condition which has to be satisfied for fusion to occur can be realised in two ways:

- either opting for a very high density and a modest confinemement time, or

- by ensuring confinement is optimized, which then relaxes on the required density.

Not relying on any confining force other than the particles' own inertia necessitates working at very high densities. Confinement of charged particles can be drastically improved by magnetic force. Whereas one needs densities much higher than the normal solid state density when relying on inertia, the gas density only needs to be a small fraction of the atmospheric air density at sea level when using magnets. Because of the two distinct options on the road to fusion, two types of experiments are performed to realize fusion on earth: fusion machines are either based on inertial, or on magnetic confinement. So far, the latter is the most successful.