The major problem on earth for fusion reaction is holding plasma at high temperature for extended period of time.
Lawson criterion for fusion reactor
In order to get an energy output greater than the energy input, a fusion reactor should achieve
n τ >10^14 s/cm³
n = the density of the interacting particles
τ = confinement time
The quantity n τ in s/cm³ is called Lawson number
In this design, the deuterium plasma is contained in a toroidal region by specially designed magnetic field. The directions and magnitudes of the magnetic field are so managed in the toroidal space that whenever a charge plasma particle attempts to go out qv×B force tends to push it back into the toroidal volume.
With such designs, confinement of the plasma has been achieved for short duration of few microseconds.
A large fusion machine known as Joint European Torus (JET) is designed to achieve fusion energy on this principle.
At the Institute of Plasma Research (IPR) Ahmedabad, a small machine named Aditya is functioning on the Tokamak design. This machine is being used to study properties of plasma.
It is an alternate method of confinement of plasma. A small solid pellet is made that contains deuterium and tritium. Intense laser beams are directed on the pellet from many directions on all the sides. The laser vaporizes the pellet converting it into plasma and then compresses it. The density increases by 10^3 to 10^4 time the initial density and temperature raises to high values. Fusion occurs in these conditions. In this method also so far, confinement for very small duration is only achieved.
The source of fuel for fusion is water only and water is abundant in oceans. Also these reactions do not result in radioactive emissions like that of fission.