Fusion reactors
The main difficulty with fusion is that the small hydrogen atoms will only combine and 'fuse' at a very high temperature (150 million掳C). At this temperature hydrogen is a plasma rather than a gas.
As the plasma is too hot to come into contact with any material it must be contained in a toroidal (doughnut-shaped) magnetic field. A tokamak is a device that does this.
The technology to provide this containment is highly complex and still under development.
The new ITER standing for International Thermonuclear Experimental Reactor is a tokamak fusion reactor located in southern France.
The main issues with containment of fusion are keeping the plasma stable and in equilibrium while being able to inject more hydrogen 'fuel' and remove the heat produced.
An introduction to nuclear fusion