First, it is difficult to hold enough gas at this temperature for a long enough time to release more energy than it took to heat it, because the plasma would melt its container walls and cool very rapidly. Second, a reliable fusion reactor would require perfecting new types of materials. A strong, twisted magnetic field inside the doughnut shaped chamber forms an invisible fence that prevents plasma from escaping and allows the nuclei of two isotopes of hydrogen to fuse. The deuterium and tritium ions acquire enough energy that fusion takes place when they collide, producing inert helium ions. Fusion reactions similar to these are the source of heat in the core of the sun.
Fusion energy has several advantages.
First, there is an almost limitless source of fuel. Deuterium is plentiful in the ocean. Furthermore, unlike fission, which produces radioactive byproducts, the fusion process produces only a neutron and benign helium gas. Incidental activation of structural materials by the neutrons does cause some radioactivity, but much less than in fission reactors. Scientifically, fusion is a controversial area.