Fusion reactors attempt to utilize the same nuclear reaction that occurs within the sun. Two deuterium nuclei will combine together to form helium if they are close enough, releasing vast amounts of energy. The nuclear force causes them to stick together; its range is 1 fm. To get this close they have to be going at incredibly high speeds to overcome the Coulomb repulsive force. At these incredibly high speeds they would incinerate anything that has mass, like a cement containment vessel. Magnetic fields are the only way to contain these nuclei at high speeds.
How fast must the deuterium nuclei be going to touch (1 fm)
How strong must the magnetic field be to get them going this fast in a cyclotron (particle accelerator in which particles travel around in a circle)?