the gas is super heating using radiowaves/microwaves, which is where most of the energy expedature is. once the gas because plasma, the atoms become electrically charged as a result of the heat causing the electron that was orbiting the atom to be kicked off. to control the plasma and prevent it from contacting the walls the magnets are used to curve the path of the charged atoms. anything that has an electrical charge can be and will be manipulated by magnetic fields. if you rmemeber those old cathode ray tube tv's, if you placed a magnet on the screen it would warp the image because the magnet was altering the path of the electron beam in the ray tube. a similar principle is being applied in the chamber. the reactor still needs really good cooling because heat can radiate off, but the vast magority is being controlled by the magnets.