The main source of heating in JET is provided by two systems, neutral beam injection and ion cyclotron resonance heating. The former uses small particle accelerators to shoot fuel atoms into the plasma, where collisions cause the atoms to ionize and become trapped with the rest of the fuel. These collisions deposit the kinetic energy of the accelerators into the plasma. Ion cyclotron resonance heating is essentially the plasma equivalent of a microwave oven, using radio waves to pump energy into the ions directly by matching their cyclotron frequency. JET was designed so it would initially be built with a few megawatts of both sources, and then later be expanded to as much of 25 MW of neutral beams and 15 MW of cyclotron heating.[36]
JET's power requirements during the plasma pulse are around 500 MW[37] with peak in excess of 1000 MW.[38] Because power draw from the main grid is limited to 575 MW, two large flywheel generators were constructed to provide this necessary power.[38] Each 775-ton flywheel can spin up to 225 rpm and store 3.75 GJ.[39] Each flywheel uses 8.8 MW to spin up and can generate 400 MW (briefly).