Spherical tokamak, illustration. A tokamak device uses magnetic fields to keep plasma (ionized gases, purple) in a nearly spherical shape. The plasma is heated to high temperatures for nuclear fusion. Nuclear fusion is the joining (fusing) of light elements to form heavier elements, which releases large amounts of energy. It is hoped that fusion will be a clean, renewable energy source for the future. Spherical tokamaks differ from traditional tokamaks in that they are more compact and have higher output powers.