Abstract: A tokamak fusion reactor. The tokamak fusion reactor comprises a toroidal plasma chamber and a plasma confinement system arranged to generate a magnetic field for confining a plasma in the plasma chamber. The plasma confinement system comprises toroidal field magnets, which generate a magnetic field, BT0, in the centre of the plasma. The toroidal field magnets are configured such that, in use, the magnetic field, on conductor of the toroidal field magnets is at least 20 Tesla. The plasma confinement system is configured such that, in use, the plasma has: an aspect ratio, A, of 2 or less; an elongation, K, of at least 2; a major radius R0 of 3.5 meters or less; a normalised beta of at least 3; an engineering safety factor, qeng, of at least 2.0; wherein the engineering safety factor qeng is defined as: geng=5 BT0R0K/A2IP where Ip is the plasma current; a ratio of the fusion gain, Qfus to the fusion power, Pfus, greater than 0.03 MW?1 at fusion power, Pfus, less than 500 MW.