A compact nuclear fusion reactor for use as a neutron source is described. The reactor comprises a toroidal plasma chamber (34) and a plasma confinement system (31) arranged to generate a magnetic field for confining a plasma in the plasma chamber (34). The plasma confinement system (31) is configured so that a major radius of the confined plasma is 0.75 m or less. The reactor is configured to operate with a plasma current of 2 MA or less. The magnetic field includes a toroidal component of 5 T or less. Despite these low values, the reactor can generate a neutron output of 1 MW or more.