Igniting and sustaining the fusion of light nuclei requires an astonishingly high temperature, on the scale of 100 million kelvin. No material can tolerate such an environment, and for some 60 years scientists have been studying plasmas bound within a cage of magnets to insulate the walls of a reaction vessel from the intense heat inside it. Equally important, the confinement helps avoid cooling the bulk plasma to the point of quenching the reaction.
In a tokamak—an acronym derived from the Russian toroidal’naya kamera s magnitnymi katushkami, meaning toroidal chamber with magnetic coils—the charged particles circulate within a helical magnetic field that winds around and delimits a bagel-shaped confinement region. The helical field arises from two components: a toroidal field whose lines extend the long way around the torus and are produced by a set of ring-shaped electromagnets, and a poloidal field whose lines form circles orthogonal to the...
