In its original and still-unrealized conception, a tokamak traps steadily fusing plasma within a helical magnetic field that winds around, and delimits, a bagel-shaped confinement vessel nested inside a metal chamber.
Two components create the helical field: a toroidal component, which arises from ring-shaped magnets positioned at regular intervals around the chamber, and a poloidal component, which arises from the circulation of the plasma’s fast electrons and slower ions.
Confinement can never be perfect because Coulomb collisions cause electrons and ions to drift across magnetic field lines.
In the mid-1990s, plasma physicists began to realize that a range of instabilities could be mitigated if the current-carrying plasma were made to rotate inside the chamber. The stabilization would be more effective if the rotation had shear, for then it could tear apart nascent eddies before they grew to burst-out size.
Experiments at various tokamaks around the world found that plasmas rotate...
