plasma current is the electric current, measured in millions of amperes, that flows the long way around the plasma ring in a tokamak, generating the poloidal magnetic field the machine needs to confine its fuel.
The current exists because a 📝plasma is an excellent electrical conductor, and it is what separates the 📝Tokamak from its near-neighbor the stellarator: a tokamak drives a large current through the plasma itself to produce the twist in the magnetic field, while a stellarator builds that twist entirely into external coils and needs no plasma current at all. In a tokamak the current is typically driven by induction — the plasma acts as the secondary winding of a transformer whose primary is the central solenoid — which heats the plasma resistively (ohmic heating) but makes the machine inherently pulsed unless other current-drive methods take over.
The magnitudes are industrial: JET carried plasma currents of several megaamperes, ITER is designed for 15 megaamperes, and 📝SPARC for 8.7 megaamperes in a machine a fraction of ITER's size. The current is also the tokamak's characteristic liability — if confinement is suddenly lost in a disruption, the stored magnetic energy of the current dumps into the surrounding structure, which is why current control and disruption avoidance are central problems of tokamak operation. The current interacts with the 📝toroidal field to give every field line its confining helical path.