The large toroidal electric current — measured in megaamperes — driven through a tokamak plasma, which generates the poloidal magnetic field necessary for stable confinement.
In a tokamak, the plasma itself carries a large electric current that flows toroidally — the long way around the doughnut. This current, typically measured in megaamperes (MA), produces a poloidal magnetic field that combines with the externally applied toroidal field to create the helical field-line structure essential for stable plasma confinement. Without a plasma current, a simple toroidal field would cause particles to drift vertically out of the plasma in milliseconds.[1]
Inductive drive. The classic method uses the central solenoid as a transformer primary. By ramping the solenoid current, an increasing magnetic flux threads the plasma loop, inducing a toroidal voltage (loop voltage) that drives current through the resistive plasma. This method is inherently pulsed: once the solenoid's flux swing is exhausted, the pulse must end unless other current-drive methods take over.[2]
Non-inductive drive. For steady-state operation, several auxiliary techniques can sustain the plasma current without the central solenoid: neutral beam injection (NBI) imparts directed momentum to plasma ions; lower-hybrid current drive (LHCD) and electron-cyclotron current drive (ECCD) use radio-frequency waves to preferentially accelerate electrons in one toroidal direction. Additionally, the neoclassical bootstrap current — a self-generated current arising from pressure gradients and trapped-particle orbits — can supply a substantial fraction (30–80%) of the total current in high-performance plasmas.[3]
The magnitude and radial profile of the plasma current determine several critical aspects of tokamak performance:
Safety factor (q). The ratio of toroidal to poloidal field-line turns is inversely related to plasma current. Operating with q > 1 everywhere and q95 > 2–3 at the plasma edge is necessary to avoid destructive MHD instabilities such as kink modes.[1]
Energy confinement. Empirical scaling laws show that energy confinement time increases with plasma current. Higher current generally means better confinement — but also greater stored energy released during disruptions.
Disruption risk. When the current profile becomes unstable (for example, through excessive peaking or a locked magnetic island), the plasma can undergo a disruption — a sudden loss of confinement that dumps the full plasma current onto the vessel structures in milliseconds, generating enormous electromagnetic forces and localized heat loads.[2]
Plasma current is measured externally using Rogowski coils — toroidally wound pickup loops encircling the plasma — and magnetics diagnostics mounted on the vacuum vessel wall. Internal current-density profiles are reconstructed from polarimetry, motional Stark effect (MSE) measurements, and equilibrium reconstruction codes such as EFIT.