The mega-ampere river of charged particles circling the long way around a tokamak — the current that creates the poloidal field and makes confinement possible
In a tokamak the plasma itself carries a large electric current that flows toroidally — the long way around the doughnut-shaped vacuum vessel. This toroidal plasma current, typically ranging from one to fifteen mega-amperes in modern devices, is not an incidental byproduct but the central mechanism that distinguishes a tokamak from other magnetic confinement concepts. The current generates the poloidal component of the magnetic field, which combines with the externally applied toroidal field to produce the helical field-line geometry required for stable confinement.[1]
The classical method uses the central solenoid as a transformer primary: ramping the solenoid current changes the magnetic flux threading the plasma torus, inducing a toroidal electric field that drives current through the resistive plasma. Because the solenoid can supply only a finite flux swing, purely inductive operation limits pulse length. Modern tokamaks therefore supplement inductive drive with non-inductive techniques — neutral beam injection (NBI), electron cyclotron current drive (ECCD), and lower hybrid current drive (LHCD) — and exploit the self-generated bootstrap current, which arises from pressure gradients in the confined plasma.[2]
The magnitude and radial profile of the plasma current determine the safety factor q, which must remain above critical thresholds (especially q > 1 on axis and q95 > 2) to avoid destructive MHD instabilities. When the current profile becomes too peaked or the total current exceeds stability limits, the plasma can undergo a disruption — a sudden, violent loss of confinement that deposits the stored magnetic and thermal energy onto the first wall in milliseconds. Disruption avoidance and mitigation are among the highest-priority engineering challenges for ITER and future power plants.[3]
The total plasma current is measured by Rogowski coils encircling the vacuum vessel. These coils sense the time derivative of the enclosed current via Ampère’s law. The internal current-density profile is reconstructed from arrays of magnetic diagnostics combined with internal measurements such as motional Stark effect (MSE) polarimetry, which probes the local magnetic field pitch angle inside the plasma.[4]