Methods to sustain tokamak plasma current non-inductively for steady-state operation
A tokamak confines plasma inside a toroidal magnetic cage, but the poloidal component of that cage is generated by a toroidal plasma current. In a pulsed tokamak this current is driven inductively by a central solenoid acting as a transformer primary—but a transformer can only swing flux for a finite time before it must reset. Steady-state fusion power therefore demands non-inductive current drive: techniques that maintain the plasma current indefinitely without relying on transformer action.1
Non-inductive current drive falls into two broad families. Radio-frequency (RF) methods inject electromagnetic waves that transfer directed momentum to resonant electrons. Lower hybrid current drive (LHCD) launches slow waves at 1–8 GHz that Landau-damp on suprathermal electrons, producing efficient off-axis current. Electron cyclotron current drive (ECCD) uses millimeter-wave beams at the electron cyclotron frequency for highly localized deposition, valuable for suppressing neoclassical tearing modes. Ion cyclotron current drive and helicon wave injection are additional RF options under active development.2
Neutral beam injection (NBI) fires energetic neutral atoms into the plasma; once ionized, the fast ions carry a directed current as they slow down on background electrons and ions. NBI is robust and well-understood, but its current-drive efficiency drops at higher densities because the beam penetration depth shrinks.3
Current-drive efficiency is conventionally quoted as γ = n̅e R ICD / P, with units of 1020 A m−2 W−1. Typical experimental values range from 0.1–0.4 for LHCD, 0.02–0.05 for ECCD, and 0.2–0.4 for NBI, all well below the Fisch–Boozer theoretical ceiling. Reactor designs must combine multiple methods—often LHCD for bulk drive, ECCD for local profile control, and bootstrap current for the majority fraction—to close the steady-state power balance.4