A self-generated plasma current driven by pressure gradients in a toroidal plasma — crucial for steady-state tokamak operation because it reduces the need for external current drive.
Bootstrap current is a neoclassical effect arising from the combination of pressure gradients and trapped-particle orbits in a toroidal magnetic field. In a tokamak, a fraction of particles are “trapped” in banana orbits on the outboard side. Collisions between these trapped particles and passing particles in the presence of a density or temperature gradient produce a net toroidal current — the bootstrap current.[1]
Since a tokamak’s central solenoid can only drive current inductively for a limited pulse duration, achieving steady-state operation requires non-inductive current drive. Bootstrap current provides “free” current (driven by the plasma’s own pressure gradient rather than external power), making it the most energy-efficient path to steady-state. Maximizing bootstrap fraction is a key goal of advanced tokamak research.[2]
The bootstrap current profile is determined by the pressure profile, which may not match the optimal current profile for MHD stability. Misalignment can drive neoclassical tearing modes. Active current profile control (via ECCD or NBI) is needed to tailor the total current profile.[3]