Neoclassical Transport
Collisional transport enhanced by toroidal geometry and trapped-particle orbits, the irreducible confinement floor.
Beyond classical diffusion
Classical transport estimates cross-field diffusion from collisions displacing particles by a gyroradius. In a torus the real step size is much larger, because trapped particles execute wide banana orbits and passing particles follow drift orbits. Accounting for this toroidal geometry gives neoclassical transport, which exceeds classical transport by a large factor.
Regimes
- Banana regime (low collisionality): transport set by trapped-particle banana-orbit width
- Plateau regime (intermediate): transport nearly independent of collisionality
- Pfirsch-Schluter regime (high collisionality): transport enhanced by the geometric factor q^2
The dimensionless collisionality nu-star selects the regime, and the neoclassical diffusivity is largest, relative to classical, in the collisionless banana regime.
Products of the theory
Neoclassical theory predicts not only enhanced particle and heat diffusion but also the bootstrap current and the neoclassical ion thermal conductivity, which is often the floor for ion heat transport. It also gives the neoclassical resistivity enhancement over the Spitzer value.
How it is computed
Neoclassical coefficients come from solving the drift-kinetic equation on a flux surface, or from validated analytic fits (Chang-Hinton, Sauter). Codes evaluate them from the local geometry, collisionality, and profiles as inputs to the transport equations.
The turbulence gap
In practice, turbulent (anomalous) transport usually exceeds the neoclassical level for electron and ion heat, so neoclassical theory is the lower bound rather than the full answer. Ion heat transport sometimes approaches the neoclassical floor in improved-confinement regimes. Both neoclassical and turbulent channels are evaluated for the Hyperion breeder, whose low aspect ratio makes trapped-particle effects and wide banana orbits especially significant.