Collisional particle and heat transport modified by toroidal geometry, magnetic trapping, and drift-orbit effects—the irreducible minimum that real turbulence far exceeds.
In a uniform magnetic field, collisions between charged particles produce “classical” cross-field diffusion that scales as the square of the Larmor radius times the collision frequency. In a torus, however, the magnetic field varies along a field line—stronger on the inboard (high-field) side, weaker outboard. This inhomogeneity creates two profoundly important effects: magnetic trapping and drift orbits. Neoclassical transport theory accounts for both, yielding transport coefficients that can exceed classical values by one to two orders of magnitude.1
Particles with insufficient parallel velocity to traverse the high-field side are magnetically trapped, bouncing back and forth in “banana”-shaped orbits when projected onto a poloidal cross-section. The width of a banana orbit is typically √(ε) × q × ρi, where ε is the inverse aspect ratio and q the safety factor—much larger than the Larmor radius itself. Because these trapped particles execute random walks with the banana width as the step size rather than the Larmor radius, their contribution to diffusion dominates in the low-collisionality “banana regime” relevant to hot fusion plasmas.2
Neoclassical transport divides into three collisionality regimes. In the banana regime (low collisionality, ν* < 1), trapped-particle diffusion dominates and the diffusion coefficient scales as q2ρi2νii/ε3/2. In the plateau regime (intermediate collisionality), transport becomes independent of collision frequency. In the Pfirsch–Schlüter regime (high collisionality), the fluid limit is recovered with an enhancement factor of (1 + 2q2) over classical transport. Burning plasmas operate firmly in the banana regime, making trapped-particle physics central to confinement prediction.4