Neoclassical transport

Neoclassical transport, also known as neoclassical diffusion and often associated with banana orbits, is a type of diffusion seen in fusion power reactors. It is a modification of classical diffusion, adding in effects due to the geometry of the reactor that give rise to new diffusion effects.

Classical transport models a plasma in a magnetic field as a large number of particles travelling in helical paths around a line of force. In typical reactor designs, the lines are roughly parallel, so particles orbiting adjacent lines may collide and scatter. This results in a random walk process which eventually leads to the particles finding themselves outside the magnetic field.

Neoclassical transport adds the effects of the geometry of the fields. In particular, it considers the field inside the tokamak and similar toroidal arrangements, where the field is stronger on the inside curve than the outside simply due to the magnets being closer together in that area. To even out these forces, the field as a whole is twisted into a helix, so that the particles alternately move from the inside to the outside of the reactor.

In this case, as the particle transits from the outside to the inside, it sees an increasing magnetic force. If the particle energy is low, this increasing field may cause the particle to reverse directions, as in a magnetic mirror. The particle now travels in the reverse direction through the reactor, to the outside and then towards the inside where the same process occurs. This leads to a population of particles bouncing back and forth between two points, tracing out a path that looks like a banana from above, the so-called banana orbits.

Since any particle in the long tail of the Maxwell–Boltzmann distribution is subject to this effect, there is always some natural population of such banana particles. Since these travel in the reverse direction for half their orbit, they have a much higher collisional cross section and will scatter with the rest of the fuel mass. This gives rise to an additional diffusion term on top of the classical one.