Neoclassical transport

The Neoclassical Transport Model is one of the pillars of the physics of magnetically confined plasmas. It provides a model for the transport of particles, momentum, and heat in complex magnetic geometries. The difference between the Neoclassical and the Classical models lies in the incorporation of geometrical effects, which give rise to complex particle orbits and drifts that were ignored in the latter.

Achievements

Neoclassical models have been used with success to predict transport under certain specific conditions. (Citation needed) In experimental studies, Neoclassical transport estimates are often used as a "baseline" transport level - even though experimental values often exceed Neoclassical estimates by an order of magnitude or more. In any case, this "baseline" level facilitates the comparison between devices. Neoclassical theory is also used in the process of machine design and optimisation. (Citation needed)

Limitations

Neoclassical theory is based on a set of assumptions that limit its range of applicability and explain why it is not capable of predicting transport in all magnetic confinement devices and under all circumstances. These are:

• Maxwellianity. This assumption implies that a certain minimum level of collisionality is needed in order to ensure that Maxwellianisation is effective. The strong drives and resulting gradients that characterise fusion-grade plasmas often lead to a violation of this assumption.
• A fixed geometry. Neoclassical transport is calculated in a static magnetic geometry. In actual experiments (especially Tokamaks), the magnetic field evolves along with the plasma itself, leading to a modification of net transport. While a slow evolution (with respect to typical transport time scales) should not e problematic, rapid changes (such as magnetic reconnections) are outside of the scope of the theory.
• The remoteness of system boundaries. Particle orbits in complex geometries can be rather wide, in some circumstances, so that the presence of system boundaries is "felt" by the plasma. Presumably, and since Neoclassical Theory assumes that the system is essentially infinite, this explains why Neoclassical Theory tends to fail near the edge of the plasma. Internal Transport Barriers may also produce non-Neoclasical effects.
• The linearity of the system. Neoclassical theory is a linear theory in which profiles are computed from sources and transport coefficients (that depend linearly on the profiles). No non-linear feedback of the profiles on the transport coefficients is contemplated. However, there are many exprimental studies that show that the profiles feed back non-linearly on the profiles (via Turbulence), leading to some degree of Self-Organised Criticality.