Pfirsch-Schluter Transport

The high-collisionality neoclassical regime, where parallel return currents enhance cross-field diffusion by roughly q squared.

The Mechanism

In a torus the vertical grad-B and curvature drifts push ions and electrons in opposite directions, tending to separate charge and create a vertical electric field. In the collisional Pfirsch-Schluter regime this is neutralized by parallel currents that flow along field lines from the top to the bottom of each flux surface, the so-called Pfirsch-Schluter currents. These currents close the drift-driven circuit but themselves cause additional radial transport.

The q-Squared Enhancement

The parallel return path has length of order q R, the connection length, where q is the safety factor and R the major radius. Following the diffusive step over this longer path enhances the cross-field particle and heat diffusivity over the classical value by a factor of order 1 + 2 q^2. Because q is typically above one across most of the plasma, Pfirsch-Schluter transport is several times classical.

When It Applies

This regime holds when the collision frequency is high enough that a particle cannot complete a poloidal circuit or a trapped banana orbit before being scattered, quantified by the normalized collisionality nu-star being much greater than one. This is typical of the cool, dense plasma edge, whereas the hot core usually sits in the low-collisionality banana regime.

Relevance

Pfirsch-Schluter transport dominates the collisional edge and contributes to the neoclassical floor on confinement. The q-squared scaling ties transport directly to the current profile. For the Hyperion breeder concept, edge collisionality and safety-factor profile are design inputs to neoclassical estimates; the machine remains a simulation study.