Neoclassical Transport Codes
Neoclassical codes compute collisional transport in toroidal geometry, including the bootstrap current that self-generates from the pressure gradient.
Collisions in a torus
Classical transport theory treats collisions in a straight magnetic field. In a torus, particle drifts and the trapping of particles in the outboard magnetic well enhance collisional transport substantially. Neoclassical transport codes compute this toroidal collisional transport, which sets a floor below which turbulence cannot reduce the losses and which produces the crucial bootstrap current.
Neoclassical transport is the irreducible collisional baseline; turbulent transport, computed by gyrokinetics, adds on top. In some regimes, such as the plasma core or strongly shaped edges, neoclassical effects dominate.
The bootstrap current
A pressure gradient in a torus drives a self-generated toroidal current, the bootstrap current, through the momentum exchange between trapped and passing particles. Neoclassical codes compute the bootstrap fraction, which is central to steady-state operation because it reduces the externally driven current the machine must supply.
Solving the drift-kinetic equation
Neoclassical codes solve the drift-kinetic equation with a collision operator, either through analytic transport coefficients valid in limiting collisionality regimes or through direct numerical solution that spans all regimes and arbitrary geometry. The latter, often Monte Carlo or continuum, handles the strong shaping of modern designs.
Design relevance
For the Hyperion breeder, neoclassical modeling predicts the bootstrap current fraction that helps sustain the 9.86 MA plasma current and sets the collisional transport floor. At low aspect ratio the trapped fraction is large, so neoclassical effects are pronounced, making these codes important for the design done in simulation.
- Toroidal collisional transport above classical
- Computes the self-generated bootstrap current
- Solves the drift-kinetic equation with collisions
- Sets the transport floor beneath turbulence