On neoclassical transport near the magnetic axis

The theory of neoclassical transport near the magnetic axis in a tokamak is discussed. It is shown that the ordinary treatment of transport in the plateau regime holds in most of the region within a trapped orbit width of the magnetic axis, and is not modified by "potato" orbits. It is also demonstrated that transport at low collisionality is not diffusive in nature and cannot be described independently of the sources of particles and heat in the region near the magnetic axis. A variational principle is formulated for the near-axis transport problem. [S1070-664X(00)00107-5].