Neoclassical ripple transport in tokamaks

Abstract

The usual ripple transport calculations lead to a ν−1 scaling of the transport coefficients with collision frequency ν. The paper extends and clarifies this scaling by taking into account the fact that the dominant contributions to transport come from particles in the high-energy tail of the distribution function. This means that particles with energy E ∼(4–6)T dominate, and this restricts the ν−1 scaling range to ν(T) > (E/T)5/2 δωd(T) ∼ 100 δωd(T), where T is the thermal energy, δ is the ripple-well depth, and is the time to drift over the region where the localized ripple well exists. In addition, transport coefficients are derived in weak-ripple (α ≡ ϵ/Nqδ ≫ 1, with ϵ the inverse aspect ratio, N the number of toroidal coils, q the safety factor), low-collisionality regimes where the radial step size is determined by the distance a particle drifts before it ‘collisionlessly detraps’ from a ripple well. The maximum transport rate is found to be about two orders of magnitude smaller than usually assumed, primarily because of the restriction of the ν−1 scaling regime to the much higher collision frequencies.