On transport in edge islands

Magnetic islands are spontaneously present in the reversed-field pinch (RFP) and the stellarator edge, and they are produced via magnetic perturbations (MP) in the tokamak edge for various purposes (ideal mode stabilization, ELM suppression, etc.). In a chaotic edge, the ion and electron drifts yield a predominantly electron driven radial diffusion when approaching the island X-point, while ion diffusivities are generally an order of magnitude smaller. This results in a strong, positive radial electric field Er > 0 pointing outward the island O-point. This mechanism has been often neglected on the grounds that electron drifts are small [1], but this is not the case in a chaotic environment. The electric field is associated with a convective cell, which can dominate transport within the island with respect to the ordinary chaotic radial diffusion. The convective cell has, at a first approximation, the same symmetry m/n as the parent island. An analytical model for the plasma potential is implemented in the Hamiltonian, guiding-centre code ORBIT [2], and analyses of the ambipolar flow/radial electric field (in RFX-mod [3] and TEXTOR devices [4]) show that both ion- and electron-dominated transport regimes can exist, known as ion and electron roots in stellarators. This gives the hint that localized ECRH/ICRH heating can modify the ambipolar solution and therefore greatly influence transport near the island. In addition to this, recent results [5] show that the electrostatic response to the island has symmetry m/n only at a first approximation: higher harmonics (m?k)/n, k=1,2..., with the same n, toroidally couple to the base mode, influencing the electrostatic and kinetic response to the MP. We will show that this phenomenon is deeply rooted in the magnetic topology itself, by using the standard analysis of connection lengths, plus a more sophisticated metric, such as the Poincaré recurrence time analysis.