The electrostatic response to edge islands induced by Resonant Magnetic Perturbations

Measurements of plasma potential have been experimentally determined in great detail in the edge of the RFX reversed-field pinch (RFP), and of the TEXTOR tokamak, with applied magnetic perturbations (MP's). Generally speaking, the potential has the form ?(r, ?, ?) = ?0 * sin u, with u the helical angle: this fact implies a strong correlation between the magnetic field topology and the poloidal/toroidal modulation of the measured plasma potential. In a chaotic tokamak 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. In the RFP the picture is more complicated, since X-points can act both as drivers of electron diffusion, or dynamical traps (reduced electron diffusion), depending on the helicity of the dominant island [3]. In both devices, this differential electron-to-ion diffusion, causes a strong radial electric field structure pointing outward (inward) from the island O-point. An analytical model for the plasma potential is implemented in the code Orbit [4], and analyses of the ambipolar flow shows that both ionand electron-dominated transport regimes can exist, which are known as ion and electron roots in stellarators. Moreover, the good agreement found between measured and modeled plasma potential supports that a magnetic island in the plasma edge can act as convective cell. These findings and comparison with stellarator literature, suggests that the role of magnetic islands as convective cells and hence as major radial particle transport drivers could be a generic mechanism in 3D plasma boundary layers.