3D topology of RFX-mod edge

The edge of magnetic confined plasmas is often characterized by a 3D topology due the distortion of magnetic flux surfaces along the toroidal and poloidal directions shaped by the presence of magnetic islands. These islands are created purposely in the tokamak to stabilize edge localized modes (ELMs), while they are spontaneously present in reversed-field pinches (RFPs) and stellarators. In the RFX-mod (R=2m, a=0.46m) RFP, high current plasma regimes (If>1MA, n/nG<0.35, with nG the Greenwald density) are characterized by the resonance of an m/n = 1/7 tearing mode in the plasma core (r/a~0.3), that molds all plasma column in a 3D helical topology [1]. The resulting edge magnetic ripple (~1%) is enough to modulate edge floating potential, electron density and temperature, particle flux and flow. The presence of this magnetic island in the central region produces, thanks to the coupling with the Shafranov shift, a chain of "remnant" islands (m/n = 0/7) on the q = 0 surface in the edge region, where the typical m = 0 resonances develop. The resulting magnetic topology is quite complex: in the low-field side (LFS) the m = 0 O-point (OP) is aligned toroidally with the m = 1 X-point (XP), the converse in the high-field side (HFS), where the m = 0 OP is aligned with the m = 1 OP [2]. A useful tool for this analysis is the introduction of a frame of reference travelling with the dominant mode, i.e. the helical angle u = m - n + t [1], in order to correctly relate different toroidal and poloidal measurements with respect to the plasma helix. Considering measurements resolved both in the toroidal and the poloidal directions (floating potential and electron density and temperature), the results indicate that the modulation in the edge plasma is not a simple 1/7 harmonic but the shape of plasma response shows the presence of sidebands in the (m,n) spectrum.