Edge plasma perturbations in toroidal magnetic confinement devices

The edge of magnetically confined plasmasin toroidal configurationsischaracterized by topological pertubations with helicity m/n (spontaneous in the Reversed Field Pinch[1], or produced by purpose in Tokamaks and Stellarators, with m,n the poloidal and toroidal mode numbers, respectively). The resulting magnetic topology is deeply influenced by higher harmonics(m ± 1)/nwith the same n, due to toroidal coupling. This means that poloidaland toroidalsidebands, though smaller than the base mode, can have an effect on the kinetic response of edge plasma and on the related issues(plasma transport and stability).Only afull 3D coverage of diagnostic measurements along? (poloidal) and? (toroidal) angles can reveal the real topology of the plasma.The response of the edge plasma to a magnetic perturbation (MP) is studied in the RFX-mod(R=2m, a=0.46m)device, in both Reversed Field Pinch (RFP) and Tokamak discharges, bytoroidallyand poloidally resolvedmeasurements of electron pressure, floating potential and plasma flow.In particular,in RFP discharges with a magnetic spectrum almost monochromatic and a dominant m=1, n=7 mode, the connection lengthof magnetic field lines to the wallLcwpresents a more complicated poloidal behaviour. For this reason, plasma-wall interaction,floating potential and electron pressure are still moulded as an m=1 helix, but this helix is strongly distorted along the poloidal angle by the toroidally coupled m=0mode....