FisMat2017 - Submission - View

Abstract's title: Edge plasma perturbations in toroidal magnetic confinement devices
Submitting author: Paolo Scarin
Affiliation: Consorzio RFX
Affiliation Address: Corso Stati Uniti 4, 35127 Padova
Country: Italy
Oral presentation/Poster (Author's request): Poster
Other authors and affiliations: M. Agostini, G.Spizzo, L.Carraro, M.Spolaore, N.Vianello Consorzio RFX-Padova, Italy
Abstract

The edge of magnetically confined plasmas in toroidal configurations is characterized by topological magnetic pertubations with helicity m/n (with m, n the poloidal and toroidal mode numbers, respectively). These features appear spontaneously in the Reversed Field Pinch [1] but can also be produced on purpose in Tokamaks [2] and Stellarators [3]. The resulting magnetic topology is deeply influenced by higher harmonics (m ± 1)/n with the same n, due to toroidal coupling. This means that poloidal and toroidal sidebands, though smaller than the base mode, can affect the kinetic response of edge plasma and the related issues (plasma transport and stability). Only a full 3D coverage of diagnostic measurements along θ (poloidal) and φ (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, by toroidally and poloidally resolved measurements of floating potential, electron pressure and plasma flow (the last two only in RFP discharges). In particular, in RFP discharges with a magnetic spectrum almost monochromatic and a dominant m=1, n=7 mode, the connection length of magnetic field lines to the wall Lcw does not follow exactly the helicity of the dominant mode butdifferent poloidal harmonics are present. For this reason, plasma-wall interaction, floating potential and electron pressure are still moulded as an m=1 helix, but this helix is strongly perturbed along the poloidal angle by the toroidally coupled m=0 mode.

The perturbations of magnetic flux surfaces, in the toroidal and poloidal directions, are due to magnetic islands and create a complex 3D structure of Lcw which modulates the electron and ion diffusion, locally characterized by a negative charge excess (electron accumulation) at longer Lcwvalues. Ions instead are characterized by larger drifts with respect to magnetic surfaces and their diffusion is more uniform. The resulting ambipolar potential tends to balance the charge excess, and therefore presents the same symmetry as the magnetic field connection length resulting by the mode coupling.

These types of studies share a common ground between RFPs and Tokamaks where edge magnetic chaos is produced by purpose to control plasma exhaust and power flow to the walls and theseresults could be even greater in large Tokamaks since the MP coils are only in the outboard midplane, and so the poloidal helicity produced is intrinsically composed by many modes.

 

[1]  N. Vianello et al. Nucl. Fusion 53 (2013) 073025

[2] T.E. Evans, Plasma Phys. Control. Fusion 57 (2015) 123001

[3] M. Kobayashi et al. Nucl. Fusion 53 (2013) 093032