3D helical flow and turbulence in RFX-mod device

The effects of non-axysimmetric magnetic field or magnetic field ergodization on the external region of the plasma is becoming a fundamental issue in the fusion community, both for intrinsic 3D configuration such as stellarators [1], and previously assumed 2D ones such as tokamaks, where magnetic perturbations are used for ELM control [2]. Interesting analogies can be drawn with the helical configuration observed in high-current regimes in RFPs [3] where the small residual helical ripple is sufficient to modulate all the kinetic properties of the edge [4]. Aim of the contribution is to present the most recent experimental results and physical interpretation of the phenomena observed in the edge region of the RFX-mod experiment, with a strong emphasis on the effects of helical deformation on the flow and turbulence. The symmetry breaking caused by the presence of a non axy-symmetric perturbation causes the appearance of an E B flow as a result of ambipolar response to balance different ion/electron diffusion. The detailed phase relationship between electric field and magnetic perturbation is in-vestigated, and to correctly handle the intrinsic 3D configuration we introduce the helical angle u m;n = mq n? ?m;n where ?m;n is the proper mode phase. The chosen framework correctly highlights the helical dependence of the plasma parameters on the existing perturbation. Ex-perimental evidences of the modulation of flow and turbulence properties, in terms of radial electric field ripple, parallel current perturbation, blobs dimension and intrinsic vorticity will be reported. The contribution will try to highlight similarities and differences between helical RFP bound-ary and analogous investigation in Tokamak RMP experiments in order to underline the benefi-cial effects arising from cross-device comparison of similar physical problems. 1 R König et al., Plasma Physics and Controlled Fusion 44, 2365 (2002). 2 T. E. Evans et al., Nature Physics 2, 419-423 (2006). 3 R Lorenzini et al., Nature Physics 5, 570-754 (2009). 4 N Vianello et al., to be published in Nuclear Fusion (2013).