Microturbulence studies in RFX-mod

Present-days Reversed Field Pinches (RFPs) are characterized by quasi-laminar magnetic configurations in their core, whose boundaries feature sharp internal transport barriers, in analogy with tokamaks and stellarators. The abatement of magnetic chaos leads to the reduction of associated particle and heat transport along wandering field lines. At the same time, the growth of steep temperature gradients may trigger drift microinstabilities. In this work we summarize the work recently done in the RFP RFX-mod in order to assess the existence and the impact upon transport of such electrostatic and electromagnetic microinstabilities as Ion Temperature Gradient (ITG), Trapped Electron Modes (TEM) and microtearing modes.Comment: Work presented at the 2010 Varenna workshop "Theory of Fusion Plasmas". To appear in Journal of Physics Conference Serie

Alfven waves in reversed-field pinch and tokamak ohmic plasmas: nonlinear 3D MHD modeling and comparison with RFX-mod

The properties and possible triggering mechanisms of Alfven waves in the reversed-field pinch (RFP) and circular tokamak configurations are discussed in the framework of nonlinear 3D magnetohydrodynamics (MHD) modeling. Numerical simulations are performed with the SpeCyl code (Cappello and Biskamp 1996 Nucl. Fusion) that solves the equations of the viscoresistive MHD model in cylindrical geometry. Configurations with increasing levels of complexity are analyzed. First, single-wave numerical solutions are compared with analytical ones in the simplest case of a uniform axial magnetic field: an excellent agreement is obtained for both the shear Alfven wave (SAW) and the compressional Alfven eigenmodes (CAEs). Then, tokamak and RFP configurations are studied. Phenomena such as phase mixing of SAW, resonant absorption of CAEs and the appearance of the global Alfven eigenmode are described. Finally, the fully 3D RFP case with typical sawtoothing activity is investigated, showing for the first time in nonlinear RFP simulations the excitation of Alfven waves by magnetic reconnection events. The modeling results appear to be consistent with the experimental characterization of Alfvenic activity observed in RFX-mod

Nonlinear verification of the resistive-wall boundary modules in the specyl and pixie3d magneto-hydrodynamic codes for fusion plasmas

A nonlinear verification benchmark is reported between the three-dimensional magneto-hydrodynamic (3D MHD) codes specyl [Cappello and Biskamp, Nucl. Fusion 36, 571 (1996)] and pixie3d [Chacón, Phys. Plasmas, 15, 056103 (2008)]. This work substantially extends a former successful verification study between the same two codes [Bonfiglio et al., Phys. Plasmas, 17, 082501 (2010)] and focuses on the verification of thin-shell resistive-wall boundary conditions, recently implemented in both codes. Such boundary conditions feature a thin resistive shell in contact with the plasma and an ideal wall placed at a finite distance, separated from the resistive shell by a vacuum region, along with a 3D boundary flow consistent with Ohm’s law. This setup allows the study of MHD modes that are influenced by the plasma magnetic boundary, such as external kink modes. The linear growth and nonlinear saturation of external kink modes are studied in both the tokamak and reversed-field pinch magnetic configurations, demonstrating excellent agreement between the two codes. For the tokamak, we present a comparison with analytical linear stability results for the external kink mode, demonstrating remarkable agreement between numerical and analytical growth rates

Sawtooth mitigation in 3D MHD tokamak modelling with applied magnetic perturbations

International audienceThe effect of magnetic perturbations (MPs) on the sawtoothing dynamics of the internal kink mode in the tokamak is discussed in the framework of nonlinear 3D MHD modelling. Numerical simulations are performed with the pixie3d code (Chacon 2008 Phys. Plasmas 15 056103) based on a D-shaped configuration in toroidal geometry. MPs are applied as produced by two sets of coils distributed along the toroidal direction, one set located above and the other set below the outboard midplane, like in experimental devices such as DIII-D and ASDEX Upgrade. The capability of n = 1 MPs to affect quasi-periodic sawteeth is shown to depend on the toroidal phase difference Delta phi between the perturbations produced by the two sets of coils. In particular, sawtooth mitigation is obtained for the Delta phi = pi phasing, whereas no significant effect is observed for Delta phi = 0. Numerical findings are explained by the interplay between different poloidal harmonics in the spectrum of applied MPs, and appear to be consistent with experiments performed in the DIII-D device. Sawtooth mitigation and stimulation of self-organized helical states by applied MPs have been previously demonstrated in both circular tokamak and reversed-field pinch (RFP) experiments in the RFX-mod device, and in related 3D MHD modelling

Helical self-organization in 3D MHD modelling of fusion plasmas

17th International Congress on Plasma Physics, Lisbon, PORTUGAL, SEP ă 15-19, 2014International audienceThe effect of external magnetic perturbations (MPs) on the helical ă self-organization in nonlinear three-dimensional magnetohydrodynamic ă modelling of fusion plasmas is discussed. The tokamak, stellarator and ă reversed-field pinch (RFP) toroidal configurations for magnetic ă confinement are considered. In the case of current-carrying RFP and ă tokamak plasmas, MPs are predicted to stimulate the bifurcation to ă long-lived helical states, as confirmed by experimental observations in ă the RFX-mod device. For all the configurations, the effect of MPs on the ă magnetic topology is discussed and interpreted on the basis of the ă safety factor profile computed by taking into account the dominant ă helical component of the magnetic field