RFX-mod [Sonato et al., Fusion Eng. Des. 66, 161 (2003)] exploits its 192 active coils in both reversed-field pinch (RFP) and tokamak configurations with varying degrees of 3D shaping, providing also a test bed for validating stellarator codes and 3D nonlinear magnetohydrodynamic codes. This makes RFX-mod a unique and flexible facility for comparative studies on 3D shaping and control. The paper discusses how 3D fields allow access to RFP and tokamak advanced regimes. 3D fields are used to feedback control Single Helicity (SH) RFP equilibria with 1/7 helicity up to $2 MA. They also allow accessing SH regimes with higher density (Greenwald fraction up to 0.5), presently inaccessible in spontaneous SH regimes. Feedback on the 2/1 resistive-wall mode in RFX-mod tokamak plasmas allows for safe operation at qðaÞ < 2, an almost unexplored promising regime. Forcing the 2/1 mode to saturate at finite but small level, a helical tokamak equilibrium with significant n ¼ 1 modulation is produced and a new way to tailor sawteeth is found. The effects of different levels of 3D shaping on momentum transport in both RFP and tokamak helical states are discussed. [http://dx.
We present the results of a benchmark study involving the Orbit and Nemato codes. The two codes have been used to compare magnetic structure in a reversed-field pinch (RFP), where conserved magnetic structures/islands appear both in the core (dominated by m ¼ 1 modes) and in the edge (dominated by m ¼ 0 modes). As input, a snapshot of a 3D nonlinear MHD visco-resistive simulation (produced by the SpeCyl code) has been used. The first test is given by the reconstruction via Poincar e surface of section plot of an island generated by a single mode. In this case, the magnetic field topology corresponds to a time-independent Hamiltonian and shows conserved flux-surfaces used as a reference. Both codes successfully yield field lines which follow the same flux surfaces, in both the m ¼ 1 and m ¼ 0 cases. The benchmark between the codes has then been extended to a more complex configuration with chaotic magnetic field, using as input a fully 3D multiple mode RFP condition, characterized by the typical chain of edge magnetic islands providing a transport barrier. Finally, a quantitative benchmark has been performed, using the same 3D input, by estimating the correlation length of the magnetic field line in a bounded stochastic domain. The conclusion is that both codes yield consistent Poincar e plot structure on one hand, and very good quantitative agreement in correlation length estimate. This gives confidence to the application of both codes to magnetic topology in the RFX-mod device, for which they are used routinely, as well as for a generic fusion device. [http://dx.
The ITER scenarios and the project of DEMO involve stable operation above the Greenwald density, which justifies efforts to understand and overcome the density limit, this last observed as a disruptive termination of tokamak discharges and a thermal crash (with no disruption) of stellarator and reversed-field pinch (RFP) ones. Both in the tokamak and the RFP, new findings show that the high density limit is not governed by a unique, theoretically well-determined physical phenomenon, but by a combination of complex mechanisms involving two-fluid effects, electrostatic plasma response to magnetic islands and plasma-wall interaction. In this paper we will show new evidence challenging the traditional picture of the 'Greenwald limit', in particular with reference to the role of thermal instabilities and the edge radial electric field E r in the development of this limit.
A complete description of the effects of magnetic perturbation on the edge region of RFXmod is here reported. The flexibility of the RFX-mod device [1] allows for the operation of the machine both as a reversed field pinch (RFP, with maximum current 2 MA) and as a low-current, circular ohmic tokamak (I p,max = 0.15 MA). The present paper summarizes the most recent results obtained in both configurations with either spontaneous or induced edge radial magnetic perturbation. Emphasis will be devoted to the experimental characterization of the edge flow, focusing on the phase relation between flow and perturbed magnetic field. These informations are provided for natural and stimulated helical discharges in RFPs, and for tokamak safely operated, thanks to the unique RFX-mod MHD control system, in a wide range of edge safety factor 1.9 ≲ q(a) ⩽ 3.4 with externally imposed helical boundary. For the first time a detailed comparison between this phenomenology in tokamaks and RFPs will be presented, providing experimental measurement of the streamline of E × B flow around the magnetic perturbation and of the density modulation which exhibits the same periodicity of the perturbation. Strong new indication of the modification of the small scale turbulence in presence of magnetic perturbation is reported: this modification is deeply connected to the variation of turbulence induced particle transport.
A series of issues with toroidally confined fusion plasmas are related to the generation of 3D flow patterns by means of edge magnetic islands, embedded in a chaotic field and interacting with the wall. These issues include the Greenwald limit in Tokamaks and reversed-field pinches, the collisionality window for ELM mitigation with the resonant magnetic perturbations (RMPs) in Tokamaks, and edge islands interacting with the bootstrap current in stellarators. Measurements of the 2D map of the edge electric field E r ðr ¼ a; h; /Þ in the RFX reversed-field pinch show that E r has the same helicity of the magnetic islands generated by a m/n perturbation: in fact, defining the helical angle u ¼ mh À n/ þ xt, maps show a sinusoidal dependence as a function of u, E r ¼Ẽ r sin u. The associated E Â B flow displays a huge convective cell with vðaÞ 6 ¼ 0 which, in RFX and near the Greenwald limit, determines a stagnation point for density and a reversal of the sign of E r . From a theoretical point of view, the question is how a perturbed toroidal flux of symmetry m/n gives rise to an ambipolar potential U ¼Ũsin u. On the basis of a model developed with the guiding center code ORBIT and applied to RFX and the TEXTOR tokamak, we will show that the presence of an m/n perturbation in any kind of device breaks the toroidal symmetry with a drift proportional to the gyroradius q, thus larger for ions (q i ) q e ). Immediately, an ambipolar potential arises to balance the drifts, with the same symmetry as the original perturbation.
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