Boundary plasma physics in RFX-mod: Radial electric field and transport topology

Scarin, P.; Agostini, M.; Carraro, L.; Scaggion, A.; Spizzo, G.; Spolaore, M.; Vianello, N.

doi:10.1016/j.jnucmat.2013.01.114

Cited by 7 publications

(8 citation statements)

References 18 publications

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“…Also, modifications in the edge turbulence were observed and related to the local deformation of the magnetic flux surfaces [19,47,48]. The measurements presented in this paper confirm in a more detailed way that a helical structure imposes a strong modification in the electric field components and in the toroidal flow: in particular, the E r behaviour with respect to the edge topology is consistent with that highlighted through optical measurements in spontaneous high-current QSH states [54]; always with respect to the QSH states the toroidal flow distribution presented here is in very good agreement with the measurements obtained in [19] for r a with the v φ absolute value maximum located in correspondence with the maximum of 1,7 .…”

Section: Discussion Of the Resultssupporting

confidence: 81%

Electrostatic properties and active magnetic topology modification in the RFX-mod edge plasma

et al. 2013

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A better understanding of the edge phenomena regulating plasma transport and turbulence is of primary importance for the whole fusion community. In particular, in reversed field pinches (RFPs) the edge properties are found to have a strong relation with the magnetic topology. The flexibility of the RFX-mod RFP experiment (Sonato et al 2003 Fusion Eng. Des. 66–68 161–8) enables exploring different topological regimes, and the insertion of a probe equipped with a two-dimensional array of electrostatic sensors allows the characterization of the outermost plasma regions (up to 10% of the minor radius). The electric field radial profiles and its dynamics are found to be highly influenced by the presence of edge magnetic islands modelled with a Field Line Tracing code (FLiT). When a helical perturbation is applied through the saddle coil feedback system a strong modification of the edge electrostatic properties is observed. The E × B velocity field results in a convective cell-like structure according to the externally imposed toroidal periodicity. Finally, an analysis of the long-range correlations in the floating potential fluctuations (measured by two toroidally separated probes) is presented and discussed along with the electric field measurements and the edge topology.

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Section: Discussion Of the Resultssupporting

confidence: 81%

Electrostatic properties and active magnetic topology modification in the RFX-mod edge plasma

et al. 2013

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“…Moreover, the transport in the edge region proves to be smaller than the Bohm diffusivity, as already observed in a previous work [50].…”

Section: Edge Transportsupporting

confidence: 84%

Characterization of particle confinement properties in RFX-mod at a high plasma current

Auriemma¹,

Lorenzini²,

Agostini³

et al. 2015

Nucl. Fusion

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Transport phenomena affect the performance of fusion devices, reducing heat and particle confinement and consequently leading to a lower fusion triple product and higher fluxes on the plasma facing material. Special attention is devoted in the fusion community to the study of transport mitigation phenomena which lead to the onset of transport barriers in tokamaks, stellarators and reverse field pinch plasmas. The aim of this study is to deepen our understanding of the mechanisms driving transport in the presence of electron internal transport barriers in the RFX-mod single helical axis (SHAx) state (Lorenzini et al 2009 Nature Phys. 5 570). We discuss whether transport in the core can be described within the theory of chaotic transport. Light is also shed on the mechanisms acting at the plasma edge. The particle source is calculated using the 2D Monte-Carlo code NENE, considering the pattern of plasma-wall interaction. In the core region of the SHAx we found that the diffusivity along 90% of the radius is reduced with respect to the standard case: D ∼ 1 m 2 s −1 fits well with the density profile, compared with the ∼50 m 2 s −1 of a standard plasma. This result confirms a strong mitigation of magnetic chaos, although comparison with the neoclassical diffusivity D neo ∼ 0.01-0.4 m 2 s −1 indicates that transport in the SHAx is still anomalous. The same picture describes well both the particle and energy transport in the plasma core: comparison with the effective thermal diffusivity χ eff indicates that the ratio χ eff /D ∼ (m i /m e ) 0.5 is consistent with the theory of chaos-dominated transport.The plasma edge is affected by a different mechanism. In the external region (the final 10% of the radius) magnetic chaos is suppressed and the edge diffusivity is likely to be ruled by electrostatic turbulence, in particular a direct link with the presence of pressure coherent structures is found.

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“…Whenever disentangled the modulation of density and temperature as obtained from THB at r/a = 0.98 exhibits opposite behaviour [45], one with respect to the other, with respect to the phase of the dominant mode, as shown in figure 7(a). Indeed density and temperature appear to be outof-phase although it is worth noting that this behaviour appears to have a radial dependence: indeed if the same analysis is performed using a triple langmuir probe at r/a = 1, density and temperature result in-phase (see figure 7(b)).…”

Section: Pressure and Potentialmentioning

confidence: 95%

3D effects on the RFX-mod boundary

Vianello¹,

Spizzo²,

Agostini³

et al. 2013

Nucl. Fusion

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In present fusion research a strong effort is devoted to the studies of non-axisymmetric magnetic perturbations and consequent field ergodization on the external region of the plasma. On this topic interesting results can be drawn from the helical configuration observed in high-current regimes in reversed field pinches (RFPs) where the small edge helical ripple is sufficient to modulate the plasma–wall interaction and the plasma kinetic properties. This paper presents the most recent experimental results and physical interpretation of the phenomena observed in the edge region of the RFX-mod RFP device. Experimental observations indicate that plasma pressure and floating potential are spatially modulated according to the helical deformation. Helical flow is observed at the edge as a consequence of an ambipolar electric field. Emphasis will be devoted to the determination of the actual phase relation between magnetic perturbation and velocity perturbation. Evidence of the influence of the helical ripple on turbulence properties at the edge is also reported.

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Boundary plasma physics in RFX-mod: Radial electric field and transport topology

Cited by 7 publications

References 18 publications

Electrostatic properties and active magnetic topology modification in the RFX-mod edge plasma

Electrostatic properties and active magnetic topology modification in the RFX-mod edge plasma

Characterization of particle confinement properties in RFX-mod at a high plasma current

3D effects on the RFX-mod boundary

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