Magnetic self organization, MHD active control and confinement in RFX-mod

Marrelli, L.; Zanca, P.; Valisa, M.; Marchiori, G.; Alfier, A.; Bonomo, F.; Gobbin, M.; Piovesan, P.; Terranova, D.; Agostini, M.; Alessi, Carol A.; Antoni, V.; Apolloni, L.; Auriemma, F.; Barana, O.; Bettini, Paolo; Bolzonella, T.; Bonfiglio, D.; Brombin, M.; Buffa, A.; Canton, A.; Cappello, S.; Carraro, L.; Cavazzana, R.; Cavinato, M.; Chitarin, G.; Bello, S. Dal; Lorenzi, A. De; Escande, Dominique; Fassina, A.; Franz, P.; Gadani, G.; Gaio, E.; Gazza, E.; Giudicotti, L.; Gnesotto, F.; Grando, L.; Guo, S. C.; Innocente, P.; Lorenzini, R.; Luchetta, A.; Malesani, G.; Manduchi, G.; Marcuzzi, D.; Martin, P.; Martini, S.; Martines, E.; Masiello, A.; Milani, F.; Moresco, M.; Murari, A.; Novello, L.; Ortolani, S.; Paccagnella, R.; Pasqualotto, R.; Peruzzo, S.; Piovan, R.; Pizzimenti, A.; Pomaro, N.; Predebon, I.; Puiatti, M. E.; Rostagni, G.; Sattin, F.; Scarin, P.; Serianni, G.; Sonato, P.; Spada, E.; Soppelsa, A.; Spizzo, G.; Spolaore, M.; Taccon, C.; Taliercio, C.; Toigo, V.; Vianello, N.; Zaccaria, P.; Zaniol, B.; Zanotto, L.; Zilli, E.; Zollino, G.; Zuin, M.

doi:10.1088/0741-3335/49/12b/s33

Cited by 59 publications

(62 citation statements)

References 29 publications

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“…The presence of two chains of islands in TEXTOR, compared to only one big island in the RFX edge, is due to the different q profile, and to the unavoidable presence of higher sidebands produced by the ergodic divertor of TEXTOR, as compared to the more sophisticated system of feedback control of RFX-mod, where individual modes can be triggered and sidebands are damped while acting on a single mode. 50 Even if the geometry of the fields in RFX and TEXTOR is different, the results are similar: electrons (energy 30 À 100 eV, depending on radius, and thermal collisions with the background e s tor ¼ 1.5) stick in limited portions of the plasma, with a $3 order of magnitude modulation of L k that retains the geometry of the parent island [see Fig. 5(a)].…”

Section: Test-particle Simulationsmentioning

confidence: 99%

Edge ambipolar potential in toroidal fusion plasmas

Spizzo¹,

Vianello²,

White³

et al. 2014

Physics of Plasmas

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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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Section: Test-particle Simulationsmentioning

confidence: 99%

Edge ambipolar potential in toroidal fusion plasmas

Spizzo¹,

Vianello²,

White³

et al. 2014

Physics of Plasmas

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“…The surface q(r rev ) = 0 is called reversal surface and is characterized by rather peculiar transport properties: it has been shown that, due to the resonance of the m = 0 modes, the reversal surface hosts a group of islands with their O and X-points [26] aligned in the toroidal direction at r ≈ r rev which reduce ion transport in the RFP edge (a sort of edge 'transport barrier'). In addition to this, the core of the RFP, especially at high current and with the help of the feedback system of active coils [27], shows the development of a helical equilibrium (single helical axis topology, i.e. SHAx regime) which features an internal transport barrier for electrons [4].…”

Section: Overview Of Density Limit Phenomenologymentioning

confidence: 99%

Investigation on the relation between edge radial electric field asymmetries in RFX-mod and density limit

Spizzo

Scarin

Agostini

et al. 2010

Plasma Phys. Control. Fusion

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In all major confinement devices (tokamaks, stellarators, spheromaks and reversed-field pinches-RFPs), a density limit has been found. Results summarized in a recent work by Puiatti et al (2009 Nucl. Fusion 49 045012) show that in the RFP high density does not cause a disruption, but a sequence of increasingly critical phenomena. First, at intermediate density there is the disappearance of the high-confinement quasisingle helicity/single helical axis regimes. Then, at densities close to the Greenwald limit, toroidally and radially localized density accumulation and radiation condensation are observed, together with a fast resistive decay of the plasma current, which constitutes the real operative limit of the device. In this paper we discuss the effect of the magnetic ripple on test particle motion, showing that the accumulation of electrons in the X-points of the edge m = 0 islands is responsible for a modulation of the radial electric field E r which is at the core of the density limit mechanism. These results can be also relevant for the explanation of X-point multifaceted asymmetric radiation from the edge formation, observed in L-mode density limit discharges of JET.

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“…This state has been achieved experimentally in the reversed-field experiment (RFXmod) [10], by increasing the plasma current up to $1:5 MA [11].…”

mentioning

confidence: 93%

Neoclassical Transport in the Helical Reversed-Field Pinch

et al. 2010

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Test particle evaluation of the diffusion coefficient in a fusion plasma in the reversed-field pinch (RFP) configuration shows distinct similarities with stellarators when the plasma spontaneously evolves towards a helical shape. The almost total absence of superbanana particles at the levels of helical deformation seen in experiment (Bh/B=10%) causes transport to be proportional to collision frequency (at low collisions). This fact excludes the possibility that the minimum conceivable transport could be inversely proportional to collision frequency, which is typical of unoptimized stellarators. This result strengthens the perspectives of the helical RFP as a fusion configuration.

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Magnetic self organization, MHD active control and confinement in RFX-mod

Cited by 59 publications

References 29 publications

Edge ambipolar potential in toroidal fusion plasmas

Edge ambipolar potential in toroidal fusion plasmas

Investigation on the relation between edge radial electric field asymmetries in RFX-mod and density limit

Neoclassical Transport in the Helical Reversed-Field Pinch

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