Helical equilibria and magnetic structures in the reversed field pinch and analogies to the tokamak and stellarator

Puiatti, M. E.; Alfier, A.; Auriemma, F.; Cappello, S.; Carraro, L.; Cavazzana, R.; Bello, S. Dal; Fassina, A.; Escande, Dominique; Franz, P.; Gobbin, M.; Innocente, P.; Lorenzini, R.; Marrelli, L.; Martin, P.; Piovesan, P.; Predebon, I.; Sattin, F.; Spizzo, G.; Terranova, D.; Valisa, M.; Zaniol, B.; Zanotto, L.; Zuin, M.; Agostini, M.; Antoni, V.; Apolloni, L.; Baruzzo, M.; Bolzonella, T.; Bonfiglio, D.; Bonomo, F.; Boozer, Allen H.; Brombin, M.; Canton, A.; Delogu, R.; Masi, G.; Gaio, E.; Gazza, E.; Giudicotti, L.; Grando, L.; Guo, S. C.; Manduchi, G.; Marchiori, G.; Martines, E.; Martini, S.; Menmuir, S.; Momo, B.; Moresco, M.; Munaretto, S.; Novello, L.; Paccagnella, R.; Pasqualotto, R.; Piovan, R.; Piron, L.; Pizzimenti, A.; Pomphrey, N.; Scarin, P.; Serianni, G.; Spada, E.; Soppelsa, A.; Spagnolo, S.; Spolaore, M.; Taliercio, C.; Vianello, N.; Zamengo, A.; Zanca, P.

doi:10.1088/0741-3335/51/12/124031

Cited by 44 publications

(45 citation statements)

References 61 publications

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“…The investigation of the ITB formation process recently got a new clue when electron ITBs were found to exist in the RFX-mod reversed field pinch experiment (R ¼ 2m, a ¼ 0:459m) [6]. During most of the duration of the flat top of RFXmod high current discharges (up to 1.7 MA [7]), the plasma self-organizes into a helical state characterized by magnetic surfaces nested around a single helical axis, enclosed in an almost axisymmetric boundary. This state is the result of two successive bifurcations occurring when the current is progressively increased.…”

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“…Electron transport barriers in Tokamaks may also occur together with the ion ITBs: the latter are characterized by a flow shear which further suppresses the turbulence in the barrier region [3]. It is unclear, at present, whether an ion transport barrier is also present in RFX-mod SHAx states as it may occur in Tokamaks: passive spectroscopy measurements indicate that T i =T e ¼ 0:7 for ion species located near the barrier but no information on the T i spatial profile is currently available [7]. On the other hand, RFX-mod eITB occur when the core topology is helical, similarly to improved confinement regimes (core electron-root confinement [25]) in Stellarator devices.…”

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Vanishing Magnetic Shear And Electron Transport Barriers In The RFX-Mod Reversed Field Pinch

et al. 2011

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We define the safety factor q for the helical plasmas of the experiment RFX-mod by accounting for the actual three-dimensional nature of the magnetic flux surfaces. Such a profile is not monotonic but goes through a maximum located in the vicinity of the electron transport barriers measured by a high resolution Thomson scattering diagnostic. Helical states with a single axis obtained in viscoresistive magnetohydrodynamic numerical simulations exhibit similar nonmonotonic q profiles provided that the final states are preceded by a magnetic island phase, like in the experiment.

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Vanishing Magnetic Shear And Electron Transport Barriers In The RFX-Mod Reversed Field Pinch

et al. 2011

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“…A theoretical interpretation proposed dealt with pellet deposition in a magnetic island localized at the q ¼ 1 surface [14]. The reversed field experiment-mod (RFX-mod) device has detected helical internal structures when the current is sufficiently large that quasi-single-helicity (QSH) structures with a m ¼ 1, n ¼ 7 periodicity form that correspond to the helicity of the most internally resonant mode of an axisymmetric configuration [15]. These were successfully modeled with helical MHD equilibrium states computed with the VMEC code [8].…”

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Tokamak Magnetohydrodynamic Equilibrium States with Axisymmetric Boundary and a 3D Helical Core

et al. 2010

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Magnetohydrodynamic (MHD) equilibrium states with imposed axisymmetric boundary are computed in which a spontaneous bifurcation develops to produce an internal three-dimensional (3D) configuration with a helical structure in addition to the standard axisymmetric system. Equilibrium states with similar MHD energy levels are shown to develop very different geometric structures. The helical equilibrium states resemble saturated internal kink mode structures. The essential confinement of particles and energy in magnetically enclosed plasmas is described by magnetohydrodynamics (MHD). The tokamak is the leading fusion energy research concept in which the plasma is expected to be contained in an essentially axisymmetric configuration with relatively small ripple effects from the finite toroidal coils. Symmetry of the equilibrium state along the toroidal coordinate grants the tokamak many appealing properties such as toroidal mass flow, which is known to reduce MHD instability and reduce non-MHD transport of particles and energy, as well as important conservation properties of single particles, thus enhancing confinement on various scales.It is shown in this Letter that despite imposing axisymmetry (toroidal symmetry) at the edge of the plasma, enforced by the geometry of the coil system, the preferred lowest energy state of MHD equilibrium can be nonaxisymmetric in the plasma center. The computation of threedimensional (3D) helical cores constitutes a paradigm shift for the description of tokamak equilibria that opens the way for the application of theoretical and simulation tools developed for stellarator analysis of MHD stability, guiding center particle orbits, kinetic stability, wave propagation or heating, neoclassical transport, gyrokinetics, etc., to determine the impact of these novel 3D states on a large range of magnetic confinement physics phenomena.The ignorable toroidal angle coordinate in axisymmetric magnetic confinement systems allows a simplified description of the MHD equilibrium state through the GradShafranov equation [1,2]. A more sophisticated approach invokes the minimization of the MHD energy to achieve an equilibrium state which can be naturally extended to model 3D systems with the imposition of nested magnetic flux surfaces and a single magnetic axis [3][4][5][6][7][8][9]. Tokamak devices, though nominally axisymmetric, display internal plasma reorganization phenomena that can break the symmetry of the system. In the tokamak à configuration variable (TCV) [10], a transition is observed where core sawteeth relaxations are replaced by global oscillations with low poloidal and toroidal mode numbers [11,12]. In these discharges, the inverse rotational transform q profiles are nearly flat or slightly reversed. One possible explanation for this transition is that q min , the minimum value of safety factor q within the plasma, becomes greater than unity. Equilibria with q min near unity are of interest in the present contribution. Similarly saturated ideal modes in the MAST device have also been re...

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“…1(e) and 1(f)], corresponding to an internal transport barrier with steep electron temperature gradients (up to 4-5 keV=m [17]). SHAx states share a complex 3D geometry with stellarators, but, unlike stellarators, RFPs are selforganized, low plasmas [8] with a safety factor q monotonically decreasing from the core (q % 0:12) to the edge [q % 0, see Fig.…”

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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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Helical equilibria and magnetic structures in the reversed field pinch and analogies to the tokamak and stellarator

Cited by 44 publications

References 61 publications

Vanishing Magnetic Shear And Electron Transport Barriers In The RFX-Mod Reversed Field Pinch

Vanishing Magnetic Shear And Electron Transport Barriers In The RFX-Mod Reversed Field Pinch

Tokamak Magnetohydrodynamic Equilibrium States with Axisymmetric Boundary and a 3D Helical Core

Neoclassical Transport in the Helical Reversed-Field Pinch

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