Magnetic order and confinement improvement in high-current regimes of RFX-mod with MHD feedback control

Piovesan, P.; Zuin, M.; Alfier, A.; Bonfiglio, D.; Bonomo, F.; Canton, A.; Cappello, S.; Carraro, L.; Cavazzana, R.; Escande, Dominique; Fassina, A.; Gobbin, M.; Lorenzini, R.; Marrelli, L.; Martin, P.; Martines, E.; Pasqualotto, R.; Puiatti, M. E.; Spolaore, M.; Valisa, M.; Vianello, N.; Zanca, P.

doi:10.1088/0029-5515/49/8/085036

Cited by 72 publications

(103 citation statements)

References 40 publications

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“…Neoclassical effects and/or micro-turbulence might be ruling transport in the helical core and across the gradient of the internal barriers. In particular, the large gradients could induce the growth of instabilities leading to the sudden (but temporary) back transitions to the axisymmetric configuration as experimentally observed [PZ09].…”

Section: Particle and Energy Transportmentioning

confidence: 69%

“…Although the helical states observed in RFX-mod last for several energy confinement times [LOR09b], temporary back transitions through reconnection events lead to a more chaotic, lower confinement state [PZ09,ZU09]. This at the moment prevents the helical equilibrium from being fully stationary.…”

Section: Rfp Helical Equilibria and Their Stabilitymentioning

confidence: 99%

“…In RFX-mod [SON03] helical states appear regularly at high plasma current (I p 1 MA) [LOR09,PZ09]: differently from the typical multiple helicity case here a single mode dominates the magnetic fluctuation spectrum with amplitude of the order of 4-5% of the total magnetic field. The whole plasma core changes its shape, expelling the native axisymmetric magnetic axis that is replaced by a single helical axis.…”

Section: Introductionmentioning

confidence: 99%

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A 3D approach to equilibrium, stability and transport studies in RFX-mod improved regimes

Terranova

Bonfiglio

Boozer

et al. 2010

Plasma Phys. Control. Fusion

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The full three-dimensional (3D) approach is now becoming an important issue for all magnetic confinement configurations. It is a necessary condition for the stellarator but also the tokamak and the reversed field pinch (RFP) now cannot be completely described in an axisymmetric framework. For the RFP the observation of self-sustained helical configurations with improved plasma performances require a better description in order to assess a new view on this configuration. In this new framework plasma configuration studies for RFX-mod have been considered both with tools developed for the RFP as well as considering codes originally developed for the stellarator and adapted to the RFP. These helical states are reached through a transition to a very low/reversed shear configuration leading to internal electron transport barriers. These states are interrupted by MHD reconnection events and the large T e gradients at the barriers indicate that both current and pressure driven modes are to be considered. Furthermore the typically flat T e profiles in the helical core have raised the issue of the role of electrostatic and electromagnetic turbulence in these reduced chaos regions, so that a stability analysis in the correct 3D geometry is required to address an optimization of the plasma setup. In this view the VMEC code proved to be an effective way to obtain helical equilibria to be studied in terms of stability and transport with a suite of well tested codes. In this work, the equilibrium reconstruction technique as well as the experimental evidence of 3D effects and their first interpretation in terms of stability and transport are presented using both RFP and stellarator tools.

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Section: Particle and Energy Transportmentioning

confidence: 69%

Section: Rfp Helical Equilibria and Their Stabilitymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A 3D approach to equilibrium, stability and transport studies in RFX-mod improved regimes

Terranova

Bonfiglio

Boozer

et al. 2010

Plasma Phys. Control. Fusion

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“…Toroidal sideband coupling constitutes a significant contribution. In RFX-mod, the SHAx state periodically relaxes [36], and ideal MHD modes are conjectured as a possible cause.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

Magnetohydrodynamic equilibrium and the stability of tokamak and reversed-field pinch systems with 3D helical cores

Cooper¹,

Graves²,

Sauter³

et al. 2011

Plasma Phys. Control. Fusion

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Bifurcated magnetohydrodynamic (MHD) equilibrium states are computed for ITER hybrid scenario and RFX-mod SHAx configurations with very flat or reversed core magnetic shear conditions. In the ITER studies, the minimum inverse rotational transform q min is near unity, while for RFX-mod it is 1/8. Two equilibrium states are obtained: one is axisymmetric, the other displays a 3D helical core. In tokamak devices, the structure resembles a saturated ideal MHD internal kink mode. In the reversed-field pinch, the structure is seven-fold toroidally periodic. The equilibrium magnetic field spectrum in the Boozer coordinate frame is calculated in both the ITER and RFX-mod configurations and the implications are discussed. The RFX-mod equilibria are strongly unstable to external ideal MHD kink modes, which become stabilized with a closely fitting conducting shell when the equilibrium state has a weak reversed core shear. It is marginally unstable with a monotonic q-profile. Unstable modes are driven by the Ohmic current, with pressure and PfirschSchlüter currents having a very weak effect. The external kink mode spectrum is dominated by coupled m = 1, n = 6 and m = 2, n = 13 Fourier components, which revert to m = 1, n = 8 and m = 2, n = 15 terms with a conducting wall in proximity to the plasma-vacuum interface.

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“…5,6 Experimentally, spontaneous transitions occur above a critical value of the Lundquist number, or equivalently, a critical value of the plasma current. 7,8 The physics of the spontaneous transition is likely distinct from that of the induced transitions, at least those described above. This is in keeping with the significant differences in the way the plasma is prepared for the two types of transitions.…”

Section: Introductionmentioning

confidence: 99%

Thermal transport dynamics in the quasi-single helicity state

McKinney

Terry

2017

Physics of Plasmas

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A dynamical model describing oscillations between multiple and single helicity configurations in the quasi-single helicity (QSH) state of the reversed field pinch [P. W. Terry and G. G. Whelan, Plasma Phys. Controlled Fusion 56, 094003 (2014)] is extended to include electron temperature profile dynamics. It is shown that QSH dynamics is linked to the electron temperature profile because the suppression of mode coupling between tearing modes proposed to underlie QSH also suppresses magnetic-fluctuation-induced thermal transport. Above the threshold of dominant-mode shear that marks the transition to QSH, the model produces temperature-gradient steepening in the strong shear region. Oscillations of the dominant and secondary mode amplitudes give rise to oscillations of the temperature gradient. The phasing and amplitude of temperature gradient oscillations relative to those of the dominant mode are in agreement with experiment. This provides further evidence that the model, while heuristic, captures key physical aspects of the QSH state.

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Magnetic order and confinement improvement in high-current regimes of RFX-mod with MHD feedback control

Cited by 72 publications

References 40 publications

A 3D approach to equilibrium, stability and transport studies in RFX-mod improved regimes

A 3D approach to equilibrium, stability and transport studies in RFX-mod improved regimes

Magnetohydrodynamic equilibrium and the stability of tokamak and reversed-field pinch systems with 3D helical cores

Thermal transport dynamics in the quasi-single helicity state

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