Single-Helical-Axis States in Reversed-Field-Pinch Plasmas

Lorenzini, R.; Terranova, D.; Alfier, A.; Innocente, P.; Martines, E.; Pasqualotto, R.; Zanca, P.

doi:10.1103/physrevlett.101.025005

Cited by 91 publications

(119 citation statements)

References 22 publications

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“…(2) The second type are closer to a helical equilibrium, since the dominant m=1 mode grows up to the point where its helical axis becomes the main magnetic axis, and the original symmetric magnetic axis ceases to exist. These type 2 helical states are of the same kind of SHAx transiently found during Oscillating Poloidal Current Drive plasmas [Lorenzini 2008], but the important difference is that they appear as a natural outcome of the high current operation, with positive perspectives for the confinement, as discussed later.…”

Section: High Current Rfx-mod Plasmas: the Natural Environment For Simentioning

confidence: 90%

“…In [Marrelli 2007] it was shown that the probability to develop QSH increases with current and that above 1 MA QSH occupies a significant fraction of the current flat top. One important and recent experimental result obtained in a transient way by oscillating the toroidal flux ] is that the helical structure associated with the dominant m=1 mode grows up to the point where a Single Helical Axis (SHAx) becomes the main magnetic axis, and the original symmetric magnetic axis ceases to exist [Lorenzini 2008]. A broad volume of conserved helical flux surfaces is present.…”

Section: The Rfp: a Paradigm For Self-organized Plasmamentioning

confidence: 99%

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High current regimes in RFX-mod

Valisa

Bolzonella

Buratti

et al. 2008

Plasma Phys. Control. Fusion

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Abstract.Optimization of machine operation, including plasma position control, density control, and especially feedback control on multiple magnetohydrodynamic modes has led RFX-mod to reliably operate at 1.5 MA, the highest current ever achieved on a Reversed Field Pinch (RFP). At high current and low density the magnetic topology spontaneously self-organizes in an Ohmical helical symmetry, with the new magnetic axis helically twisting around the geometrical axis of the torus. The separatrix of the island disappears leaving a wide and symmetric thermal structure with large gradients in the electron temperature profile. The new topology still displays an intermittent nature but its overall presence has reached 85 % of the current flat top period. The large gradients in the electron temperature profile appear to be marginal for the destabilization of Ion Temperature Gradient modes on the assumption that ions and electrons have the same gradients. There are indications that higher currents could provide the conditions where to prove the existence of a true helical equilibrium as the standard RFP configuration

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Section: High Current Rfx-mod Plasmas: the Natural Environment For Simentioning

confidence: 90%

Section: The Rfp: a Paradigm For Self-organized Plasmamentioning

confidence: 99%

High current regimes in RFX-mod

Valisa

Bolzonella

Buratti

et al. 2008

Plasma Phys. Control. Fusion

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“…The reduction of magnetic chaos is crucial in reducing D and cancelling the pinch: this happens e.g. in the RFP approaching the single-helicity condition [4,5,7].…”

Section: A Steady State Distributionsmentioning

confidence: 99%

“…For purposes of thermonuclear fusion, the reversed field pinch (RFP) [4,5] is of interest particularly because of the existence of single helicity states [6,7], which offer good confinement (for a recent review of the results on single helicity see [8]). However at low current the normal state of the RFP is the multi-helicity state, dominated by a known but large spectrum of relatively stable saturated tearing modes.…”

Section: Introductionmentioning

confidence: 99%

Nonlocal transport in the reversed field pinch

Spizzo

White

Cappello

et al. 2009

Plasma Phys. Control. Fusion

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Several heuristic models for nonlocal transport in plasmas have been developed, but they have had a limited possibility of detailed comparision with experimental data. Nonlocal aspects introduced by the existence of a known spectrum of relatively stable saturated tearing modes in a low current reversed field pinch offers a unique possibility for such a study. A numerical modelling of the magnetic structure and associated particle transport is carried out for the reversed-field pinch experiment at the Consorzio RFX, Padova, Italy. A reproduction of the tearing mode spectrum with a guiding center code 1 reliably reproduces the observed soft X-ray tomography. Following particle trajectories in the stochastic magnetic field shows the transport across the unperturbed flux surfaces to be due to a spectrum of Lévy flights, with the details of the spectrum position dependent. The resulting transport is subdiffusive, and cannot be described by Rechester-Rosenbluth diffusion, which depends on a random phase approximation. If one attempts to fit the local transport phenomenologically, the subdiffusion can be fit with a combination of diffusion and inward pinch 2 . It is found that whereas passing particles explore the stochastic field and hence participate in Lévy flights, the trapped particles experience normal neoclassical diffusion.A two fluid nonlocal Montroll equation is used to model this transport, with a Lévy flight defined as the motion of an ion during the period that the pitch has one sign. The necessary input to the Montroll equation consists of a time distribution for the Lévy flights, given by the pitch angle scattering operator, and a distribution of the flight distances, determined numerically using a guiding center code. Results are compared to experiment. The relation of this formulation to fractional kinetics is also described.

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“…However, RFP plasmas in the axisymmetric Reversed Field eXperiment (RFX) [6] are observed [7], [8] to self-organize into a long-lived helical state with greatly improved transport coefficients. The ability to design helically-shaped plasmas to achieve given physics goals has greatly improved over the last quarter century [9], [10], [11].…”

Section: Introductionmentioning

confidence: 99%

Use of Helical Fields to Allow a Long Pulse Reversed Field Pinch

Boozer¹,

Pomphrey²

2008

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The maintenance of the magnetic configuration of a Reversed Field Pinch (RFP) is an unsolved problem. Even a toroidal loop voltage does not suffice to maintain the magnetic configuration in axisymmetry but could if the plasma had helical shaping. The theoretical tools for plasma optimization using helical shaping have advanced, so an RFP could be relatively easily designed for optimal performance with a spatially constant toroidal loop voltage. A demonstration that interesting solutions exist is given.

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Single-Helical-Axis States in Reversed-Field-Pinch Plasmas

Cited by 91 publications

References 22 publications

High current regimes in RFX-mod

High current regimes in RFX-mod

Nonlocal transport in the reversed field pinch

Use of Helical Fields to Allow a Long Pulse Reversed Field Pinch

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