Role of a continuous MHD dynamo in the formation of 3D equilibria in fusion plasmas

Piovesan, P.; Bonfiglio, D.; Cianciosa, M.; Luce, T. C.; Taylor, N.Z.; Terranova, D.; Turco, F.; Wilcox, R.S.; Wingen, A.; Cappello, S.; Chrystal, C.; Escande, Dominique; Holcomb, C.T.; Marrelli, L.; Paz-Soldan, C.; Piron, L.; Predebon, I.; Zaniol, B.; Teams, RFX-Mod

doi:10.1088/1741-4326/aa700b

Cited by 32 publications

(36 citation statements)

References 47 publications

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“…Experiments performed in RFX-mod tokamak plasmas confirmed the predictions [20]. These results have been proved to be relevant also for high- DIII-D plasmas when a helical core forms induced by an external n=1 MP and can explain the observed central current redistribution [21] Experiments have been performed to investigate the role of the ion mass M i on plasma dynamics in the RFP. Interestingly, the existing main magnetic configurations are characterized by largely different responses to the change of the working gas.…”

Section: Internal Transport Barriers and Helical State Formationsupporting

confidence: 68%

Overview of the RFX-mod fusion science activity

Zuin¹,

Bello²,

Marrelli³

et al. 2017

Nucl. Fusion

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This paper reports the main recent results of the RFX-mod fusion science activity. The RFX-mod device is characterized by a unique flexibility in terms of accessible magnetic configurations. Axisymmetric and helically shaped Reversed-field pinch equilibria have been studied, along with tokamak plasmas in a wide range of q(a) regimes (spanning from 4 down to 1.2 values). The full range of magnetic configurations in between the two, the so-called ultra low-q ones, has been explored, with the aim of studying specific physical issues common to all equilibria, such as, for example, the density limit phenomenon. The powerful RFX-mod feedback control system has been exploited for MHD control, which allowed to extend the range of experimental parameters, as well as to induce specific magnetic perturbations for the study of 3D effects. In particular, transport, edge and isotope effect in 3D equilibria have been investigated, along with runaway mitigations through induced magnetic perturbations. The first transitions to an improved confinement scenario in circular and D-shaped tokamak plasmas have been obtained thanks to an active modification of the edge electric field through a polarized electrode. The experiments are supported by intense modelling with 3D MHD, gyrokinetic, guiding center and transport codes. Proposed modifications to the RFX-mod device, which will enable further contributions to the solution of key issues in the roadmap to ITER and DEMO, are also briefly presented.

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Section: Internal Transport Barriers and Helical State Formationsupporting

confidence: 68%

Overview of the RFX-mod fusion science activity

Zuin¹,

Bello²,

Marrelli³

et al. 2017

Nucl. Fusion

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“…In many tokamaks such as DIII-D, it was reported the sawtooth-free hybrid plasma was realized by "flux pumping" with HCs [6,7]. In this study, the sawtooth-free phase is also observed with an m/n = 2/1 NTM as seen in figure 3.…”

Section: Discussionsupporting

confidence: 57%

“…Helical cores (HCs) in tokamaks have been an important research topic for the fusion reactor because HCs can affect transports of impurities [1][2][3][4], energetic particles [5], or toroidal momentum [5]. In addition to the effect on transport by helical structures, recent studies suggested that magnetohydrodynamics (MHD) dynamo accompanied with HCs redistributes the current profile in the core and realizes sawtooth-free plasmas of hybrid scenarios as observed in DIII-D [6,7], in which the minimum value of the safety factor profile, qmin., is kept slightly above unity by "flux pumping".…”

Section: Introductionmentioning

confidence: 99%

“…If the beta value is sufficiently high, the saturated m/n = 1/1 quasi-interchange instability is excited and induce the dynamo loop voltage using 3D nonlinear simulations [15]. On the other hand, in recent experimental studies, HCs are induced by n = 1 magnetic perturbation with the external coils and characteristics of HCs are investigated with taking account of reconstructed three-dimensional MHD equilibria in DIII-D [7,16,17]. The EAST team recently showed that the two-dimensional mode structure of a long-lived helical mode by combined use of tomography and a singular value decomposition, showing an m/n = 1/1 mode structure and a substantial m/n = 2/2 component where qmin is kept slightly above unity [18].…”

Section: Introductionmentioning

confidence: 99%

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Non-Resonant n = 1 Helical Core Induced by m/n = 2/1 Neoclassical Tearing Mode in JT-60U

Bando

Inoue

Shinohara

et al. 2021

Plasma and Fusion Research

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We have investigated the excitation mechanism of n = 1 helical cores (HCs) with m/n = 2/1 neoclassical tearing modes (NTMs) in JT-60U. It is found that the n = 1 HC is observed with the mode frequency from several Hz to 6 kHz. This indicates that the resistive wall and the plasma control system do not induce n = 1 HCs because the both time scales are different from the mode frequency. In addition, n = 1 HCs appear to be the non-resonant mode from the two observations: n = 1 HCs do not rotate with the plasma around the q = 1 surface in the core and an n = 1 HC is observed even when q min. > 1. It is also observed that the fluctuation due to an n = 1 HC in the core region disappears with the stabilization of an m/n = 2/1 NTM, implying that n = 1 HCs are driven by m/n = 2/1 NTMs. We revisit a quasi-linear MHD model where the n = 1 HC is induced directly by the sideband of the current for the m/n = 2/1 NTM, which potentially excites the non-resonant m/n = 1/1 mode.

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“…In fact in the absence of this term a purely helically deformed plasma could hardly satisfy the constraint imposed by the single fluid electron momentum balance [9]. A dynamo electric field generated by a single dominant mode has been estimated recently in [19], where the radial profile of the dynamo electric potential, as deduced by equilibrium reconstructions, was estimated (see figure 8 of [19]), with a very similar shape to the case with 40% dissipation (and with Δ=0) shown in figure 2(b) of this paper. In [20,21] it was assumed that the α-dynamo contributed to the transport of total helicity from the fluctuations to the macroscale.…”

Section: Discussionmentioning

confidence: 99%

Helicity transport and dynamo sustainment for helical plasma states

Paccagnella

2019

New J. Phys.

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Plasma states dominated by single helical modes are often observed in the Reverse Field Pinch (RFP) plasma confinement devices. In this paper the properties of these states are studied on the basis of a relaxation model that assumes the existence of several topological invariants related to the dominant mode. It is hypothesized that the value of the first invariant in this chain, is determined by the existence of a plasma dynamo mechanism that transport helicity. This hypothesis enables us to determine the steady state properties of the plasma equilibrium and some other interesting physical consequences. Further, by considering the properties of the transfer of helicity from the mesoscale (fluctuations) to the macroscale (equilibrium), through the dynamo field, a nonlinear dynamical model can be constructed, that evolves in time to a steady state with a non-vanishing dynamo field, when helicity is injected in the system, as in the case of the ohmic sustained RFP, while the dynamo oscillates initially, but is damped later in time, for vanishing helicity input.Magnetic helicity conservation is an important plasma property in ideal magneto-hydro-dynamics, describing the invariance of the degree of twist and linkage of the magnetic field lines [1]. In presence of even a small amount of dissipation, it becomes an approximate invariant. In fact in a resistive plasma, reconnection can produce magnetic field topological rearrangements. Even in this case magnetic helicity can play a crucial role in determining the properties of large scale fields in astrophysics and/or in laboratory plasmas, since it is generally better conserved than energy during reconnection and therefore can determine the plasma relaxation pattern toward minimum magnetic energy states, as was first observed by Woltjer [2] for astrophysical plasmas and by Taylor [3] in interpreting the results obtained in a pinch machine. Moreover, the role played by the topological invariants is considered to be very important for the origin and sustainment of large scale magnetic fields in several natural systems, as galaxies, stars and planets. In fact, it is believed that in most, maybe in all of these cases, a MHD dynamo mechanism is needed to produce and/or amplify the magnetic field. Therefore the study of the relationship between the helicity related invariants and the dynamo amplification and sustainment of the magnetic field, is of fundamental importance for a deeper understanding of different natural phenomena, covering a large variety of space and time scales. While helical symmetric turbulence is generally associated with the dynamo process, in principle, also a single large helical mode could amplify an existing magnetic field. Examples of the presence of large scale helical fields are found in planets [4], in solar eruptions [5] and in active galactic nuclei [6] and also in RFP experiments, where, under certain conditions, a single dominant helical mode emerges [7] over a generally much wider fluctuations spectrum. In a series of recent papers [8-10] thi...

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Role of a continuous MHD dynamo in the formation of 3D equilibria in fusion plasmas

Cited by 32 publications

References 47 publications

Overview of the RFX-mod fusion science activity

Overview of the RFX-mod fusion science activity

Non-Resonant n = 1 Helical Core Induced by m/n = 2/1 Neoclassical Tearing Mode in JT-60U

Helicity transport and dynamo sustainment for helical plasma states

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