Improved Particle Confinement in Transition from Multiple-Helicity to Quasi-Single-Helicity Regimes of a Reversed-Field Pinch

Frassinetti, L.; Predebon, I.; Koguchi, Haruhisa; Yagi, Y.; Hirano, Y.; Sakakita, Hajime; Spizzo, G.; White, R. B.

doi:10.1103/physrevlett.97.175001

Cited by 25 publications

(15 citation statements)

References 20 publications

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“…1 These studies consist both in ͑i͒ the development of techniques for active control of plasma instabilities 2,3 and ͑ii͒ the discovery of a spontaneous plasma regime, termed quasi single helicity ͑QSH͒, 4 with improved performance. 5 The RFP is a magnetic configuration for the confinement of thermonuclear fusion plasma in which the toroidal component of the magnetic field B , of the same order of magnitude as the poloidal field B , reverses near the edge. This configuration, spontaneously reached with a relaxation process, is maintained in time by the plasma itself, which selfgenerates the necessary toroidal magnetic flux through the dynamo mechanism.…”

Section: Introductionmentioning

confidence: 99%

Spontaneous quasi single helicity regimes in EXTRAP T2R reversed-field pinch

et al. 2007

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In recent years, good progress toward a better understanding and control of the plasma performance in reversed-field pinch devices has been made. These improvements consist both of the discovery of spontaneous plasma regimes, termed the quasi single helicity ͑QSH͒ regime, in which part of the plasma core is no longer stochastic, and of the development of techniques for active control of plasma instabilities. In this paper, a systematic study of spontaneous QSH in the EXTRAP T2R device ͓P. R. Brunsell, H. Bergsaker, M. Cecconello et al., Plasma Phys. Control. Fusion 43, 1457 ͑2001͔͒ is presented. In this device, QSH states can occur spontaneously and it is associated with magnetic and thermal structures. A statistical analysis to determine the most favorable experimental conditions to have a transition to the QSH regime will be presented. The results described here are useful to understand the underlying properties of QSH regimes in view of future applications of the QSH active control in EXTRAP T2R; they are also important to have a comparison with the QSH studied in other devices.

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Section: Introductionmentioning

confidence: 99%

Spontaneous quasi single helicity regimes in EXTRAP T2R reversed-field pinch

et al. 2007

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“…Unfortunately all the experimental data are still obtained within limited time windows and the experiments are likely far from being operated in an asymptotical relevant (in the sense described in section 3) regime. Beside the SH scenario, RFPs operate commonly also in the so called Multi-Helical (MH) regime, where many MHD modes are simultaneously excited [7,25,26]. Since the experimental operational spaces in SH and in MH are similar, it can be hypothesized that the shape of the dynamo electric field would be also similar in the two cases, although in MH possibly more modes can contribute to it.…”

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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“…where the second term on the left-hand side represents the cooperative works of the turbulent velocity and magnetic field fluctuations denotedṽ andB, respectively. A number of experiments have been performed to clarify the dynamo mechanisms observed in RFP plasmas [143][144][145][146]. Like turbulence-driven particle and heat fluxes, turbulence-driven momentum fluxes can be intermittent and poloidally asymmetric.…”

Section: Turbulence and Electric Field Structureflow Generationmentioning

confidence: 99%

A Review -Observations of Turbulence and Structure in Magnetized Plasmas

Fujisawa

2010

Plasma and Fusion Research

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Magnetized plasma is an non-equilibrium matter rich in nonlinear phenomena; its structural formations are dominated by turbulence. This article provides a brief review of experiments and observations of phenomena occurring in turbulent plasmas, with an emphasis on the methodologies for characterizing turbulence, and visualizing the invisible structure created by turbulence and the internal couplings between the elemental waves that constitute turbulence. Zonal flows, streamers, blobs, and other phenomena are investigated by using analytical methods such as Fourier transformation, wavelet analysis, probabilistic density function analysis, bicoherence, wavelet bicoherence. Finally, the contemporary view of plasma turbulence is presented with discussion of unsolved transport issues in fusion plasmas, such as transport barrier formation and nonlocal transport.

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Improved Particle Confinement in Transition from Multiple-Helicity to Quasi-Single-Helicity Regimes of a Reversed-Field Pinch

Cited by 25 publications

References 20 publications

Spontaneous quasi single helicity regimes in EXTRAP T2R reversed-field pinch

Spontaneous quasi single helicity regimes in EXTRAP T2R reversed-field pinch

Helicity transport and dynamo sustainment for helical plasma states

A Review -Observations of Turbulence and Structure in Magnetized Plasmas

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