Bifurcation to 3D Helical Magnetic Equilibrium in an Axisymmetric Toroidal Device

Bergerson, W.F.; Auriemma, F.; Chapman, B. E.; Ding, W. X.; Zanca, P.; Brower, D. L.; Innocente, P.; Lin, L.; Lorenzini, R.; Martines, E.; Momo, B.; Sarff, J. S.; Terranova, D.

doi:10.1103/physrevlett.107.255001

Cited by 37 publications

(39 citation statements)

References 16 publications

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“…When broad electron temperature profiles were found in quasi single helicity states of RFX-mod, experimentalists confirmed immediately it was present [3]. The same phenomenology was found in MST [95,96], and the Double Axis (DAx) state was also found in EXTRAP T2R [97]. Quasi single helicity states were also found in TPE-RX [98] and in RELAX [99,100].…”

Section: From Double To Single Magnetic Axissupporting

confidence: 58%

What is a Reversed Field Pinch?

Escande¹

2015

Rotation and Momentum Transport in Magnetized Plasmas

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Section: From Double To Single Magnetic Axissupporting

confidence: 58%

What is a Reversed Field Pinch?

Escande¹

2015

Rotation and Momentum Transport in Magnetized Plasmas

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“…10 The second technique is a transition to a single helicity state: the amplitude of the core-most magnetic mode grows rapidly as the amplitudes of secondary modes are suppressed. 11 This so-called Single Helicity Axis state (SHAx) has a helical (as opposed to axisymmetric) core, surrounded by good magnetic surfaces and is often accompanied by an improved confinement state. 12 A neutral beam injector optimized for MST is now routinely used to study fast ion physics in the RFP.…”

Section: The Rfp As a Test Bed For Fast Ion Physicsmentioning

confidence: 99%

Fast ion confinement and stability in a neutral beam injected reversed field pinch

et al. 2013

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The behavior of energetic ions is fundamentally important in the study of fusion plasmas. While well-studied in tokamak, spherical torus, and stellarator plasmas, relatively little is known in reversed field pinch plasmas about the dynamics of fast ions and the effects they cause as a large population. These studies are now underway in the Madison Symmetric Torus with an intense 25 keV, 1 MW hydrogen neutral beam injector (NBI). Measurements of the time-resolved fast ion distribution via a high energy neutral particle analyzer, as well as beam-target neutron flux (when NBI fuel is doped with 3-5% D 2 ) both demonstrate that at low concentration the fast ion population is consistent with classical slowing of the fast ions, negligible cross-field transport, and charge exchange as the dominant ion loss mechanism. A significant population of fast ions develops; simulations predict a super-Alfv enic ion density of up to 25% of the electron density with both a significant velocity space gradient and a sharp radial density gradient. There are several effects on the background plasma including enhanced toroidal rotation, electron heating, and an altered current density profile. The abundant fast particles affect the plasma stability. Fast ions at the island of the core-most resonant tearing mode have a stabilizing effect, and up to 60% reduction in the magnetic fluctuation amplitude is observed during NBI. The sharp reduction in amplitude, however, has little effect on the underlying magnetic island structure. Simultaneously, beam driven instabilities are observed as repetitive $50 ls bursts which coincide with fast particle redistribution; data indicate a saturated core fast ion density well below purely classical predictions. V C 2013 AIP Publishing LLC [http://dx. INTRODUCTIONThe envisioned burning plasma experiment, regardless of magnetic concept, relies on sufficient confinement of the charged fusion product for plasma self heating. As such, the confinement of fast ions and their impact on the bulk plasma are crucial issues.A tremendous body of work demonstrates that fast ions in a tokamak plasma (born from fusion reactions, ICRF, or NBI) are generally well confined and thermalize via classical Coulomb collisions. However, a sufficiently intense fast ion population can excite collective instabilities that can lead to resonant fast ion transport. 1 A new body of work on the effects of a large fast ion population in the reversed field pinch (RFP) configuration has recently been opened. Despite the RFP's weak toroidal field and multiple resonant tearing modes which could diminish fast ion confinement, 2,3 NBI-born fast ions in low concentration are observed to slow classically and have a confinement time much larger than thermal particles. 4 The dearth of transport within the modestly stochastic magnetic field is understood to result from the decoupling of the fast ion orbits from the magnetic perturbations. The ions are routinely confined for up to a classical slowing time. 5 In this work, we investigate the effect of...

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“…More recently, fine structures of internal magnetic field and current density have been measured with the Faraday effect [14,15], and direct measurement of the internal magnetic field structure associated with a three-dimensional (3D) helical equilibrium generated spontaneously in the core of an axisymmetric toroidal plasma clearly demonstrated the bifurcation from a stochastic magnetic field to a helical flux surface [16]. Alternatively, the change of magnetic topology, for example the change from a nested magnetic flux surface to a magnetic island, can be identified by heat pulse propagation [17,18].…”

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confidence: 99%

Topology bifurcation of a magnetic flux surface in magnetized plasmas

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Bifurcation phenomena in the radial propagation of a heat pulse driven by modulated electron cyclotron heating are observed in the Large Helical Device. There are two patterns of heat pulse propagation observed in the flat temperature region. One is a bi-directional slow heat pulse propagation observed in the fast magnetic shear drop and the other is a fast heat pulse propagation observed in the slow magnetic shear drop. This bifurcation in the heat pulse propagation suggests the topology bifurcation of a magnetic flux surface in the plasma.The transition from regular to chaotic motion is very often represented as a topological bifurcation of a torus in phase space [1]. Islands are formed in nested tori by resonant perturbations, and if global stochasticity sets in, turbulence-driven transport (such as superdiffusion) can occur [2]. This topological bifurcation has been considered to happen in hightemperature toroidal plasmas, e.g. during disruption in tokamaks [3,4], by a large deformation such as a spontaneous transition from the stochastic state to helical equilibrium in reversed-field pinch (RFP) plasmas [5][6][7], or at the plasma edge as seen in the edge-localized-modes mitigation

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Bifurcation to 3D Helical Magnetic Equilibrium in an Axisymmetric Toroidal Device

Cited by 37 publications

References 16 publications

What is a Reversed Field Pinch?

What is a Reversed Field Pinch?

Fast ion confinement and stability in a neutral beam injected reversed field pinch

Topology bifurcation of a magnetic flux surface in magnetized plasmas

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