Saturated ideal modes in advanced tokamak regimes in MAST

Chapman, I. T.; Hua, M.-D.; Pinches, S. D.; Akers, R.; Field, A. R.; Graves, J. P.; Hastie, R. J.; Michael, Clive

doi:10.1088/0029-5515/50/4/045007

Cited by 155 publications

(247 citation statements)

References 83 publications

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“…Equilibria with q min near unity are of interest in the present contribution. Similarly saturated ideal modes in the MAST device have also been recently reported [13]. The ''snake'' structures in JET constitute another symmetrybreaking internal structure triggered by pellet injection.…”

supporting

confidence: 58%

“…We therefore predict that the TCV tokamak with large elongation and flat or hollow q profiles (with large shear reversal radius) could acquire a 3D helical core. The existence of these new 3D equilibrium states within axisymmetric tokamak geometry opens new research possibilities, in particular, for explaining the radical change of the sawtooth behavior in highly elongated [11,12] and oval-shaped [23] plasmas, the robustness of the ''snake'' structures to internal perturbations [24] and the ubiquitous appearance of the ideal mode perturbing MAST discharges [13]. The confinement of particles in reactor systems in the presence of internal helical structures will become a highly relevant issue with implications for the ''ITER hybrid scenario'' [25].…”

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confidence: 99%

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Tokamak Magnetohydrodynamic Equilibrium States with Axisymmetric Boundary and a 3D Helical Core

et al. 2010

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Magnetohydrodynamic (MHD) equilibrium states with imposed axisymmetric boundary are computed in which a spontaneous bifurcation develops to produce an internal three-dimensional (3D) configuration with a helical structure in addition to the standard axisymmetric system. Equilibrium states with similar MHD energy levels are shown to develop very different geometric structures. The helical equilibrium states resemble saturated internal kink mode structures. The essential confinement of particles and energy in magnetically enclosed plasmas is described by magnetohydrodynamics (MHD). The tokamak is the leading fusion energy research concept in which the plasma is expected to be contained in an essentially axisymmetric configuration with relatively small ripple effects from the finite toroidal coils. Symmetry of the equilibrium state along the toroidal coordinate grants the tokamak many appealing properties such as toroidal mass flow, which is known to reduce MHD instability and reduce non-MHD transport of particles and energy, as well as important conservation properties of single particles, thus enhancing confinement on various scales.It is shown in this Letter that despite imposing axisymmetry (toroidal symmetry) at the edge of the plasma, enforced by the geometry of the coil system, the preferred lowest energy state of MHD equilibrium can be nonaxisymmetric in the plasma center. The computation of threedimensional (3D) helical cores constitutes a paradigm shift for the description of tokamak equilibria that opens the way for the application of theoretical and simulation tools developed for stellarator analysis of MHD stability, guiding center particle orbits, kinetic stability, wave propagation or heating, neoclassical transport, gyrokinetics, etc., to determine the impact of these novel 3D states on a large range of magnetic confinement physics phenomena.The ignorable toroidal angle coordinate in axisymmetric magnetic confinement systems allows a simplified description of the MHD equilibrium state through the GradShafranov equation [1,2]. A more sophisticated approach invokes the minimization of the MHD energy to achieve an equilibrium state which can be naturally extended to model 3D systems with the imposition of nested magnetic flux surfaces and a single magnetic axis [3][4][5][6][7][8][9]. Tokamak devices, though nominally axisymmetric, display internal plasma reorganization phenomena that can break the symmetry of the system. In the tokamak à configuration variable (TCV) [10], a transition is observed where core sawteeth relaxations are replaced by global oscillations with low poloidal and toroidal mode numbers [11,12]. In these discharges, the inverse rotational transform q profiles are nearly flat or slightly reversed. One possible explanation for this transition is that q min , the minimum value of safety factor q within the plasma, becomes greater than unity. Equilibria with q min near unity are of interest in the present contribution. Similarly saturated ideal modes in the MAST device have also been re...

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supporting

confidence: 58%

mentioning

confidence: 99%

Tokamak Magnetohydrodynamic Equilibrium States with Axisymmetric Boundary and a 3D Helical Core

et al. 2010

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“…Rather, reducing the density at fixed H tends to rapidly lower the central safety factor, as the central neutral beam current drive drives down q min . As noted above, maintaining q min >1 is critical for the avoidance of n=1 kink and coupled core/kink tearing modes, as documented in Refs [34,41,42,44,[106][107][108][109]. Hence, this trend in q min provides a low-density limit for scenarios with fully relaxed current profiles.…”

Section: Importance Of the Plasma Density And Confinement Levelmentioning

confidence: 82%

Exploration of the equilibrium operating space for NSTX-Upgrade

Gerhardt¹,

André²,

Ménard³

2012

Nucl. Fusion

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This paper explores a range of high-performance equilibrium scenarios achievable with neutral beam heating in the NSTX-Upgrade device (Menard J.E. 2012 Nucl. Fusion 52 083015). NSTX-Upgrade is a substantial upgrade to the existing NSTX device (Ono M. et al 2000 Nucl. Fusion 40 557), with significantly higher toroidal field and solenoid capabilities, and three additional neutral beam sources with significantly larger current-drive efficiency. Equilibria are computed with free-boundary TRANSP, allowing a self-consistent calculation of the non-inductive current-drive sources, the plasma equilibrium and poloidal-field coil currents, using the realistic device geometry. The thermal profiles are taken from a variety of existing NSTX discharges, and different assumptions for the thermal confinement scalings are utilized. The no-wall and ideal-wall n = 1 stability limits are computed with the DCON code. The central and minimum safety factors are quite sensitive to many parameters: they generally increase with large outer plasma-wall gaps and higher density, but can have either trend with the confinement enhancement factor. In scenarios with strong central beam current drive, the inclusion of non-classical fast-ion diffusion raises qmin, decreases the pressure peaking, and generally improves the global stability, at the expense of a reduction in the non-inductive current-drive fraction; cases with less beam current drive are largely insensitive to additional fast-ion diffusion. The non-inductive current level is quite sensitive to the underlying confinement and profile assumptions. For instance, for BT = 1.0 T and Pinj = 12.6 MW, the non-inductive current level varies from 875 kA with ITER-98y,2 thermal confinement scaling and narrow thermal profiles to 1325 kA for an ST specific scaling expression and broad profiles. Scenarios are presented which can be sustained for 8–10 s, or (20–30) τCR, at βN = 3.8–4.5. The value of qmin can be controlled at either fixed non-inductive fraction of 100% or fixed plasma current, by varying which beam sources are used, opening the possibility for feedback control of the current profile. In terms of quantities like collisionality, neutron emission, non-inductive fraction, or stored energy, these scenarios represent a significant performance extension compared with NSTX and other present spherical torii.

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“…Another experimental observation of long-lived mode has been observed on MAST 8 and HL-2A 9 devices. Neutral beam injection (NBI) heated plasmas in MAST with an advanced tokamak scenario safety factor profile above unity open exhibited long-lived saturated n ¼ 1 magnetohydrodynamic (MHD) instabilities.…”

Section: Introductionmentioning

confidence: 78%

Long-lived pressure-driven coherent structures in KSTAR plasmas

et al. 2016

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Highly coherent structures associated with an extremely long-lived saturated magnetohydrodynamic instability have been observed in KSTAR tokamak under a long-pulse and steady-state operation. They persist essentially unchanged for the full duration of a discharge up to 40 s, much longer than any dynamical or dissipative time scales in the system. Analysis of the data, supported by numerical simulations, indicates that they may be associated with a pressure-driven mode causing some degradation in the toroidal rotation, electron, and ion energy confinement.

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Saturated ideal modes in advanced tokamak regimes in MAST

Cited by 155 publications

References 83 publications

Tokamak Magnetohydrodynamic Equilibrium States with Axisymmetric Boundary and a 3D Helical Core

Tokamak Magnetohydrodynamic Equilibrium States with Axisymmetric Boundary and a 3D Helical Core

Exploration of the equilibrium operating space for NSTX-Upgrade

Long-lived pressure-driven coherent structures in KSTAR plasmas

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