Fast magnetic reconnection and driven plasma rotation in reversed central magnetic shear configuration

Yu, Q.; Günter, S.

doi:10.1088/1741-4326/ac984f

Cited by 5 publications

(3 citation statements)

References 71 publications

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“…In addition to the m/n = 1/1 mode, strong dynamo effect can also be generated by other MHD modes. For nonmonotonic equilibrium q-profiles with the minimal q value below 2, the local q-profile between two q = 2 surfaces was flattened to be about 2 by the m/n = 2/1 double tearing modes, as found both in experiments and in numerical simulations [59,60].…”

Section: Discussion and Summarysupporting

confidence: 56%

Numerical modelling of sawteeth and sawtooth-free regime

Yu,

Günter,

Lackner

2024

Nucl. Fusion

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To better understand the sawteeth physics and the sawtooth-free regime associated with the hybrid scenario in tokamak experiments, numerical calculations up to quasi-steady state have been carried out for realistic middle-size tokamak plasma parameters, including the bootstrap current perturbation and basing on both the single- and two-fluid equations with the large aspect ratio approximation. Two types of the sawtooth crash are found in multiple sawteeth simulations: (1) For a low equilibrium bootstrap current fraction, the crash is caused by the internal kink mode, as expected; (2) When the bootstrap current density fraction is larger than 10% in the core region, however, the crash is caused by the non-ideal double kink mode, in contrary to the conventional understanding. In this case, a non-monotonic radial profile of the safety factor q with two q = 1 surfaces emerges before the crash, caused by the bootstrap current density and plasma resistivity perturbations, although the original equilibrium has only single q = 1 surface. In both types of sawtooth crashes, the crash time in two-fluid simulations is tens of microseconds, as observed in experiments. Furthermore, for a relatively low ion density and finite bootstrap current density fraction, a transition from the sawtooth to the sawtooth-free regime is found, in which flat q profiles with the q value being about unity in the central region, similar to that observed in hybrid scenario experiments, are maintained by the dynamo effect. To enter into the sawtooth-free regime in two-fluid simulations, a much larger Alfvén velocity than that in single-fluid simulations is required due to the diamagnetic drift.

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Section: Discussion and Summarysupporting

confidence: 56%

Numerical modelling of sawteeth and sawtooth-free regime

Yu,

Günter,

Lackner

2024

Nucl. Fusion

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“…9,11 Several authors have used 3D MHD codes to study the MHD stability properties of reversed-shear discharges in conventional aspect ratio tokamaks, including DIII-D, 12 TFTR, [13][14][15] Tore-Supra, 16,17 JET, 18,19 JT-60SA, 20 and some generic configurations. 21,22 These and other calculations have highlighted the role of resistive interchange modes, 12 the (2,1) double tearing mode (DTM), 15,23,24 the (3,1) DTM, 25,26 and ballooning, infernal, and resonant modes. 20 These unstable modes can lead to localized pressure crashes, 15,24 explosive bursts, [21][22][23]26 reconnection, 19,25 and/or confinement degradation.…”

Section: Introductionmentioning

confidence: 82%

“…21,22 These and other calculations have highlighted the role of resistive interchange modes, 12 the (2,1) double tearing mode (DTM), 15,23,24 the (3,1) DTM, 25,26 and ballooning, infernal, and resonant modes. 20 These unstable modes can lead to localized pressure crashes, 15,24 explosive bursts, [21][22][23]26 reconnection, 19,25 and/or confinement degradation. 18,20 In Sec.…”

Section: Introductionmentioning

confidence: 82%

MHD stability of spherical tokamak equilibria with non-monotonic q-profiles

Jardin,

Munaretto,

Ferraro

et al. 2024

Physics of Plasmas

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We use the 3D magnetohydrodynamic (MHD) code M3D-C1 [Jardin et al., Comput. Sci. Discovery 5, 014002 (2012)] to examine the MHD stability and subsequent evolution of NSTX shot 129169. This discharge had a period with a non-monotonic safety factor profile, q (reversed shear), which was terminated by a MHD event that abruptly lowered the central safety factor, q0, and greatly reduced the peakedness of the pressure profile. We show that the equilibrium just before the MHD event occurred was linearly unstable to many pressure-driven infernal modes. Modes with toroidal mode number n≥3 all had rational surfaces very close to the minimum value of q. However, a non-resonant pressure-driven (1, 1) mode was also present, and this dominated the nonlinear evolution. The final state in the simulation, after the MHD activity subsided, had a reduced and flattened pressure profile and a nearly monotonic q-profile, in qualitative agreement with experimental results. The initial state was also unstable to the resistive interchange criteria in the reversed-shear region, but the final state was stable everywhere. The “double tearing mode” (DTM) does not appear to play a role in the MHD activity of this discharge. In Appendix A, we show that in a torus, the DTM is strongly stabilized by pressure, but it is destabilized in cylindrical geometry (which has been the most extensively analyzed in the literature).

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Fast pressure crash related to m/n = 3/2 double tearing mode

Lu,

Guo,

et al. 2024

Nucl. Fusion

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The CLT code is employed to quantitatively investigate how a single destabilization of the m/n = 3/2 double tearing mode (DTM) and sequential destabilizations of multi-DTMs affect the pressure crash. The focus is on the explosive reconnection process, characterized by kinetic energy bursts and the position exchange of islands on two resonant surfaces. For the first time, it has been demonstrated that pressure crashes related to the 3/2 DTM can be manipulatable by modifying the q-profile. Precursor bursts impact the behavior of the 3/2 DTM but do not significantly contribute to pressure collapse, which mainly occurs during the destabilization of the 3/2 DTM. The sequential reconnecting DTMs of different helicities culminating in a 3/2 DTM with a pressure crash may decrease the overall amplitude of the pressure crash, thereby mitigating the destructive impact of the pressure crash on plasma confinement.

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Fast magnetic reconnection and driven plasma rotation in reversed central magnetic shear configuration

Cited by 5 publications

References 71 publications

Numerical modelling of sawteeth and sawtooth-free regime

Numerical modelling of sawteeth and sawtooth-free regime

MHD stability of spherical tokamak equilibria with non-monotonic q-profiles

Fast pressure crash related to m/n = 3/2 double tearing mode

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