Magnetic reconnection rate during sawtooth crashes in ASDEX Upgrade

Samoylov, O.; Igochine, V.; Yu, Q.; Zohm, H.

doi:10.1088/1741-4326/ac6617

Cited by 5 publications

(8 citation statements)

References 33 publications

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“…The profiles are also hollow right after the crash. It is worth to mention that this time scale is comparable to that of the fast sawtooth crash found in experiments and two-fluid simulations [51,66,67].…”

Section: Core Crashsupporting

confidence: 74%

“…While the linear ideal mode could be unstable driven by a sufficiently large plasma pressure gradient, magnetic islands will be formed in the nonlinear phase when the mode growth is not fast enough such that non-ideal effects cannot be neglected [69]. The nonideal effects are important for a fast mode growth in tens of microseconds for the electron temperature of a few keV, as also found from the fast sawtooth crash due to the internal kink mode [51,66,67].…”

Section: Discussion and Summarymentioning

confidence: 98%

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

Günter²

2022

Nucl. Fusion

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Numerical calculations have been carried to study the nonlinear growth of the double tearing mode (DTM) in the reversed central magnetic shear configuration for medium-size tokamak plasma parameters, based on two-fluid equations and large aspect ratio approximation. Three different regimes of the DTM growth are found: (a) Annular crash regime, existing for a small distance but a not too large plasma rotation frequency difference between two resonant surfaces. In this regime the plasma pressure between two resonant surfaces is flattened due to the fast magnetic reconnection in tens of microseconds, in agreement with experimental observations. In addition, a large plasma rotation shear is generated around the edge of the pressure flattening region right after the fast magnetic reconnection. (b) Core crash regime, existing for a medium distance and a relatively low rotation frequency difference between two resonant surfaces, in which the plasma pressure is flattened over a large region up to the magnetic axis during the fast magnetic reconnection in tens of microseconds, in agreement with experimental observation too. (c) No crash regime, existing for a sufficiently large distance and/or frequency difference between two resonant surfaces. In this regime the mode grows slowly in the nonlinear phase and saturates at a finite amplitude, causing a local flattening of the plasma pressure at the resonant surface but without fast crashes.

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Section: Core Crashsupporting

confidence: 74%

Section: Discussion and Summarymentioning

confidence: 98%

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

Günter²

2022

Nucl. Fusion

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“…The difference between them depends on the sawteeth period and the heat source amplitude. The sawtooth crash time, found from the time interval between the solid black and green curves, is 238 µs, being in the same order of magnitude as that found in single crash simulation [20] but several times longer than that observed in experiments [3][4][5][6][7][8]. The profile is hollow right after the crash (green) due to the outwards radial motion of the hot core.…”

Section: Sawteeth and Sawtooth-free Regimesupporting

confidence: 52%

“…The ion polarization current associated with diamagnetic drift is included. Equations (A1)-(A9) are solved simultaneously using the TM1 code, which had been used for modelling single sawtooth crash for realistic tokamak plasma parameters [6,19,20].…”

Section: Discussion and Summarymentioning

confidence: 99%

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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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Magnetic reconnection during sawteeth crashes

Igochine

2023

Physics of Plasmas

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Sawteeth oscillations are periodic relaxations of the core plasma density and temperature in tokamaks. The rise of the temperature due to external heating is terminated by the crash phase, which involves magnetic reconnection. This is the case of fast magnetic reconnection in collisionless plasmas (Lundquist number S≥108) with a strong guide field. (The toroidal magnetic field in a tokamak is a few orders larger compared to the reconnected helical field.) Experimental measurements show non-linear behavior before and during the crash phase. Simplified single-fluid models are not able to explain the reconnection dynamics during the crash, and two-fluid effects have to be considered. In this case, numerical simulations give good agreement with the observations for the crash duration. At the same time, the present simulations explain experimentally observed phenomena only partially, and several questions remain an area of active research: evolution during the crash, the onset of the fast phase, the existence of the post-cursors, the degree of stochasticity, and others. This overview paper summarizes the current understanding of the crash process, highlights remaining problems, and shows connections to magnetic reconnection research in other plasmas.

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Magnetic reconnection rate during sawtooth crashes in ASDEX Upgrade

Cited by 5 publications

References 33 publications

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

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

Numerical modelling of sawteeth and sawtooth-free regime

Magnetic reconnection during sawteeth crashes

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