Evolution Of Double Tearing Modes Research Articles

Nonlinear evolution and secondary island formation of the double tearing mode in a hybrid simulation

Double tearing modes (DTMs), induced by double current sheet configurations or two neighboring rational surfaces with the same safety factor in tokamaks, are widely observed in solar, space, and fusion plasmas. In this paper, the evolution of DTMs without a guide field is investigated numerically using a hybrid model (electron fluid + ion PIC). The overall evolution processes of DTMs are qualitatively consistent with previous works using other models. The particle dynamics during the evolution of DTMs is analyzed in detail. Behaviors of ions and electrons present different characteristics around the reconnection region which gives rise to Hall effects producing the out-of-plane quadrupole magnetic field. In the explosive reconnection process with interactions between two DTMs islands, the asymmetric drive and the thin current layer feature lead to the emergence of secondary magnetic islands which develop with the late evolution of the DTMs.

Effects of resonant magnetic perturbation on the triggering and the evolution of double-tearing mode

The effects of resonant magnetic perturbation on the triggering and the evolution of the double-tearing mode are investigated by using nonlinear magnetohydrodynamics simulations in a slab geometry. It is found that the double-tearing mode can be destabilized by boundary magnetic perturbation. Moreover, the mode has three typical development stages before it reaches saturation: the linear stable stage, the linear-growth stage, and the exponential-growth stage. The onset and growth of the double-tearing mode significantly depend on the boundary magnetic perturbations, particularly in the early development stage of the mode. The influences of the magnetic perturbation amplitude on the mode for different separations of the two rational surfaces are also discussed.

Effect of plasmoids on nonlinear evolution of double tearing modes

The nonlinear evolution of the double tearing mode is numerically investigated in 2D geometry. Long and thin current sheets are found to be formed and become tearing unstable in the high Lundquist number regime during the fast reconnection phase, leading to the onset of the secondary and tertiary islands (plasmoids). Eventually the system saturates at a quasi-stationary state with those islands coexisting in two pairs. Interestingly, a new evolution process, characterized by two fast reconnection phases, is also discovered for an intermediate distance between the two resonant surfaces due to the evolution of current ribbons associated with the secondary island. These results are obtained only when the symmetry of magnetic configuration is strictly guaranteed in numerical calculations.

Non-linear growth of double tearing mode: Explosive reconnection, plasmoid formation, and particle acceleration

The nonlinear evolution of double tearing modes (DTMs) is investigated within the framework of resistive magnetohydrodynamic (MHD) simulations in a two-dimensional Cartesian geometry. We have explored the explosive reconnection phase associated with the growth of the secondary structure-driven instability for a range of resistivity values. The time scale of the explosive phase (that is of order of a few Alfvénic time scales) is shown to be quasi-independent of the resistivity, even when fast growing plasmoids develop for the highest enough Lundquist number cases. Test particle accelerations are performed using the MHD explosive simulations as input parameters. Our results show that reconnection DTM dynamics is able to provide an efficient process for accelerating charged particles far beyond characteristic thermal velocities within the reconnection layers. The main acceleration mechanism is attributed to the strong inductive electric field generated by the island structure-driven instability, with an additional smaller contribution due to the presence of plasmoids. Finally, our results are used to discuss some features of the accelerated particle spectra during flaring activity in the solar corona.

Influence of asymmetric magnetic perturbation on the nonlinear evolution of double tearing modes

The effects of asymmetric magnetic perturbation on the triggering and evolution of double tearing modes (DTMs) are investigated using nonlinear magnetohydrodynamics simulations in a slab geometry. We find that for reversed magnetic shear plasmas the resistive reconnection process induced by the initial perturbation at one rational surface can drive a new island at the other rational surface with the same mode number. The four typical states of the mode for the time evolution are found, and include: (i) a linear growth stage; (ii) a linear/nonlinear stable stage; (iii) an interactively driving stage; and (iv) a symmetric DTM stage. These differ from previous simulation results. Moreover, nonlinear DTM growth is found to strongly depend on the asymmetric magnetic perturbation, particularly in the early nonlinear phase. The initial perturbation strength scale of island width suggests that the left island enters into a Sweet–Parker growth process when the right island is sufficiently large to effectively drive the other. These results predict that although externally applied magnetic perturbations can suppress the neoclassical tearing mode they can also trigger new instabilities such as asymmetric DTMs.

Effects of electron cyclotron current drive on the evolution of double tearing mode

The effects of electron cyclotron current drive (ECCD) on the double tearing mode (DTM) in slab geometry are investigated by using two-dimensional compressible magnetohydrodynamics equations. It is found that, mainly, the double tearing mode is suppressed by the emergence of the secondary island, due to the deposition of driven current on the X-point of magnetic island at one rational surface, which forms a new non-complete symmetric magnetic topology structure (defined as a non-complete symmetric structure, NSS). The effects of driven current with different parameters (magnitude, initial time of deposition, duration time, and location of deposition) on the evolution of DTM are analyzed elaborately. The optimal magnitude or optimal deposition duration of driven current is the one which makes the duration of NSS the longest, which depends on the mutual effect between ECCD and the background plasma. Moreover, driven current introduced at the early Sweet-Parker phase has the best suppression effect; and the optimal moment also exists, depending on the duration of the NSS. Finally, the effects varied by the driven current disposition location are studied. It is verified that the favorable location of driven current is the X-point which is completely different from the result of single tearing mode.

The effects of a shear flow on nonlinear evolutions of double tearing mode in Hall magnetohydrodynamics

Abstract The effects of a shear flow on nonlinear evolution of double tearing mode in Hall magnetohydrodynamics are investigated. The parallel shear flow displays a suppressing effect on the double tearing mode. Due to the effect of the shear flow, the relative displacements between the magnetic islands are changed, and therefore their mutual interactions become weak. Furthermore, the nonlinear impulsive growth phase of the double tearing mode is delayed with increasing the shear flow velocity. When the magnetic islands are pushing against each other, the width of current sheets decreases drastically, and therefore the Hall effects are dominant and the magnetic reconnection enhances greatly.

Nonlinear Evolution of Shear Flows in Double Tearing Modes

Shear flows induced by the nonlinear evolution of double tearing modes in plasma are studied numerically within the framework of resistive magnetohydrodynamic theory in a slab geometry. Taking into account the plasma resistivity and the distance between two resonant surfaces in a tokamak device, the results show that the effective shear flows are generated in the magnetic island region during the process of magnetic reconnection and last for a few tens of Alfvén time. Furthermore, a few vortical structures of shear flows appear in the final stage of magnetic reconnection. A larger plasma resistivity promotes the process of magnetic reconnection while has no obvious effect on shear flows, particularly in the fast-magnetic-reconnection phase. The amplitude of the shear flows is found to increase with the increase of the initial separation between both resonant surfaces. The temporal evolution of the poloidal plasma flows is also studied in this paper.

Structure-Driven Nonlinear Instability as the Origin of the Explosive Reconnection Dynamics in Resistive Double Tearing Modes

The onset of abrupt magnetic reconnection events, observed in the nonlinear evolution of double tearing modes (DTM), is investigated via reduced resistive magnetohydrodynamic simulations. We have identified the critical threshold for the parameters characterizing the linear DTM stability leading to the bifurcation to the explosive dynamics. A new type of secondary instability is discovered that is excited once the magnetic islands on each rational surface reach a critical structure characterized here by the width and the angle rating their triangularization. This new instability is an island structure-driven nonlinear instability, identified as the trigger of the subsequent nonlinear dynamics which couples flow and flux perturbations. This instability only weakly depends on resistivity.

A study of shear flows induced by nonlinear evolution of double tearing modes in Hall magnetohydrodynamics

Shear flows induced by the nonlinear evolution of double tearing modes are studied numerically using Hall magnetohydrodynamics simulations in a slab geometry. The Hall effect is shown to plays an important role when the thickness of current sheets decrease and falls in the range of ion inertia length d i . Effective shear flows in the magnetic islands are generated during the process of magnetic reconnection and disappear finally. The induced toroidal velocity are shown to be more sensitive than the induced poloidal velocities in the Hall dominant systems. Moreover, the temporal evolution of the shear flows in the Tokamak resonant surface is studied and discussed.

Nonlinear evolution of double tearing mode with guiding magnetic field

Nonlinear dynamic evolution of the double tearing mode (DTM) with a guiding magnetic field (By0) is investigated by magnetohydrodynamical numerical simulation. The dynamic process of DTM depends weakly on the guiding field in the weak guiding field regime (By0≤1), but is suppressed by a strong guiding field (By0>2). During the explosive nonlinear phase, the maximum reconnection rate (γmax) increases weakly with the increase of the resistivity as γmax~η0.06 for By0≤1, but for By0>2, γmax is nearly independent of the resistivity. The maximum reconnection rate in the explosive growth phase increases with increase of the initial current sheet separation. A secondary tearing instability is observed at moderate current sheet separation. A strong guiding field suppresses the formation of a secondary island. Based on the simulation results, it is found that the secondary tearing instability occurs only when the length-to-thickness aspect ratio of the reconnection region exceeds about 20.

Shear flows induced by nonlinear evolution of double tearing modes

Shear flows induced by nonlinear evolution of double tearing modes are investigated in a resistive magnetohydrodynamic model with slab geometry. It is found that intensive and thin poloidal shear flow layers are generated in the magnetic island region driven by coupled reconnection process at both rational surfaces. The structure of the flow layers keeps evolving after the merging of magnetic separatrices and forms a few narrow vortices along the open field lines in the final stage of magnetic reconnection. The effects of the distance between both rational surfaces and the initial magnetic shear on the nonlinear evolution of the plasma flows are also taken into consideration and the relevant mechanism is discussed.

Fast Resistive Reconnection Regime in the Nonlinear Evolution of Double Tearing Modes

Phases of nonlinear double tearing modes are studied numerically. The first two phases lead to the formation and growth of magnetic islands and are followed by a fast reconnection phase to complete the process, driven by a process of neighboring magnetic separatrices merging and magnetic islands coupling. The fast growth can be understood as a result of the island interaction equivalent to a steadily inward flux boundary driven. Resistivity dependences for various phases are studied and shown by scaling analysis for the first time. It is found that after an early Sweet-Parker phase with a eta(1/2)-scale, a slow nonlinear phase in a Rutherford regime with a eta(1)-scale is followed by the fast reconnection phase with a eta(1/5)-scale.

Nonlinear growth of the double tearing mode

A theory for the growth of the double tearing mode in the early nonlinear stage is presented. The equations governing the nonlinear evolution of double tearing modes are formulated and are solved in different parameter regimes. When the phase difference between the two islands is π, the coupling of the two islands plays a destabilizing role. The island growth is of the form w∼t in the early nonlinear stage. The instability condition for the nonlinear double tearing mode is found, and is the same as that for the linear double tearing mode. When current profile is flattened near the two rational surfaces, the coupling plays the dominant role in determining the nonlinear growth.