A. Zocco

Reduced fluid-kinetic equations for low-frequency dynamics, magnetic reconnection, and electron heating in low-beta plasmas

A minimal model for magnetic reconnection and, generally, low-frequency dynamics in low-beta plasmas is proposed. The model combines analytical and computational simplicity with physical realizability: it is a rigorous limit of gyrokinetics for plasma beta of order the electron-ion mass ratio. The model contains collisions and can be used both in the collisional and collisionless reconnection regimes. It includes gyrokinetic ions (not assumed cold) and allows for the topological rearrangement of the magnetic field lines by either resistivity or electron inertia, whichever predominates. The two-fluid dynamics are coupled to electron kinetics—electrons are not assumed isothermal and are described by a reduced drift-kinetic equation. The model, therefore allows for irreversibility and conversion of magnetic energy into electron heat via parallel phase mixing in velocity space. An analysis of the exchanges between various forms of free energy and its conversion into electron heat is provided. It is shown how ...

Fast collisionless reconnection and electron heating in strongly magnetized plasmas

Magnetic reconnection in strongly magnetized (low-beta), weakly collisional plasmas is investigated by using a novel fluid-kinetic model [Zocco and Schekochihin, Phys. Plasmas 18, 102309 (2011)] which retains nonisothermal electron kinetics. It is shown that electron heating via Landau damping (linear phase mixing) is the dominant dissipation mechanism. In time, electron heating occurs after the peak of the reconnection rate; in space, it is concentrated along the separatrices of the magnetic island. For sufficiently large systems, the peak reconnection rate is cE(∥)(max) ≈ 0.2v(A)B(y,0), where v(A) is the Alfvén speed based on the reconnecting field B(y,0). The island saturation width is the same as in magnetohydrodynamics models except for small systems, when it becomes comparable to the kinetic scales.

Global gyrokinetic particle-in-cell simulations of internal kink instabilities

Internal kink instabilities have been studied in straight tokamak geometry employing an electromagnetic gyrokinetic particle-in-cell (PIC) code. The ideal-MHD internal kink mode and the collisionless m=1 tearing mode have been successfully simulated with the PIC code. Diamagnetic effects on the internal kink modes have also been investigated.

Current sheet bifurcation and collapse in electron magnetohydrodynamics

Inertial effects in nonlinear magnetic reconnection are studied within the context of 2D electron magnetohydrodynamics (EMHD) with resistive and viscous dissipation. Families of nonlinear solutions for relevant current sheet parameters are predicted and confirmed numerically in all regimes of interest. Electron inertia becomes important for current sheet thicknesses

Fundamental role of ion viscosity on fast magnetic reconnection in large-guide-field regimes

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Unified theory of the semi-collisional tearing mode and internal kink mode in a hot tokamak: implications for sawtooth modelling

below the inertial length

Fast ion stabilization of the ion temperature gradient driven modes in the Joint European Torus hybrid-scenario plasmas: a trigger mechanism for internal transport barrier formation

d_{e}

Kinetic microtearing modes and reconnecting modes in strongly magnetised slab plasmas

. In this case, in the absence of electron viscosity, the sheet thickness experiences a nonlinear collapse. Viscosity regularizes solutions at small scales. Transition from resistive to viscous regimes shows a nontrivial dependence on resistivity and viscosity, featuring a hysteresis bifurcation. In all accessible regimes, the nonlinear reconnection rate is found to be explicitly independent of the electron inertia and dissipation coefficients.

Global linear gyrokinetic particle-in-cell simulations including electromagnetic effects in shaped plasmas

Nonlinear analytical theory of magnetic reconnection with a large guide field is presented for the first time. We confirm that two distinct steady-state reconnection regimes are possible depending on the relative size of the diffusion region thickness δ versus the sound gyroradius ρs. The reconnection is slow (Sweet–Parker-like) for δ≳ρs, and fast otherwise. However, unlike earlier work, we find that ion viscosity μ plays a fundamental role in the fast regime. In particular, for δ<ρs we obtain δ∝Ha−1, with Ha∝1/ημ as the Hartmann number, and the reconnection rate Ez∝Pr−1/2, with Pr=μ/η as the Prandtl number and η as the resistivity. If the perpendicular ion viscosity is employed for μ, the reconnection rate becomes independent of plasma β and collision frequencies, and therefore potentially fast.

Linear collisionless Landau damping in Hilbert space

In large hot tokamaks like JET, the width of the reconnecting layer for resistive modes is determined by semi-collisional electron dynamics and is much less than the ion Larmor radius. Firstly a dispersion relation valid in this regime is derived which provides a unified description of drift-tearing modes, kinetic Alfv?n waves and the internal kink mode at low beta. Tearing mode stability is investigated analytically recovering the stabilizing ion orbit effect, obtained previously by Cowley et al (1986 Phys. Fluids 29 3230), which implies large values of the tearing mode stability parameter ?? are required for instability. Secondly, at high beta it is shown that the tearing mode interacts with the kinetic Alfv?n wave and that there is an absolute stabilization for all ?? due to the shielding effects of the electron temperature gradients, extending the result of Drake et al (1983 Phys. Fluids 26 2509) to large ion orbits. The nature of the transition between these two limits at finite values of beta is then elucidated. The low beta formalism is also relevant to the m?=?n?=?1 tearing mode and the dissipative internal kink mode, thus extending the work of Pegoraro et al (1989 Phys. Fluids B 1 364) to a more realistic electron model incorporating temperature perturbations, but then the smallness of the dissipative internal kink mode frequency is exploited to obtain a new dispersion relation valid at arbitrary beta. A diagram describing the stability of both the tearing mode and dissipative internal kink mode, in the space of ?? and beta, is obtained. The trajectory of the evolution of the current profile during a sawtooth period can be plotted in this diagram, providing a model for the triggering of a sawtooth crash.