129 research outputs found
Phase Diagrams of Forced Magnetic Reconnection in Taylor's Model
Recent progress in the understanding of how externally driven magnetic
reconnection evolves is organized in terms of parameter space diagrams. These
diagrams are constructed using four pivotal dimensionless parameters: the
Lundquist number , the magnetic Prandtl number , the amplitude of the
boundary perturbation , and the perturbation wave number .
This new representation highlights the parameters regions of a given system in
which the magnetic reconnection process is expected to be distinguished by a
specific evolution. Contrary to previously proposed phase diagrams, the
diagrams introduced here take into account the dynamical evolution of the
reconnection process and are able to predict slow or fast reconnection regimes
for the same values of and , depending on the parameters that
characterize the external drive, never considered so far. These features are
important to understand the onset and evolution of magnetic reconnection in
diverse physical systemsComment: Comments: 13 pages, 2015 Workshop "Complex plasma phenomena in the
laboratory and in the universe
Formation of Plasmoid Chains in Fusion Relevant Plasmas
The formation of plasmoid chains is explored for the first time within the
context of the Taylor problem, in which magnetic reconnection is driven by a
small amplitude boundary perturbation in a tearing-stable slab plasma
equilibrium. Numerical simulations of a magnetohydrodynamical model of the
plasma show that for very small plasma resistivity and viscosity, the linear
inertial phase is followed by a nonlinear Sweet-Parker evolution, which gives
way to a faster reconnection regime characterized by a chain of plasmoids
instead of a slower Rutherford phase
Extended theory of the Taylor problem in the plasmoid-unstable regime
A fundamental problem of forced magnetic reconnection has been solved taking
into account the plasmoid instability of thin reconnecting current sheets. In
this problem, the reconnection is driven by a small amplitude boundary
perturbation in a tearing-stable slab plasma equilibrium. It is shown that the
evolution of the magnetic reconnection process depends on the external source
perturbation and the microscopic plasma parameters. Small perturbations lead to
a slow nonlinear Rutherford evolution, whereas larger perturbations can lead to
either a stable Sweet-Parker-like phase or a plasmoid phase. An expression for
the threshold perturbation amplitude required to trigger the plasmoid phase is
derived, as well as an analytical expression for the reconnection rate in the
plasmoid-dominated regime. Visco-resistive magnetohydrodynamic simulations
complement the analytical calculations. The plasmoid formation plays a crucial
role in allowing fast reconnection in a magnetohydrodynamical plasma, and the
presented results suggest that it may occur and have profound consequences even
if the plasma is tearing-stable.Comment: Accepted for publication in Physics of Plasma
Gyro-induced acceleration of magnetic reconnection
The linear and nonlinear evolution of magnetic reconnection in collisionless
high-temperature plasmas with a strong guide field is analyzed on the basis of
a two-dimensional gyrofluid model. The linear growth rate of the reconnecting
instability is compared to analytical calculations over the whole spectrum of
linearly unstable wave numbers. In the strongly unstable regime (large \Delta
'), the nonlinear evolution of the reconnecting instability is found to undergo
two distinctive acceleration phases separated by a stall phase in which the
instantaneous growth rate decreases. The first acceleration phase is caused by
the formation of strong electric fields close to the X-point due to ion
gyration, while the second acceleration phase is driven by the development of
an open Petschek-like configuration due to both ion and electron temperature
effects. Furthermore, the maximum instantaneous growth rate is found to
increase dramatically over its linear value for decreasing diffusion layers.
This is a consequence of the fact that the peak instantaneous growth rate
becomes weakly dependent on the microscopic plasma parameters if the diffusion
region thickness is sufficiently smaller than the equilibrium magnetic field
scale length. When this condition is satisfied, the peak reconnection rate
asymptotes to a constant value.Comment: Accepted for publication on Physics of Plasma
Generation of near-equipartition magnetic fields in turbulent collisionless plasmas
The mechanisms that generate "seed" magnetic fields in our Universe and that
amplify them throughout cosmic time remain poorly understood. By means of
fully-kinetic particle-in-cell simulations of turbulent, initially unmagnetized
plasmas, we study the genesis of magnetic fields via the Weibel instability and
follow their dynamo growth up to near-equipartition levels. In the kinematic
stage of the dynamo, we find that the rms magnetic field strength grows
exponentially with rate , where
is the driving scale and is the rms turbulent velocity. In the
saturated stage, the magnetic field energy reaches about half of the turbulent
kinetic energy. Here, magnetic field growth is balanced by dissipation via
reconnection, as revealed by the appearance of plasmoid chains. At saturation,
the integral-scale wavenumber of the magnetic spectrum approaches . Our results show that turbulence -- induced by, e.g., the
gravitational build-up of galaxies and galaxy clusters -- can magnetize
collisionless plasmas with large-scale near-equipartition fields.Comment: 10 pages, 10 figures, PRL in pres
Numerical comparison between a Gyrofluid and Gyrokinetic model investigating collisionless magnetic reconnection
The first detailed comparison between gyrokinetic and gyrofluid simulations
of collisionless magnetic reconnection has been carried out. Both the linear
and nonlinear evolution of the collisionless tearing mode have been analyzed.
In the linear regime, we have found a good agreement between the two approaches
over the whole spectrum of linearly unstable wave numbers, both in the drift
kinetic limit and for finite ion temperature. Nonlinearly, focusing on the
small- regime, with indicating the standard tearing
stability parameter, we have compared relevant observables such as the
evolution and saturation of the island width, as well as the island oscillation
frequency in the saturated phase.The results are basically the same, with small
discrepancies only in the value of the saturated island width for moderately
high values of . Therefore, in the regimes investigated here, the
gyrofluid approach can describe the collisionless reconnection process as well
as the more complete gyrokinetic model.Comment: Accepted for publication on Physics of Plasma
Magnetic island evolution under the action of electron cyclotron current drive
The magnetic island evolution under the action of an externally current generated by electron cyclotron (ECCD) wave beams is studied using a reduced resistive magnetohydrodynamics (RRMHD) plasma model. We found interesting and somewhat unexpected features of the actual nonlinear 2-D evolution of the magnetic perturbation depending on the injection time of the radio frequency control. In particular in the linear phase of the magnetic island growth we observe that the complete annihilation of the island width is followed by a spatial phase shift of the island, referred as «flip»instability. On the other hand, a current drive deposition in the Rutherford regime can be accompained by the occurrence of a Kelvin-Helmholtz type shear flow instability, responsible for the onset of a plasma turbulent behavior
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