Magnetic field reconnection in collisionless plasmas

Magnetic reconnection in collisionless regimes, where electron inertia is responsible for the decoupling of the plasma motion from that of the magnetic field, can now be observed in laboratory plasmas produced by tokamaks such as JET. At the high plasma temperatures of these experiments, internal plasma relaxations can occur on a time scale shorter than the electron-ion collision time. Recently, the linear theory of the m = 1 kink-tearing modes has been extended to experimentally relevant regimes, leading to the conclusion that these modes can remain virulent at low collisionality with an initial growth rate that compares favorably with that observed in the experiments. The nonlinear evolution of this process can be analyzed on the basis of a two-dimensional incompressible fluid model. A numerical and analytical solution of this model that neglects ion Larmor radius and electron pressure effects has been presented. The reconnection process is accompanied by the formation of a current density sublayer narrower than the skin depth and that keeps shrinking with time. This numerical solution was obtained with a pseudospectral code that can no longer resolve the sublayer after the early nonlinear phase is entered. With the aim of resolving this sublayer we adopted a different approach based on a finite-difference method. (1) Under the same initial and boundary conditions, we can reproduce the results obtained with the pseudospectral code. In particular we find the same quasi-explosive behavior of the reconnection process in the early non linear phase. (2) The code is implemented with a non-uniform grid and this increases our ability to follow the evolution of the microscale inside the current-sheet. (3) Now, we can extend this model and generalize its equations so as to include the effect of the thermal ion Larmor radius which is of the order of, or larger than, the electron skin depth in the regimes of interest