Computer simulations of plasma expansion across a magnetic field

A two-dimensional electrostatic particle-in-cell code is used to simulate the thermal expansion of a cylindrical plasma across a uniform magnetic field. The simulations show that at early times (i.e., short compared to an ion gyroperiod), the plasma develops three characteristic regions: a charge neutral core surrounded by a negatively charged region (the electron charge layer), and an external halo composed only of ions (the ion charge layer). The radii of these non-neutral regions increase when either the electron plasma to electron cyclotron frequency ratio ωpe/‖Ωe‖ increases or when the electron to ion temperature ratio, Te/Ti, decreases. These simulations, which have been run with an ion-to-electron mass ratio of 100, show that the plasma develops structure as a result of the growth of a nonoscillatory instability in the electron charge layer. This structure exhibits growth times short compared to the ion gyroperiod, and wavelengths of the order of the ion gyroradius. The mode number of this structure increases with decreasing ωpe/‖Ωe‖, with increasing initial radius of the plasma relative to the electron Debye length, and with increasing Te/Ti. The linear theory of the lower hybrid drift instability, driven by the E×B and ∇n×B electron drift velocities, provides a good description of the parametric variation of the structure wavelength. It is therefore concluded that this instability is the most likely source of the structure observed in these simulations.