Nonideal Magnetohydrodynamic Turbulent Decay in Molecular Clouds

It is well known that non-ideal magnetohydrodynamic effects are important in the dynamics of molecular clouds: both ambipolar diffusion and possibly the Hall effect have been identified as significant. We present the results of a suite of simulations with a resolution of 512-cubed of turbulent decay in molecular clouds incorporating a simplified form of both ambipolar diffusion and the Hall effect simultaneously. The initial velocity field in the turbulence is varied from being super-Alfvénic and hypersonic, through to trans-Alfvénic but still supersonic

A Three‐Dimensional Numerical Method for Modelling Weakly Ionized Plasmas

Astrophysical fluids under the influence of magnetic fields are often subjected to single- or two-fluid approximations. In the case of weakly ionized plasmas, however, this can be inappropriate due to distinct responses from the multiple constituent species to both collisional and non-collisional forces. As a result, in dense molecular clouds and protostellar accretion discs, for instance, the conductivity of the plasma may be highly anisotropic leading to phenomena such as Hall and ambipolar diffusion strongly influencing the dynamics

An Explicit Scheme for Multifluid Magnetohydrodynamics

When modelling astrophysical fluid flows, it is often appropriate to discard the canonical magnetohydrodynamic approximation, thereby freeing the magnetic field to diffuse with respect to the bulk velocity field. As a consequence, however, the induction equation can become problematic to solve via standard explicit techniques. In particular, the Hall diffusion term admits fast-moving whistler waves which can impose a vanishing time-step limit.Within an explicit differencing framework, a multifluid scheme for weakly ionized plasmas is presented which relies upon a new approach to integrating the induction equation efficiently. The first component of this approach is a relatively unknown method of accelerating the integration of parabolic systems by enforcing stability over large compound time-steps rather than over each of the constituent substeps. This method, Super Time-Stepping, proves to be very effective in applying a part of the Hall term up to a known critical value. The excess of the Hall term above this critical value is then included via a new scheme for pure Hall diffusion

An Explicit Scheme for Multifluid Magnetohydrodynamics

When modelling astrophysical fluid flows, it is often appropriate to discard the canonical magnetohydrodynamic approximation, thereby freeing the magnetic field to diffuse with respect to the bulk velocity field. As a consequence, however, the induction equation can become problematic to solve via standard explicit techniques. In particular, the Hall diffusion term admits fast-moving whistler waves which can impose a vanishing time-step limit.Within an explicit differencing framework, a multifluid scheme for weakly ionized plasmas is presented which relies upon a new approach to integrating the induction equation efficiently. The first component of this approach is a relatively unknown method of accelerating the integration of parabolic systems by enforcing stability over large compound time-steps rather than over each of the constituent substeps. This method, Super Time-Stepping, proves to be very effective in applying a part of the Hall term up to a known critical value. The excess of the Hall term above this critical value is then included via a new scheme for pure Hall diffusion

Oscillatory behavior of hollow grid cathode discharges

Multiple complex space-charge structures in unmagnetized low-temperature plasmas arise from ionization phenomena near additional negatively or positively biased electrodes or due to local constraints. Because of their usually spherical form, such structures are called fireballs. If they appear inside hollow grids, they are called inverted fireballs or plasma bubbles. The temporal evolution of such structures is often accompanied by strong plasma instabilities. The dynamics of complex space-charge structures have been investigated by using single spherical grid cathode with an orifice. Langmuir probe and optical emission spectroscopy were used to diagnose the structures. Measurements delivered the axial profiles of the plasma potential, electron temperature and density, and the densities of excited atoms and ions, that confirmed the formation of a fireball in the region near the orifice (also evidenced by visual observation). Inside the grid, a plasma bubble has developed, with a high ion density inside due to the hollow cathode effect. Information on the nonlinear dynamics of the complex space charge structures was obtained from the analysis of the oscillations of the discharge current