G. Joyce

Ion distribution function in a plasma with uniform electric field

For a homogeneous partially ionized plasma subject to a uniform electric field E, several methods and models are used to calculate the distribution function f(v) for ions subject to charge-exchange collisions. The exact solution for f(v), based on the energy-dependent cross section for Ar, is obtained by Monte Carlo (MC) simulation. This is compared to the MC results for f(v), based on either a constant cross section σ or a constant collision frequency ν. The constant-σ model is found to accurately represent f(v) for any value of E, whereas the constant-ν results are qualitatively incorrect for large fields. Under the constant-σ assumption, a simple, easily solvable ordinary differential equation is obtained which reproduces the MC results with good accuracy.

Nonlinear whistler instability driven by a beamlike distribution of resonant electrons

Theory and simulation are used to study the instability of a coherent whistler parallel-propagating in a simplified model radiation belt with a background of cold electrons, as well as a ring distribution of energetic electrons. A nonlinear instability is initiated at the location z+, where the electrons are cyclotron resonant with the wave, on the side of the equator (z=0) where the wave is propagating away from the equator. The instability propagates backward toward the equator, growing both spatially and temporally. As the instability develops, frequency falls in such a way as to keep the electrons nearly resonant with the waves over the entire region 0<z<z+. The instability causes a sharp drop in the pitch angle of the resonant electrons and eventually saturates with peak amplitude near the equator.

Grain-grain interaction in stationary dusty plasma

We present a particle-in-cell simulation study of the steady-state interaction between two stationary dust grains in uniform stationary plasma. Both the electrostatic force and the shadowing force on the grains are calculated explicitly. The electrostatic force is always repulsive. For two grains of the same size, the electrostatic force is very nearly equal to the shielded electric field due to a single isolated grain, acting on the charge of the other grain. For two grains of unequal size, the electrostatic force on the smaller grain is smaller than the isolated-grain field, and the force on the larger grain is larger than the isolated-grain field. In all cases, the attractive shadowing force exceeds the repulsive electrostatic force when the grain separation d is greater than an equilibrium separation d0. d0 is found to be between 6λD and 9λD in all cases. The binding energy is estimated to be between 19u2009eV and 900u2009eV for various cases.

Structural properties of 3D complex plasmas under microgravity conditions

We report the structural properties of three-dimensional complex plasmas observed recently on board the International Space Station. A local order analysis reveals spatially resolved features that occur during the crystallization of a plasma crystal. The plasma crystal consists of hcp and fcc phases with a small fraction of bcc-like clusters. It has been shown that the observed anisotropy of the system of microparticles is due to presence of the hcp phase. Molecular-dynamics simulations of crystallization of a system of particles, interacting via Debye-Huckel (Yukawa) potential, reproduces the observed local order remarkably well.

Effective dipole moment for the mode coupling instability: mapping of self-consistent wake models

The theory of the mode coupling instability operating in two-dimensional plasma crystals is generalized, by employing the linear plasma response formalism to describe the interparticle interactions self-consistently. In this approach, the underlying ion distribution function is calculated from first principles. Subthermal and suprathermal regimes of the ion flow are considered. A mapping procedure is proposed, which relates the self-consistent coupling coefficients to the effective dipole moment of the wake—the parameter which characterizes the mode coupling in the framework of the conventionally used Yukawa/point-wake model. The importance of the self-consistent approach is demonstrated by comparing the theoretically obtained dipole moments with the values deduced from experiments.

Anisotropic plasma cyrstals: Phase diagram

Summary form only given. Laboratory complex (dusty) plasmas are low-pressure gas- discharge plasmas containing monodisperse microparticles that are highly charged due to absorption of ambient electrons and ions. The unique feature distinguishing complex plasmas is that one can vary the strength of the electrostatic coupling between particles over an extremely broad range. This allows us to explore the entire hierarchy of the phase states - ranging from gaseous to solid - and thus study all relevant generic processes occurring in strongly coupled media at the most fundamental kinetic level. Recently, the experimental discovery of electrorheological complex plasmas has been reported1. In such plasmas the interaction between microparticles can be controlled by external AC electric fields, due to distortion of the Debye spheres that surround microparticles. We show that interactions in electrorheological complex plasmas are equivalent to those in conventional dipolar fluids. Results of the recent microgravity experiments and complementary molecular dynamics simulations reveal variety of phase transitions controlled by the amplitude of the field, resulting in different anisotropic (string-like) solid structures. We also report on a comprehensive thermodynamic study of anisotropic complex plasmas (with a spherically-symmetric repulsive Yukawa interaction and additional dipole-dipole interaction) induced by an external field. We propose a simple variational approach based on the Bogoliubov inequality for determining equilibrium solid phases. We show that the phase diagram of such complex plasmas exhibits numerous solid-solid phase transitions precisely controlled by the magnitude of the applied field.