Search

The objective of the Active Plasma Experiment North Star mission was to study the interaction of artificially produced aluminum ion plasma jets with the space environment. Two separate plasma jets were injected almost perpendicular to the local magnetic field during the North Star experiment. The jets were created using an explosivetype generator designed to produce a high-speed (7‐42-km/s) aluminum ion plasma jet with plasma densities exceeding 10 9 cm −3 at a distance 170 m from the plasma-jet source. The first plasma-jet injection occurred at an altitude of 360 km and was preceded by the release of an artificial air cloud. The second injection occurred at an altitude of 280 km and did not include the air cloud. Interactions of the plasma jet with the local space environment and artificial air cloud were monitored using instrumentation on three diagnostic payloads, ground-based optical sensors, and space-based optical sensors. An overview is provided of the experiment, along with a summary of the principal results from the mission.

The charge accumulation process inside a Teflon film is numerically simulated with a Particle‐in‐Cell/Monte Carlo methodology. Elastic and inelastic collision processes between irradiated electrons and Teflon are implemented based on a model that combines theoretical and semi empirical approaches. The simulations predict the charge accumulation process properly and provide predictions of the penetration depth inside the Teflon. Electron irradiation experiments are also conducted for code validation. These experiment provide the transient charge distribution inside dielectric materials such as Teflon, Kapton and PMMA and show the dependency of the penetration depth to the material density. Improvements in the computational and experimental methodologies will enable proper code validation.

The electrodynamics of particulate contamination in an ionospheric plasma near spacecraft surfaces is being investigated by a new 2D particle-in-cell (PIC) simulation. Particulate charging in the PIC simulation is treated as a collisional event and is implemented in a Monte Carlo fashion. The simulation has been applied to dusty plasmas both in an unbounded plasma and near a bounded plasma (spacecraft surface). Results from the simulation include estimates of the charge and potential structure of finite-size dust clouds. Low-density dust clouds do not perturb the electric potential of the plasma, similar to the case for isolated particulates. In high-density dust clouds, the potential forms a sheath-like structure while the charge forms a double layer on the outer boundary of the cloud. Particulates can have negative or positive charges depending on the applied surface potential, thus raising the possibility of controlling particulate contamination by controlling the spacecraft potential.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A two-dimensional particle-in-cell/Monte Carlo simulation code is developed and used to examine the electrodynamics of a finite-size dust (particulate) plasma. The model that can be applied to a dusty plasma bounded by surfaces or to a dust cloud embedded in an unbounded plasma accounts for the dynamics of plasma and dust particles, includes the collisional absorption of plasma species by the dust with the use of a multistep Monte Carlo algorithm, and solves Poisson's equation for the electric fields. Charging of isolated particulates is compared with estimates based on orbital motion-limited current collection. The charge and potential structure of finite-length dust clouds embedded in a plasma are examined, and the results are compared to previous one-dimensional solutions. The average dust charge is found to decrease with increasing dust density. Low-density dust clouds do not perturb the potential and their average charge approaches that of isolated particulates. In the case of dense dust clouds the potential forms a sheathlike structure while the charge forms a double layer with the negative charge residing on the outer boundary of the cloud. Dust clouds initialized in the sheath of a surface biased to a potential are also examined. It is found that the dusty plasma electrodynamics is affected by the applied surface potential, plasma drift energy, and dust density. Collective effects in high-density clouds alter the sheath, which can expand or collapse. It is found that particulates can be charged negatively or positively depending on the drift energy and the applied surface potential.

A fully three-dimensional model for an artificial plasma cloud in the ionosphere is developed. Such a cloud could be a result of a deliberate release or contamination about a spacecraft. Finite perpendicular and parallel lengths, all elastic collisions between the considered species, finite temperature effects, variable ionospheric densities, variable neutral winds, ambient electric fields and gravity are taken into account. The velocities of the charged species are written with the use of transport coefficient tensors. The current balance equation includes conductive currents, diffusion currents, currents due to the ambient and contaminant neutral winds, and gravitational drift currents. The numerical solution for a plasma sphere immersed in a plasma with an external electric field is obtained and compared with known analytic solutions. The electrodynamical behavior of Gaussian-like clouds is examined. The neutral cloud simulates an orbital release (U=8 km/s) with various masses. The effects of the central density of the contaminant neutrals and ions are investigated. The perturbation potential increases with increasing ion and neutral density. The perturbation electric field consists of a dipole field due to the action of the neutral wind and a diffusion field due to the density gradients. The dipole field is mapped along the magnetic field lines while the diffusion field is bounded within the density perturbation. With cloud densities of 1015 m−3 the ion cloud drifts almost with the neutral cloud velocity. The effects of the coupling of the lower E and upper F region are also investigated. The ambient plasma is expected to develop image clouds with differential rotation which will stretch the flux tube. The effect of the altitude of the release is examined. It is found that releases at higher altitudes result in faster moving ion clouds. As a result of the nonuniformity of the perturbation electric field, the ion cloud drifts with differential speed which will result in fingerlike shaping.

Large structures in low earth orbit will release neutral water or ions through outgassing, water dumps or thruster firings and thus perturb the ambient ionosphere. Neutrals and ions within the perturbed environment will undergo chemical reactions to form a contaminant cloud. It is assumed that the contaminant cloud consists of ions such as O{sup +}, H{sub 2}O{sup +}, H{sub 3}O{sup +} and neutrals such as O, H, OH and H{sub 2}O. A two-dimensional model for the motion of the contaminant cloud perpendicular to the magnetic field lines is developed. Numerical solution of the derived model equations examines the effects of Alfven wave coupling, neutral water density, ion temperature and initial conditions on the cloud motion. In low density neutral water clouds ({le} 10{sup 9} cm{sup {minus}3}) the shielding of the electric field is small. In neutral water clouds with densities of interest for shuttle conditions ({approximately} 10{sup 10} cm{sup {minus}3}) the shielding is predicted to be of the order of the motion electric field and the ratios of the line averaged densities of the ions is consistent with experimental data. The effect of ion temperature is negligible for the range of densities and temperatures considered. It is predicted that formore » symmetric initial conditions, the drifting clouds will develop fingerlike instabilities. Plasma depletions are predicted in the wake of the moving structure with enhancements in the ram direction. Asymmetric initial conditions, such that could result from a thruster firing, rotate the plasma cloud which undergoes a highly asymmetric distortion.« less