N. Singh

Arcing generated electromagnetic fields associated with a high-voltage power system of the space station

A large spacecraft such as the planned space station requires a relatively large amount of electrical power for its routine operation. For example, the power required by the space station is expected to be about 300 kW. This high power requirement entails a power system operating at a high voltage. The space station will operate at about 160 V. Such a large voltage in conjunction with the grounding scheme of the solar cell arrays, would make the potential of the space station structure highly negative with respect to the ionospheric plasma. Since the structure of the station is insulated by a 3-/spl mu/m thick coating of Al/sub 2/O/sub 3/ and the plasma shields the potential, the large negative potential drops across the thin insulating layer, creating an intense electric field (/spl sim/5/spl times/10/sup 7/ V/m) in it. This raises the possibility of dielectric breakdown and arcing from the space station structure to the ionospheric plasma. Laboratory simulations of the arcing have shown large arc currents (/spl sim/1500 A) with a rise time of about 0.1 ms and a total duration of about /spl sim/1 ms. Such a large arc current can affect the operation of the power system and generates electromagnetic waves, which cause concerns about electromagnetic interference. In this paper we report the results from a study on the electromagnetic fields generated by the arc. This problem is modeled as the radiation from a monopole antenna having a transient current in a magnetized plasma.

Nonlinear evolution of plasma instabilities in a plasma with counterstreaming ion beams

Summary form only given, as follows: Linear instabilities of counterstreaming ion beams are well known. We have studied here the nonlinear evolution of such instabilities using a 2.5-D particle-in-cell code. The instabilities lead to the generation of non-resonant zero-frequency and a resonant ion cyclotron modes. For sufficiently large counterstreaming velocities, the non-resonant mode dominates. The anomalous cyclotron resonance of this mode with the beam ions causes a strong pitch angle scattering, generating hat-shaped velocity distribution functions in the phase space perpendicular to the ambient magnetic field. The scattering can be effective in trapping the ions in a magnetic mirror.

Three-dimensional numerical simulation of ion and electron accelerations by parametric decay of fast lower hybrid waves

Summary form only given. The transverse acceleration of ions by fast lower hybrid waves in the topside auoral ionosphere has remained elusive. The linear and nonlinear propagation and evolution of lower hybrid waves are essentially a three-dimensional problem. Analytical treatment of this problem, including the essential kinetic effects for particle acceleration, is a formidable task if not impossible. We have treated this problem by a fully three-dimensional particle-in-cell simulation. It is found that a fast lower hybrid pump wave undergoes a parametric decay generating secondary waves over a broad frequency range from the ion cyclotron frequency /spl Omega//sub i/ to just above the lower hybrid frequency /spl omega//sub lh/. These secondary waves are instrumental in the accelerations of both cold ions and electrons. The ion acceleration is predominantly perpendicular to the ambient magnetic field (B), and it is accompanied by a relatively weak parallel acceleration in the direction of the phase velocity of the pump wave V/sub p/spl par/o/.

Comparison between measured and simulated features of current collection by the tethered satellite

Summary form only given, as follows. The reflight of the tethered satellite system has revealed an intriguing set of plasma processes affecting collection of electrons by a conducting body biased at a positive potential. Currents greatly in excess of that predicted from the Parker-Murphy theory have been measured, but the scaling of the current with the bias potential (/spl Phi//sub 0/) is found to be approximately the same as predicted by the theory for sufficiently large value of /spl Phi//sub 0/. Three-dimensional particle-in-cell simulations of current collection by a conducting spherical body in a magnetized plasma reveals similar behaviors of the current collection. The geometrical aspects of the current flow in the plasma contributing to the collection of electrons by the body and the structure of the sheath will be discussed.

A three-dimensional numerical model for satellite-magnetoplasma interactions

Numerical simulations of the electrodynamic interactions between a satellite and magnetoplasma have been carried out using a 3-D particle-in-cell code in cylindrical geometry. The satellite is a finite-sized cylindrical body, which can be biased at a potential. The plasma is composed of electrons and hydrogen ions. Included in the model is a magnetic field, which represents the geomagnetic field and which in the model can be oriented as required. Simulation cases have been performed with and without a relative motion between the satellite and plasma. For the situation without motion, the current collected by the satellite is compared with the available analytical prediction from the theory of L. W. Parker and B. L. Murphy (1967). The current collected in the simulations is found to be in agreement with the theoretical value for different system parameters. For simulations with a relative motion, the current collected by the satellite is found to be enhanced over the nondrifting case.<<ETX>>

Calculation of refractive index around a hypersonic vehicle with infrared sensors

The disturbed gas envelope around a hypersonic vehicle which carries an infrared sensor is shown. The envelope contains a compressed hot gas and plasma produced by the ionization of the gas constituents. Both gas and plasma surrounding the sensors have high densities, resulting in a sharp density gradient at the shock wave in front of the vehicle. The density gradients inside the envelope act like a multiple-lens system which refracts optical rays in the medium. Plasma effects are important only in the nose-cone region where temperatures and the densities of the compressed gas are high. In regions far from the nose-cone, plasma effects are negligible. The relative effects of the gas and the plasma on the refractive index are as follows. At low Mach numbers, the gas effects are more significant than the plasma effects. At high Mach numbers, this situation is reversed. By comparing the effects of plasma based on the ionizations obtained from Sahas equation and the CFD code, it was found that transport effects make use of the Saha equation less reliable for estimating the refractive index due to the plasma.<<ETX>>

Numerical simulations for the current collection by a tethered-satellite in the ionosphere

The current collection by a tethered-satellite in the low earth orbit and the processes involved in this ionospheric phenomenon were investigated by numerical simulations. Initially a 2-D case was considered in the cylindrical geometry. The system solved numerically was comprised of a long cylindrical satellite, an arbitrary orientation of the ambient magnetic field, and a Maxwellian plasma flowing relative to the satellite. The relative orientation between the magnetic field and the plasma flow has been found to effect the current collection, with an enhancement achieved when the magnetic field and the plasma flow are perpendicular. Results presented include the plasma distributions, sheath structures, and the current-voltage characteristics. For a typical orbit in the ionosphere, the relative plasma flow is perpendicular to the magnetic field, and this case has been extensively studied.<<ETX>>