M. J. Mandell

Nascap-2k Spacecraft Charging Code Overview

Nascap-2k is a modern spacecraft charging code, replacing the older codes NASA Charging Analyzer Program for GEosynchronous Orbit (NASCAP/GEO), NASA Charging Analyzer Program for Low-Earth Orbit (NASCAP/LEO), Potentials Of Large objects in the Auroral Region (POLAR), and Dynamic Plasma Analysis Code (DynaPAC). The code builds on the physical principles, mathematical algorithms, and user experience developed over three decades of spacecraft charging research. Capabilities include surface charging in geosynchronous and interplanetary orbits, sheath, and wake structure, and current collection in low-Earth orbits, and auroral charging. External potential structure and particle trajectories are computed using a finite element method on a nested grid structure and may be visualized within the Nascap-2k interface. Space charge can be treated either analytically, self-consistently with particle trajectories, or consistent with imported plume densities. Particle-in-cell (PIC) capabilities are available to study dynamic plasma effects. Auxiliary programs to Nascap-2k include Object Toolkit (for developing spacecraft surface models) and GridTool (for constructing nested grid structures around spacecraft models). The capabilities of the code are illustrated by way of four examples: charging of a geostationary satellite, self-consistent potentials for a negative probe in a low-Earth orbit spacecraft wake, potentials associated with thruster plumes, and PIC calculations of plasma effects on a very low frequency (about 1 to 20 kHz) antenna

The Decrease in Effective Photocurrents Due to Saddle Points in Electrostatic Potentials near Differentially Charged Spacecraft

The reported investigation had the objective to illustrate the presence of important multidimensional effects in spacecraft charging. Two-dimensional codes have been under development by Parker (1976). A description is presented of a calculation which was performed using the three-dimensional NASA Charging Analyzer Program (NASCAP). NASCAP was run to calculate the electrostatic potentials on the surface of, and in the space surrounding, a sunlit Teflon-coated sphere. Currents to the sunlit surfaces were determined on the basis of an approximate photosheath model for strong differential charging.

A Hall effect thruster plume model including large-angle elastic scattering

A new model of the plasma plume from Hall Effect Thrusters (HETs) is presented. The model includes the self-expansion of the main beam by density gradient electric fields, lowenergy ions produced by resonant charge exchange between beam ions and neutral atoms (ambient and thruster-induced), and angle-dependent elastic scattering of beam ions off neutral atoms. The variation of radial velocities across the annular thruster beam is also included. The model is an advance over previous plume models in the way it numerically models the self-expansion of the main beam, and in particular, the treatment of elastic scattering using recently calculated differential cross sections. The results are compared with recent measurements of the energy and angledependent plume from the BPT4000 Hall-Effect Thruster. Both the intensity and energy dependence of the scattering peaks are compared. The principal result is that elastic scattering is the source of the majority of ions with energy greater than E/q=50V that are observed at angles greater than 45° with respect to the thrust axis. The model underscores the need for elastic scattering cross sections for multiply charged ions, as well as a better understanding of HET propellant utilization.

Electrical Breakdown Currents on Large Spacecraft in Low Earth Orbit

An experimental and theoretical investigation of an expanding plasma generated by an arc produced by biasing a conductor underneath a thin layer of anodized aluminum 160-V negative of a laboratory plasma that can produce large peak arc currents by discharging large surface areas is presented. A simple theory shows that the time scales and observed current magnitudes are consistent with the expansion of a discharge-generated plasma. The implication for large spacecraft in low Earth orbit, such as Space Station Freedom (SSF) which can store large amounts of charge, is that arcs with the same amount of energy similar to those observed in the laboratory may occur. The energy in these arcs degrade the surface of the anodized aluminum thermal control coatings by producing large pits in the surface. These pits tend to increase the temperature of the spacecraft, and the material from the pits can become an additional source of contamination . The rise time and intensity of theses arc could produce significant EMI. To prevent the occurrence of these undesirable effects, SSF will utilize a plasma contactor that will control the structure to ambient plasma potentials.

Plasma Collection by High-Voltage Spacecraft at Low Earth Orbit

A computer model of the three-dimensional sheath formation and plasma current collection by high-voltage spacecraft has been developed. By using new space charge density and plasma collection algorithms, it is practical to perform calculations for large, complex spacecraft. The model uses objects and geometries compatible with the NASA Charging Analyzer Program (NASCAP). Results indicate that ion focusing observed in the laboratory during high-voltage collection experiments is probably due to voltage gradients on the collecting surfaces.

The Role of Unneutralized Surface Ions in Negative Potential Arcing

The observed arcing on negatively biased solar arrays exposed to plasma environments is shown to be due to an effective charge layer on the interconnect formed by ion collection from the plasma. Time scales to form this layer are shown to be in agreement with experimental observations. A quantitative theory is presented which predicts arcing threshold dependence on plasma density and external potentials. After breakdown, the discharge process is modeled as space charge limited transport to nearby coverslips. Peak currents and decay times predicted by this model are compared with experimental observations.

NASCAP, a Three-Dimensional Charging Analyzer Program for Complex Spacecraft

A computer code, NASCAP (NASA Charging Analyzer Program), has been developed by Systems, Science and Software under contract to NASA-LeRC to simulate the charging of a complex spacecraft in geosynchronous orbit. The capabilities of the NASCAP code include a fully three-dimensional solution of Poissons equation about an object having considerable geometrical and material complexity, particle tracking, shadowing in sunlight, calculation of secondary emission, backscatter and photoemission, and graphical output. A model calculation shows how the NASCAP code may be used to improve our understanding of the spacecraft-plasma interaction.

Potentials on Large Spacecraft in LEO

A system-oriented computer code is used to predict surface charging due to voltages generated within a satellite operating in the typical dense plasma environment of LEO. The use of this code is demonstrated by predicting the expansion of electric fields onto a kapton surface from a pinhole over a biased conductor in a LEO environment. The results are compared to a more-exact solution and experimental data.

Fluid Model of Plasma Outside a Hollow Cathode Neutralizer

The present study analyzes the capability of a fluid model of electron transport to explain observed properties of the external plasma of a hollow cathode neutralizer used to neutralize a beam emerging from an ion thruster. Calculations reported here show that when the effective collision frequency in such a model is near the plasma frequency, the resulting electric potential and electron temperature variations are in qualitative agreement with values measured in the plume mode of the hollow cathode. Both theory and experiment show strong variations of temperature and potential within a few centimeters of the cathode orifice. E E J k L m n n, P Q V r R T u V_ y+ V dn Nomenclature kinetic energy of an electron electric field net current density Boltzmanns constant scale length for variation of macroscopic plasma properties electron mass electron density density of ions (+) and electrons (-); / = + , scalar electron pressure magnitude of electron charge heat flux position vector collisional drag force between electrons and ions electron temperature v.v+ drift velocity of electrons drift velocity of ions jitter velocity of electrons electron density fluctuation plasma resistivity kT thermal conductivity of plasma coefficient defined by power law dependence of K on 6, K — mK 1 Om~l, with m a pure number mean free path for pair collisions between electrons Debye length effective collision frequency electron-ion collision frequency = 7J0 = electric potential (jo = frequency of oscillation of fluctuating field cjp = electron plasma frequency

High‐voltage interactions in plasma wakes: Simulation and flight measurements from the Charge Hazards and Wake Studies (CHAWS) experiment

The Charge Hazards and Wake Studies (CHAWS) flight experiment flew on the Wake Shield Facility (WSF) aboard STS-60 and STS-69. The experiment studied high-voltage current collection within the spacecraft wake. The wake-side sensor was a 45-cm-long, biasable cylindrical probe mounted on the 3.66-m-diameter WSF. Operations were performed in free flight and at various attitudes while on the shuttle orbiter remote manipulator system (RMS) arm. Preflight and postflight simulations were performed using the programs Potentials of Large Objects in the Auroral Region (POLAR) and Dynamic Plasma Analysis Code (DynaPAC) and are compared here with the flight results. Both programs perform three-dimensional, self-consistent, steady state plasma simulations. During high-voltage operations the wake-side probe collected current consistent with preflight predictions. In both the flight data and the steady state simulations the current collected has a power law dependence on the potential and has a less than linear dependence on density. Growth of the sheath beyond the WSF edge controls the high-voltage current collection, and kinematic and space charge effects both play important roles in attracting ions into the wake region. Measurements made at low voltage differ from the calculations. The preflight calculations for a pure oxygen plasma predict a collection threshold at −100 V bias. The flight data show little or no threshold, implying a source of ions not accounted for in the simulations. Possible sources for these ions are discussed.