Victoria A. Davis

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

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.

A Van der Waals‐like theory of plasma double layers

A new theory is presented that describes plasma double layers in terms of multiple roots of the charge density expression. Analogous to Van der Waals’ equation for simple fluids, the system is described using simple analytical expressions that contain the essential nonlinearity of the physics. Both theories predict multiple states and transitions between the states. Van der Waals’ theory is for the liquid–gas phase transition; the theory presented here is for double layers between two plasmas. Except within the double layer, the plasma is assumed to be quasineutral, that is, the charge density is almost zero. The expression used for charge density includes linear shielding at low potentials and current continuity at high potentials. The theory is independent of the details of the expression used for the charge density; it only requires that the charge density be a nonmonotonic function of potential. Multiple roots exist because of this nonlinearity; linear theories such as Debye shielding allow for only a...

Charge Control of Geosynchronous Spacecraft Using Field Effect Emitters

Abstract : *Space Systems Loral is conducting an lR&D program to determine the feasibility and effectiveness of field effect electron emitters for potential control of geosynchronous altitude spacecraft. This electron emitters will be based on Spindt Cathode Field Emission Array Technologies. The configuration studied here consists of two emitters, each with an area of about 1 cm2 and emitting up to 1 mA of electrons at approximately 50eV energy. We show that it appears feasible to use electron emitters to control the surface charge of a satellite. Results concerning the placement and effectiveness of emitters and the spacecraft potential configuration under substorm conditions with and without emitter operations in sunlight, in eclipse, and during eclipse exit.

Nascap-2k Spacecraft-Plasma Environment Interactions Modeling: New Capabilities and Verification

Nascap-2k is a three-dimensional computer code that models interactions between spacecraft and plasma environments in low-Earth, geosynchronous, auroral, and interplanetary orbits. The code builds on physical principles, mathematical algorithms, and user experience developed over three decades of spacecraft charging research. Nascap-2k has improved numeric techniques, a modern user interface, and a simple, interactive satellite surface definition module (Object ToolKit). 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. Previously, we reported on the accuracy of Nascap-2ks charging and current collection calculations by comparing computed currents and potentials with analytic results, and by comparing Nascap-2k results with published calculations using the earlier lower resolution codes, NASCAP/GEO, NASCAP/LEO, and POLAR. Here we examine the accuracy and limitations of two new capabilities of Nascap-2k: modeling of plasma plumes such as generated by electric thrusters and enhanced PIC computational capabilities. Nascap-2k models one or more ion engine plumes in full three-dimensional geometry, including plume-plume and plume-spacecraft interactions. The primary thruster beam, parameters describing the neutral efflux, and the initial charge-exchange plume are imported from a PlumeTool-generated file. Nascap-2k generates and tracks charge-exchange Ions to obtain plasma densities and calculates potentials consistent with plasma densities and object surfaces. Nascap-2ks PIC capability has been expanded to include boundary injection, particle splitting, and substep charge deposition. The boundary injection replaces collected particles in long running calculations. The particle splitting allows for modeling the effects of the thermal distribution of velocities, as well as accommodating particle weight to variable grid cell volume. The substep charge deposition makes possible calculations in which two effects have significantly different timescales. We use calculations for simple geometries to explore the accuracy and limitations of these capabilities.

Reverse Trajectory Approach to Computing Ionospheric Currents to the Special Sensor Ultraviolet Limb Imager on DMSP

The special sensor ultraviolet limb imager was developed by the U.S. Naval Research Laboratory and is deployed on the Defense Meteorological Satellite Program (DMSP) F16 polar-orbiting spacecraft. The instrument is experiencing a level of noise that is, at times, interfering with its proper operation. The noise is correlated with the spacecraft chassis potential. The potentials about DMSP and the resulting ionospheric current entering the instrument were computed to determine if the noise could be due to this current. In order to obtain results of sufficient accuracy, it is necessary to use a reverse trajectory technique that effectively integrates over the thermal distribution of incident ions. The reverse trajectory technique is described in detail. Once the resistance between the mirror surface and the chassis is taken into account, the current computed using this approach shows the same dependence on the chassis potential as the observed noise, which supports the conjecture that ionospheric ions are responsible for the noise

Photoemission Driven Charging in Tenuous Plasma

In the cold, tenuous plasma commonly encountered in magnetospheric and interplanetary orbits, and in the case of scientific satellites with nearly all surfaces effectively conducting, surface charging is driven by photoemission current. The differential potential of the few insulating surfaces, such as lenses or insulating grout between solar cells, can be positive or negative depending on the photoemissivity of the material. We analyze this effect for spacecraft like those of the Magnetospheric MultiScale and the Radiation Belt Storm Probes missions. This paper develops a simple theory for the potentials of sunlit insulators, focusing on insulators that make up a small part of a large conductive surface. The shape of the photoemission spectrum places an absolute limit of about 12 V of positive differential charging on sunlit insulators. For small insulating surfaces, the conventional assumption-that the photoelectronsthat cannot energetically escape return to their surface of origin-is not valid because the photoelectron trajectory path length is large compared with the surface dimension. We describe a theory that accounts for photoelectron transport between small insulators and the surrounding conductive area. These calculations are done both for the case that the insulator has photoemission similar to the conductive area and for the case that the photoemission is far less, as is the case for many insulators. If the insulator has photoemission current density similar to that of a conductor, we predict positive differential potentials of about 2 V at low chassis potential and negligible differential at high chassis potential. In the opposite case that the insulator photoemission is low, we predict no differential at low chassis potentials and negative differential potentials of up to several volts at high chassis potential.

The Best GEO Daytime Spacecraft Charging Index - Part II

Ferguson and Wimberly, in 2013, reopened the debate on what is the best daytime GEO spacecraft charging index, by concentrating mainly on differential charging, and by performing Nascap-2k simulations of charging under different assumed environments. They concluded that the total thermal electron flux was the best charging index of those formally proposed. In this paper, the authors attempt to verify the conclusions of the previous paper by comparing Nascap-2k results with charging and fluxes measured on the SCATHA, Intelsat, DSCS, and LANL GEO satellites. In addition, because the net total thermal electron flux is dependent on the secondary electron emission from incident electrons below the second crossover point, which can be as high as several keV, a refined measure of charging is presented as the total thermal electron flux above a certain energy that is well above the second crossover point. The use of this type of index will be justified by correlations between Nascap-2k simulation results and total fluxes above a range of energies. This approach is similar to those proposed previously in the interpretation of SCATHA and LANL results, and incorporated into the design of the DSCS satellites. The best minimum energy to use will be determined for spacecraft of different design and surface materials. The use of sensors on spacecraft that will allow measurement of the total flux above the optimal minimum energy will be discussed, along with in-situ spacecraft potential monitors. In addition to aiding the design of flux and charging monitors on succeeding spacecraft generations, the results found here will be shown to point the way toward spacecraft that will not arc even under severe GEO storm conditions. Finally, the optimum GEO daytime spacecraft charging index will be obtained, and its use for predicting and resolving spacecraft anomalies in real time will be discussed.

Simulation of Auroral Charging in the DMSP Environment

The Earth’s aurora provides one of the two regions where spacecraft charging is of greatest concern, along with the geosynchronous orbit. Nascap-2k is designed to model charging in both regions however the specific numerical settings for each are very different. This paper describes the physical models implemented in Nascap-2k and can serve as a guide to using the code for auroral charging simulations. Simple test objects are used to illustrate the numerical options. A full scale simulation of a satellite of interest is also included. The Air Force DMSP (Defense Meteorological Satellite Program) satellites have been flying in the auroral region for almost three decades and provide a wealth of environmental data as well as measurements of the vehicle charging.

Electric Propulsion Interactions Code (EPIC): Recent Enhancements and Goals for Future Capabilities

The Electric Propulsion Interactions Code (EPIC) is the leading interactive computer tool for assessing the effects of electric thruster plumes on spacecraft subsystems. EPIC, developed by SAIC under the sponsorship of the Space Environments and Effects (SEE) Program at the NASA Marshall Space Flight Center, has three primary modules. One is PlumeTool, which calculates plumes of electrostatic thrusters and Hall-effect thrusters by modeling the primary ion beam as well as elastic scattering and charge-exchange of beam ions with thruster-generated neutrals. ObjectToolkit is a 3-D object definition and spacecraft surface modeling tool developed for use with several SEE Program codes. The main EPIC interface integrates the thruster plume into the 3-D geometry of the spacecraft and calculates interactions and effects of the plume with the spacecraft. Effects modeled include erosion of surfaces due to sputtering, re-deposition of sputtered materials, surface heating, torque on the spacecraft, and changes in surface properties due to erosion and deposition. In support of Prometheus I (JIMO), a number of new capabilities and enhancements were made to existing EPIC models. Enhancements to EPIC include adding the ability to scale and view individual plume components, to import a neutral plume associated with a thruster (to model a grid erosion plume, for example), and to calculate the plume from new initial beam conditions. Unfortunately, changes in program direction have left a number of desired enhancements undone. Variable gridding over a surface and resputtering of deposited materials, including multiple bounces and sticking coefficients, would significantly enhance the erosion/deposition model. Other modifications such as improving the heating model and the PlumeTool neutral plume model, enabling time dependent surface interactions, and including EM1 and optical effects would enable EPIC to better serve the aerospace engineer and electric propulsion systems integrator. We review EPIC S overall capabilities and recent modifications, and discuss directions for future enhancements.