NASCAP-2K Spacecraft-Plasma Environment Interactions Modeling: New Capabilities and Verification

Abstract

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-2k's 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-2k's 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.