Charging Analysis Tools
Simple non-geometrical methods can be used to assess a given material's tendency to charge in beam or plasma environments. An example is the MATCHG (NASCAP/MATCHG) code. This solves the current balance equation to predict surface charging and current terms and is a useful preprocessor and aid to interpretation of charging or material testing results. Charging tests usually include exposing material samples to (approximately) mono-energetic electron beams and measuring surface potentials and currents from the sample. These parameters can be computed readily, provided the material data are available. Such an analysis usefully checks the consistency between materials characterisation and charging tests.
However, 3-dimensional effects are very important in spacecraft charging. These include shadowing which produces photo-emission differences, electric fields around the spacecraft (barrier formation) which affects the currents striking or leaving a surface, and inter-material currents which will affect their potentials. Therefore it is clear that a 3-D analysis is necessary and for this reason the NASCAP code was developed by S-Cubed for NASA.
NASCAP and MATCHG are valuable tools for assessing charging tests and on-orbit behaviour but require a large amount of input data and careful use.
Besides NASCAP and MATCHG, the Spacecraft Plasma Interaction System (SPIS) has been developed by ONERA & Artenum and used in paralel to these programs for plasma analysis.
Plasma Analysis Tools
The POLAR and NASCAP/LEO computer codes were developed for assessing the problems of plasma interaction at low altitude. They have the same 3-dimensional treatment as the original NASCAP code on which they are based. The same material properties are treated but they are applicable to short-Debye-length regimes (i.e. cold dense plasmas). POLAR is for evaluating charging expected in the auroral oval (and v x B effects) while LEO is for evaluating anomalous current collection - snapover.
Computational tools need validating if they are to be used for predicting on-orbit behaviour and chamber simulations need to be correctly interpreted. Therefore the close linking of numerical simulation and experiments on spacecraft-plasma interactions is important.
Last update: 29 March 2007