Table of Contents ECSS Model Page
Background Information Spacecraft charging
EQUIPOT

EQUIPOT makes a rapid assessment of the likelihood of charging for surface materials on a spacecraft. It utilises a simple geometry (a small isolated patch of material on a spherical spacecraft) to calculate the various components of current to both the body (structure) and the patch, and estimates the equilibrium potentials which will develop in order to achieve zero net current.

Two sets of input parameters are required for EQUIPOT, to define the spacecraft environment and the spacecraft materials, respectively.

When the input form has been completed, pressing the button will start the calculation and bring up the "Results" page.

Model parameters

Spacecraft environment

The spacecraft in sunlight or in eclipse. For a sunlit patch, the sun angle on the isolated patch has to be specified. The selection between isotropic and normal incident particle distributions causes different secondary electron emission functions to be used.

The user can choose between a low-altitude (LEO) environment or a high-altitude (GEO) environment. For both altitude regimes, the charged particle environment has to be specified. For low altitudes, the ram/wake configuration has to specified as well. When ram or wake effects are taken into account, the spacecraft altitude (from which the velocity is calculated) and the angle of attack on the isolated patch need to be entered.

In order to characterize the plasma environment, a number of pre-defined environments can be chosen (the environment parameters are listed in the report file), or the environment parameters can be specified by explicitly defining thermal spectra and energy spectra (individual spectra are activated by checking the corresponding check boxes). Three thermal spectra can be defined for electrons and for ions by entering a density and temperature, and a mean mass for ions. The energy spectra are entered in a two column format in the corresponding text areas, in order of increasing energy. If a user defined environment is selected, at least one thermal spectrum for electrons and ions each should be specified.

Material parameters

The materials for the structure and for the isolated patch are selected using the respective menus. The structure and patch materials are defined by a set of parameters that correspond to those used in the NASCAP (NASA Charging Analyzer Program) software. The default values are from the NASCAP data base and from measurements performed for ESA and provided by the ESA SPINE. It is important to note that the charging analysis is very sensitive to the material parameters, especially the electron induced secondary electron emission yield parameters.

The first three parameters (relative permittivity, thickness and conductivity) only apply to non-conductive materials and will thus not appear for conductive materials. The parameters defining the electron induced secondary electron emission depend on the SEE formula that is selected. The menu for the structure material includes an option to fix the structure potential. In this case, no material parameters are required.

If a conductive patch material was selected, the properties of an intermediate insulator (thickness, conductivity, and relative permittivity) have to be specified in an additional table that does not appear when the patch material is an insulator.

Results

EQUIPOT produces the files listed in the table below. A description of the format of the files can be brought up by clicking on their description in the table.

The report file spenvis_epp.html is a printable file containing the input parameters and summary tables. The spenvis_epo.txt file is used by the plotting routines to produce graphical output.

List of output files generated by EQUIPOT
File name Description
spenvis_epp.html Report file
spenvis_epo.txt Emission yields, currents and fluxes

To generate plots, click the appropriate check boxes, select the plot options and graphics format, and click the button. The current page will be updated with the newly generated plot files.


Last update: Mon, 12 Mar 2018