D.1.1.1. Overview
Charging simulations are used to find the charging state of surface materials, the spacecraft ground and the differential potentials that exist. These simulations are useful only if they are able to realistically represent the main charging currents, primary electrons, primary ions, photo-emission, secondary emission and conduction. Because of this, good material parameters (especially photo-emission and secondary yields) are necessary inputs. For initial assessments, simple 1-d codes are useful but a realistically simulation cannot be made without considering geometrical effects on field distributions and so 3-D codes are used.
There are a number of simulation codes available for performing surface charging assessments taking into account surface material processes and 3D effects. Historically the most common spacecraft charging code in use by the space industry was the NASA-USAF charging analysis code, NASCAP-GEO.
Recently, ESA has sponsored the development of several modelling codes, Equipot, PicUp3D, SPIS which are publicly available for all users and therefore well suited for being used on standard basis. The SPARCS (SPAcecRaft Charging Software) is also suitable.
D.1.1.2. Equipot
A simple charging code, called EQUIPOT
[40] performs computations assuming either thick-sheath or
thin-sheath current collection and is thus applicable to GEO and LEO charging.
It allows to estimate the differential charging expected between a dielectric
patch and the ground structure. It includes a list of material properties. It
can be run over internet on http://www.spenvis.oma.be/
D.1.1.3. PicUp3D
PicUp3D is a fully 3D PIC code that allows the exact computation of the sheath structure and of the current collected by spacecraft surfaces for rather detailed geometries. Surface interactions other than photo-electron emission are not modelled. The code source is freely available from www.spis.org and a mailing list allows a limited amount of support.
SPIS is a fully 3D PIC code that allows the exact computation of the sheath structure and the current collected by spacecraft surfaces for very detailed geometries. A large class of surface interactions including (photo-electron emission, back-scattering, secondary-electron emission and conduction) are modelled. In terms of modelling capabilities it largely supersedes NASCAP-GEO, LEO and POLAR. The code source is freely available from www.spis.org and a mailing list allows a limited amount of support.
D.1.1.5. NASCAP Family of Charging Codes
D.1.1.5.1 NASCAP-GEO
A commonly used code, for simulating surface charging in the outer magnetosphere, is NASCAP (also called NASCAP-GEO [41]). This code calculates the total current, due to all the current contributions, for each surface on a numerically modelled 3-d spacecraft, using a two Maxwellian environment for both ions and electrons. From these currents, the change in potential at each surface is calculated. The current and potential calculations can be performed iteratively until an equilibrium charging state is achieved. The NASCAP-GEO spacecraft geometric definition is quite coarse, consisting mainly of blocks .
D.1.1.5.2 MATCHG
Where spacecraft geometry is unimportant because all that is relevant is the susceptibility of a particular material to charging in a particular environment, then a simple 1-D code, MATCHG [41], based on a subset of NASCAP subroutines, can be used. Current collection in both codes is determined using the `thick sheath' approximation i.e. assuming that the Debye length is long compared to the spacecraft dimensions. This approach is valid at GEO but for charging in low-altitude auroral conditions a different approach is used.
D.1.1.5.3 NASCAP-LEO
NASCAP-LEO is aimed at the simulation of high potential objects with cold dense plasma typical of the LEO environment. Like NASCAP-GEO it employs analytical current collection equations, although these are aimed at sheath-limited current collection that is appropriate for the short Debye-length LEO plasmas. A typical use is the simulation of parasitic currents from high potential surfaces, such as solar array interconnects.
The geometrical model of the spacecraft is more sophisticated than for NASCAP-GEO, consisting of a finite-element representation. However, NASCAP-LEO does not simulate the spacecraft sheath with any great accuracy and hence for problems which involve sheaths and wake effects, e.g. natural charging, it is not well suited.
D.1.1.5.4 NASCAP-2K
The most recent NASCAP code (NASCAP-2K) is
available, free, to US citizens only. This is a comprehensive code with
realistic geometry. It is reported to combine the capabilities of NASCAP-GEO,
NASCAP-LEO and POLAR which are discussed later. However, since the code is not
easily available in
The main problem with computing charging in LEO is computing effects associated with the spacecraft sheath. The 3-D POLAR code [42] has been designed for assessment of sheath and wake effects on Polar orbiting spacecraft. This uses numerical techniques to track ambient ions inwards from the electrostatic sheath surrounding a negatively charged spacecraft, onto the spacecraft surface. Spacecraft velocity is included as an input and ram and wake effects are modelled. One or two Maxwellian components may be used to define the ambient plasma. The electron population in POLAR is a superposition of power-law, Maxwellian, and Gaussian components. Once the surface currents have been found, POLAR calculates potentials and equilibrium charging state in a similar way to NASCAP.
D.1.1.7. Other surface charging codes
There are a number of other codes available to simulate surface charging. Two Russian codes ECO-M [43] and COULOMB [43] perform a very similar function to NASCAP and POLAR and NASCAP-LEO.