How to model charging in GEO

 

 

Rationale

 

Geosynchronous Earth Orbit (GEO) has a specific environment. The behaviour of spacecraft in non quiet GEO environment (sub-storm) is of particular interest because it can lead to severe charging. This "HowTo" page summarises how to tune SPIS parameters so as to model this situation.

 

Non quiet GEO environment is characterised by the presence of relatively high energy electrons (10s of keV as far as surface charging is concerned) and the absence of significant low energy plasma (contrarily to the plasmasphere environment). The total plasma density is small (typically a few #/cm3). The physical consequences are:

-         a large Debye length, typically similar to or larger than spacecraft size

-         currents collected from the plasma are usually smaller than the photoemission current: typically less than 1 nA/cm2 versus a few nA/cm2

The next consequences for charging are:

-         the importance of photoemission (and secondary emission) and of its local variations results in differential charging (more positive where sunlit)

-         the large Debye length makes long distance electrostatic influence effective

-         this results in the well known situation of potential barrier in GEO (except for conductive spacecraft) depicted in the figures.

 

Figure 1 – Typical situation on a GEO spacecraft: inverted potential gradient (-2000V in sunlight and -3000V in shade on this example) and creation of a potential barrier blocking part of the photoemission.

Figure 2 – Example of SPIS simulation of charging in GEO: local saddle points are visible (the main difference wih the previous chart is that in reality the potential is not uniform on the sunlit side here)

 

This has mostly to do with steady state charging. Concerning time-dependent modelling the consequences are the following:

-         Spacecraft capacitance is small while coating capacitance is large

-         Absolute charging is thus fast while relative charging is slow

Using the new implicit solver of SPIS v4 is thus needed to allow a large time step (automatic in SPIS v4).

 

Another purely numerical consequence from the large Debye length is the following:

-         Modelling a population (ions or electrons) by a regular PIC method yields poor current statistics on spacecraft surfaces: too few of the particles injected at boundary of the large computation box hit the small spacecraft in it.

-         This statistics can be improved by using back tracking, i. e. by initiating particle trajectories on the spacecraft and inverting time.

 

 

Practical parameterization of SPIS

 

The major practical consequences for SPIS parameter tuning are the following:

-         Back tracking must be used for particles for which the user wants to have accurate SC currents (often ambient electrons and ions), cf. Controlling NUM from UI. The choice of composite distributions (BacktrackingBoltzmannCompositeVolDistrib and BacktrackingPICCompositeVolDistrib) allows an accurate computation of currents on spacecraft and the computation of volume density by direct PIC or Boltzmann (which is not performed by the simple BackTrackingVolDistrib, i.e. which neglects particle space charge as in vacuum).

-         The specific handling of the potential barrier must be turned on through the "barrierCSFlag" global parameter (barrier current scaler).

 

Other parameters of lesser importance can be found on the two examples of GEO charging supplied with the release.