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.