[Archive] Warning: OML approxiamation

Message by Nuno Loureiro:
I am running a simulation with a PicVolDistrib of ions and a GlobalMaxwellBoltzmannVolDistrib of elec1 for the environment. I am getting this warning message:
WARNING: GlobalMaxwellBoltzmannVolDistrib elec1 may be inaccurate due to a locally positive potential, of maximum 257.83798V (to be compared to temperature 1.0eV)
=> at positive potential locations, current and density are proportional to (1+e.pot/k.Te), which is an OML approximation for current and simply wrong for density
histogram of potential: number per interval between min = -20.7 and max = 257.83798 (unit V): 1887.0, 12.0, 5.0, 1.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0
PICVolDistrib source1: 821882 particles, last numerical time step used = 2.0E-8s (max allowed = 1.0E-7s), numerical speed up = 1.0
Last local volume potential change: 0.2119155V maximum change, and 5.604178E-4V average change (last plasma time step = 2.0E-8s)
Did anyone get this kinda of warning?
I understand that the maximum potential is very big, since e.pot should be of the order of kTe, but I am not sure what can be causing this.
I have tried to increase the electron density and that actually works, but in that case I don’t get the results I expected, since the environment density is very high. Is there another way to fix it?
Thank you,

Message by abul anuar:
I think that happens because it is quite unusual to have a positive surface when you consider that the surface is more likely to be negatively charge in maxwellian plasma. OML theory ignored the potential barrier, and in your case since the surface is highly positive, electrons are all attracted to the surface,i.e. maxwellian distribution is no longer true in the vicinity of the spacecraft surface. why don’t you try using other distribution function (like PICVolDistrib) because this distribution is kinetically modelled, unlike maxwellian which using fluid model. i might be wrong but you can always refer to books by Chen,1965 on OML theory or just find a paper by Whipple,1981.
hope this will help.