### poissonBCType 2

poissonBCType 2
Hello to the SPIS community,

I have an issue with boundary conditions on SPIS which I can't solve alone… so your support could be very helpful, I post it here in case someone has a idea.

I am currently running a simulation with electron temperature much higher than ion temperature and ion kinetic energy (10eV for electron temperature using Boltzmann model, 2eV for PIC phoelectron temperature versus 0.026 eV for PIC ion temperature and 0.035eV for ion kinetic drift energy). This means that ion trajectories are disturbed by the sheath at a much larger distance than electrons. Thus:

• I need a box much larger than 10 debye lengths-distant (debye length = 0.74m, box distance from spacecraft = 25 m)
• I need to use a boundary condition where potential is not null and ions are already accelerated when injected.
This is the reason why I am using the type 2 boundary condition (poissonBCType = 2), which I was told suits better this case. However, in the simulation I am running, I get potential on the boundary that are very negative (up to -14V when spacecraft potentiel is -63V) which does not look like a physical phenomenon to me. I did not find documentation on that option so I can't understand why it occurs. Do you know for that specific model of boundary conditions:
• How the number of PIC particles injected at each time step is computed taking into account the external boundary potential? (I was told it uses Liounville's theorem, but how does SPIS implement that exactly?)
• How does SPIS compute the potential on external boundary? What does "mimicking a 1/r² decay" means?
• How does SPIS compute the velocity distribution of PIC ions at injection? Is it normal to the surface with its energy corresponding given by external surface potential?
Thank you very much for your help.

Guillaume

posted by Guillaume Tcherniatinsky at Mar 27, 2019 2:17 PM