How to set boundary conditions for non-rectangular shapes?

[AGreatDayForReal]

Hi, I want to set up a simulation of a Hall thruster. I found an article that shows the computational domain below.

I’m a bit confused about the dielectric region. In the article, it looks like it’s treated as a wall. How can I define a boundary with this shape in WarpX and assign the proper boundary conditions? The simulation results are shown below; the dielectric region is not included in the computational domain.

Is using EmbeddedBoundary and build a stl file the only way to do this?
Thanks!

[roelof-groenewald]

Hi @AGreatDayForReal! Using the embedded boundaries would be the way to go yes. You wouldn't need to build an STL file (although that would work too), you can specify the regions to cover with an "parsable expression" - something like:

warpx.eb_implicit_function = if(z<2.5,if(x>3.5,if(x>5,0,1),0),1)

[the notation above is if(condition, when true, when false)]

The boundary condition gets a little trickier. WarpX does not have built in dielectric support, but you can specify the potential on the surface of embedded boundaries as a function of space, so you could track the charging at each cell on the EB surface and assign the corresponding potential to that location dynamically (this would have to be done via Python callbacks, similar to what is done in this example - https://github.com/BLAST-WarpX/warpx/tree/development/Examples/Physics_applications/spacecraft_charging).

Hopefully that example will help to get the ball rolling but please let us know if you have any further questions.

[AGreatDayForReal]

Thanks for the reply! I have now set up the area.
By the way, how can I set boundary conditions separately?
I would like the boundaries at x = 5 and x = 6.5 to have different conditions, such as absorbing and reflecting. However, when I use Cartesian2DGrid, it seems that only lower and upper boundary options are available.
Sorry for the interruption

[roelof-groenewald]

Glad to hear you have the geometry set up. The embedded boundaries natively only support absorbing boundaries, but using the callbacks and the particle boundary buffer, you can implement particle reflection (with your own reflection model). See for example https://github.com/BLAST-WarpX/warpx/blob/development/Examples/Tests/particle_boundary_interaction/inputs_test_rz_particle_boundary_interaction_picmi.py.

[Joseph-Thaggard]

Keep an eye on the WarpX-Hall repository being built by the University of Michigan. Their setup is axial-azimuthal (not the cross-section) but they're moving to 3D in the future. Repo: https://github.com/archermarx/warpx-hall.