Number density calculation

[Aratopn]

Greetings!

I'm new to WarpX. As far as I understand, there is no direct way to get number density as output. I came up with some ideas:
For charged particles: I can use "rho_electrons" and "rho_ions" divided by charge to get number density for these species. If I don't miss anything, these values are averaged on the cell centers since rho fields are averaged. But this method can't work with neutral particles.
For all particle species: I can use particles' position to calculate number of macroparticles in each cell. Knowing the cell volume and the macroparticle weight, I can also determine number density even for neutrals. But if particle is located on the mesh node I need somehow take into account its effect on the number density for 4 adjacent cells. I think that the macroparticle's contribution to the number density in each cell can be considered based on its position relative to the centers of these cells, hence the closer particle is to the cell center, the greater its contribution to the number density in this cell.

I want to know if these options are adequate. Maybe I'm missing some simpler way.

[JustinRayAngus]

You can add custom diagnostics to achieve this.

diag.particle_fields_to_plot = num ene
diag.particle_fields.num.do_average = 0
diag.particle_fields.ene.do_average = 0
diag.particle_fields.num(x,y,z,ux,uy,uz) = "1."
diag.particle_fields.ene(x,y,z,ux,uy,uz) = "u2=ux*ux+uy*uy+uz*uz;gamma=sqrt(1.+u2);1./(1.+gamma)*u2*clight*clight"

Then you will have the variables num and ene for each species in your output files. The density is num / cell volume. Similarly, the species energy density is species mass * ene / cell volume.

[Aratopn]

Thank you for your reply.
There are still a few questions:

• In this case "num" is the number of macroparticles or real particles?
• Which cell does the particle belong to if it is located at the cell edge or the cell node?

[dpgrote]

num will be the number of real particles.
If a particle is on the lower edge of a cell, it is considered in that cell.