Add charge in the Beam classes

[cr-xu]

This is probably an important feature that is overlooked.

Since ParticleBeam tracks macroparticles, it might make sense to add a charge associated with each particle. This could open possibilities for additional physical process modules (collective effect etc.).

ParameterBeam will just have something like a q_total.

[jank324]

I was actually thinking about the same, because it would also open the door to crude space charge estimations. I would prefer more descriptive variable names (e.g. total_charge) though. Or in Bmad they choose the charge based on particle species, which is also interesting.

[cr-xu]

total_charge sounds good. I would still decouple the charge from the particle species because the idea of using macroparticles is that each one of them would represent N real particles.

[jank324]

Good point. If we want to, an option would be to later introduce sort of a macro ... where you can pass the species and it will figure out the per particle charge on its own.

I would actually say, both beam classes get a parameter and argument charge. ParameterBeam just saves this. ParticleBeam if only charge is passed derives num_particles from that, assuming electrons for the time being. The other way round, if you only pass num_particles, it figures out the charge. If you pass both, it just takes them as passed. And then maybe ParticleBeam should have an @property for single_particle_charge (I prefer this to total_charge).

Thoughts?

[cr-xu]

That's not necessarily the case, in a lot of particle tracking codes, one macroparticle actually represents a collection of particles. So it's allowed to have different charge q per macroparticle, even in most of the cases the macroparticles contain the same charge $q = Q\text{total}/N\text{macro}$. The totalcharge (physical quantity) is not related to the number of macroparticles used for tracking ( which is chosen as a tradeoff between computational resources and the accuracy of simulation) The true number of electrons is derived $N\text{e} = Q\text{total}/ q\text{e}$