How to initialize beam from arrays with different macroparticle charges?

[Xuan0533]

Hi,

I am wondering whether IMPACTX can run with beam macroparticles with different charges (or weights). From the instruction manual, there is a section 'Initialize a Beam from Arrays', but I cannot find a way to lauch with different charges.

May I get some help or insights about this? Thank you very much!

Sincerely,
Xuan Zhang

[ax3l]

Hi @Xuan0533,

Thanks for the question! Yes, internally ImpactX does use particle weights for individual particles, which primarily has an effect on collective effects and diagnostics.

The example you mention is a bit rough still and focuses on equal-weight particle beams. The example calls a high-level API, which is implemented here and after partitioning the particles and ensuring we have them well distributed for parallelism (in-node and across compute nodes), we create the weight array in the same function and actually init the particles.
This is the part that we could certainly split up further and expose a more low-level function to Python.

We also recently implemented a new Source element that can load openPMD distributions, but in its first quick implementation we use it also to load equal-weight distributions and will generalize this at a later point, among others, because WarpX particle beams are also often weighted with different weights per particle.

Can you tell us a bit more about your use case and if you like to try an implementation yourself? We will certainly expose this detail more at some point, but it's not on the top of the priorities just yet for our use cases, but we welcome contributions if you need it earlier.

[ax3l]

Hi @Xuan0533,

I added a first implementation for the Python API in #1097 . Only need to add a test case to finalize it.

If you like to try it out, you could compile ImpactX and test it. Otherwise, I plan to ship it with ImpactX version 25.09.

[Xuan0533]

Hi @ax3l,

Thank you very much for implementing this function!

Sorry for the late reply. It took me some time to recover from overnight flight.

I just tried to compile ImpactX with your fork version - "topic-load-weight" with conda forge, and it works pretty well with weighted particles, the results not vary a lot from the case with upsampled equal weight particles. I also tried to compile with development branch, but it seems this function is still not available yet.

I will keep testing to see if there is anything wrong. Thanks again for your help!

Best Regards,
Xuan Zhang

[ax3l]

Awesome, thanks for testing @Xuan0533!

I finalized the PR in #1097 today by adding a test and we will include it in the next release (25.09) :)