Focusing force gradient artifact near axis

[n01r]

Hi, I wanted to report an artifact I found when I analyzed an LWFA simulation.
It first became visible in the gradient of the transverse focusing force inside the plasma bubble at the position of the accelerated particle beam.

Image of the normalized focusing force $$\frac{1}{E_0}\frac{\mathrm{d}(E_x - c B_y)}{\mathrm{d}r}$$

Zoom-in:

Here is a Python file (as TXT) that reproduces the run.
run_wake-t.py.txt

It changes with the numerical resolution in $r$. See here an average over all slices at all beam positions.

Here with 10x radial resolution

These spikes flatten out when the number of plasma particles is increased. (The original number of particles was 2)
With ppc=4 it is still visible, with ppc=8 and higher it is basically gone.

This artifact can cause numerical emittance growth in the beam.

Already discussed with @AngelFP and @RemiLehe. This issue is for tracking purposes and can be closed with the implementation of either documentation for users or maybe a correction.