large laser centroid position "removes" particles about fbpic HOT 3 CLOSED

Thanks for the question.

The laser centroid z0 corresponds to the position of the laser centroid at t=0. (Note that this example uses the default direct laser injection, whereby the laser is initialized entirely in the box at t=0 - as opposed to being emitted from one of the boundaries of the simulation as is often the case in other PIC codes. More details here: https://fbpic.github.io/api_reference/lpa_utilities/laser.html)

When using z0 = 38e-6, the laser centroid is outside of the simulation box (which goes from -10e-6 to +30e-6 here) by a good margin (8e-6). In this case, the laser intensity in the box corresponds to the wings of the Gaussian intensity distribution and is probably very weak (and is clipped by the righthand side of the box).

Regarding the particles in the diagnostics: in this particular example, only particles with uz higher than 1. (i.e. energetic electrons) are written in the diagnostics. In the case z0 = 38e-6, I am guessing that the laser intensity too weak for any of the electrons to reach this kind of energy. Does that seem to be the case here?

from fbpic.

Btw, here is a nice image that @MKirchen made, in order to illustrate the distinction between z0 (centroid position) and zf (focal position):

from fbpic.

Thank you @RemiLehe for providing an explanation of what happens.

I see, that I should have set zf and not z0 for the focal plane position.

from fbpic.