Here, I show the electron distribution in the x-px space. The results are from 1d PIC simulation.
The interesting thing here is that the magnetic field counter-intuitively boosts the electron expansion rather than confines the electron motion. The tendency can be found from the case of B=0006T to that of B=0020T. When the external magnetic field is high enough like 200 T, the confinement effect is dominant. The explanation for this counterintuitive phenomenon needs detailed particle tracking, which will be performed in the next step.