OK, so it’s partly that the plasmoid itself is considerably more massive than the electron stream, but are you also saying that there is some residual magnetic field that the plasmoid pushes against, ie some magnetic lines of flux still attached to the electrodes?
I’m still confused though; the charge of the electron beam has to balance the charge in the ion beam. I had imagined the electrons firing back through a hole in centre electrode to provide a negative charge path back to the outer electrodes. Rather, is the goal here to hold the electrons close to stationary trapped in the plasmoid so that the alpha particles go zooming off even faster towards the “decellerator coil”.
