The helium nuclei have no electrons, and thus carry a net charge. That charge is what the field grabs and propels out along the axis.

The electrons have a net charge as well, one that is opposite the He ions, and thus are also grabbed and propelled… but in the opposite direction.

But unlike the ions the electrons don’t get far, instead they are absorbed by the plasmoid that generated the beams, heating it further and causing more fusion events.

Added note: the forces are opposite but due to the setup the resulting effect is unbalanced… and that’s why NASA originally funded DPFs: they wanted a fusion space drive.