The magnetic field here is acting as a nozzle. The circuit driving the field is not doing work on the ions, and any power loss is just parasitic. It's generally

The magnetic field here is acting as a nozzle. The circuit driving the field is not doing work on the ions, and any power loss is just parasitic. It's generally not worth it for small thrusters, because the extra efficiency gained by having a magnetic nozzle is offset by the parasitic mass and power cost (here they eliminate the power cost).