[pfdietz]

Sure, but one still has a savings if one goes to heavier noble gas elements over the lighter ones. The noble gases are used because they don't contaminate spacecraft surfaces.

About the only place I can think of in a plasma engine where you'd want to use light elements is if the engine is thermal: making a confined plasma, heating it to very high temperature, then expanding it through a magnetic nozzle. There, you'd want to use hydrogen to minimize radiative losses from the hot plasma, especially vs. using a plasma containing partially ionized atoms of higher atomic number elements; these can radiate fiercely.