Steam is a gas, meaning the thrust is a function of the pressure. Plasma is made of ions, which can be significantly accelerated by electromagnets, meaning the

Steam is a gas, meaning the thrust is a function of the pressure. Plasma is made of ions, which can be significantly accelerated by electromagnets, meaning the thrust depends on the acceleration caused by those magnets, and can be a lot higher than steam. (Technically, the thrust generated is more complicated than just pressure, heavily affected by temperature too, but I digress)