a propulsion system for spacecraft is based on an electric engine that expels propellant to achieve thrust . the propellant is first ionized to generate a plasma . plasma particles are selectively accelerated via a pulsed laser that accelerates predominantly the electrons in the plasma . the electrons are expelled first , forming a space charge that acts as a virtual cathode to accelerate the positive ions . interactions between the laser beam and plasma electrons are predominantly through the ponderomotive force .