If the magnetron is turned off, so is the plasma that heats the fuel. In Micci's prototype the electrical hardware and microwave source, or magnetron, came from a conventional 1000W kitchen microwave oven. The system has been operated using hydrogen, nitrogen and ammonia fuel gases. To date, the research has shown that the device can be successfully operated with a number of propellants and easily create and maintain plasmas from them. The Penn State engineer notes that the thruster can potentially be operated with water as fuel.
Water is heavier than ammonia but lighter than nitrogen.
Micci has achieved his best performance with hydrogen, the lightest gas. However, hydrogen is difficult to store in space. Ammonia was closest in performance to hydrogen. Nitrogen's performance came in third. Micci and his commercial partner, Research Support Instruments Inc., have applied for patents on the prototype device.