Because it doesn’t work very well in atmosphere, so far tests seem to indicate that it really wants to be in a vacuum.
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so that’s an extra cost to create a vacuum in which the reaction can work. So once you create the vacuum you add energy and whatever else. If the result is that you get more energy out—then it would seem that this could be an energy source. the question is how expensive is that energy.
This isn’t a power generation system. It’s (if it works as advertised) a method to convert electricity to propulsion. It’s not like a nuclear reactor, it’s more like an electric motor.