What I mean is an engine whose thrust is on the order of tens to hundreds of Newtons with an ISP "well" above that of chemical engines. I don't require this engine to be space ready - if it has to plug into a mega watt generator and only works on the ground at the moment that's fine - but it must be buildable with current technology. That means that fusion power or antimatter is out. I'm ok with an engine that has been proposed but not built, or an engine that is low thrust but only because it has never been scaled up.
I do require the engine to at least have been peer reviewed and simulated in a high fidelity simulation.
So, doors such an engine concept exist?