looks like VASIMR but sounds different
Photons have a rest mass of 0; microwave photons -- doubly so.
But the idea to use microwaves in a resonating cavity was thought up by Franklin Chang-Diaz and used, not to spill those microwaves out of the open end, but to excite, accelerate and spew argon ions. These are cheap (much cheaper than Krypton and Xenon) and much heavier than light particles.
The difference between VASIMR and other ion engines is, I thought, that the other engines need some kind of electrified grid for the acceleration, so that grid gets degraded as the ions bump into it; it gets sandblasted away. VASIMR should be able to perform much longer without eating up the engine.
Years ago, NASA was supposed to launch and test Ad Astra's VASIMR on the ISS, but I haven't heard anything about it ever again.
Biting the hand that feeds IT
