[tgflynn]

Yeah, it seems like it might make more sense if they could figure out a way to use the RF fields to actually accelerate some of the ions in the plasma. Then you might be able to reach much higher exhaust velocities than with a conventional jet engine. I don't think your thermodynamic objections would apply to that case since the energy would remain organized, it wouldn't just be heat.

To do that though you'd need to find a way to maintain overall electrical neutrality.

[jcims]

>Yeah, it seems like it might make more sense if they could figure out a way to use the RF fields to actually accelerate some of the ions in the plasma.

That's essentially what they are doing, but in an oscillating path rather than a continuous path:

In the waveguide, the charged particles in the plasma start to oscillate with the microwave field (aka: RF) while rapidly heating. The ions, atoms, and electrons collide with each other frequently, spreading the energy from the ions and electrons to the neutral atoms, heating the plasma rapidly. As a result, the researchers claim that the plasma rapidly heats to well over 1,000°C.

I don't know if you can rectify RF energy like you can with voltage. That might be one way to create a more continous path. They might also be able to add an electromagnetic field to create a net velocity out the provebial barn door as they do with ion thrusters, but I think the problem there is you just can't get enough free air path to accelerate to very high speeds...it'd be like trying to drive an F1 car at full throttle in bumper to bumper traffic. You could also pass a current through the plasma to generate lorentz forces like they do in plasma driver rail guns.

One huge advantage with this design is no moving parts and presumably extremely low manufacturing costs. So it might not be awesome for commercial airliners but it could be useful for high endurance UAVs. Pack a few dozen of them and use just the ones you need.

[tgflynn]

> I don't know if you can rectify RF energy like you can with voltage.

You can in essence, because charged particles have inertia, they don't instantly follow the EM field. That's how RF particle accelerators work. But there you're working with a beam of charged particles, not a neutral plasma.

It still sounds to me that they are essentially using the RF energy just to heat the plasma and in that case the parent's objections seem valid, though I don't know much about jet engine design.

[mannykannot]

A few months ago, there was a demonstration of a lightweight model airplane propelled by the air currents produced by a corona discharge [1], and ion rocket motors have great specific impulse, but they are both low-thrust devices. I believe you are right, that these are not heat engines, any more than an electric motor is: in all these cases, the moving part is directly accelerated in the direction we want it to go.

[1] https://cosmosmagazine.com/technology/researchers-successful...