If the article is right about the scaling required then I'm very skeptical that this will ever be practical. A 1 kW magnetron is a part found in just about every microwave oven. Scaling that up by 4 orders of magnitude would need a 10 MW RF source or amplifier. I'm pretty sure those don't exist and if they did they'd be very large and heavy. A long time ago I used to do RF engineering for particle accelerators and the most powerful continuous wave RF amplifiers I ever heard about were on the order of 1 MW.
An even bigger problem would be efficiency, which this article doesn't even mention (haven't looked at the original paper). High power RF amplifiers aren't particularly efficient, I would guess around 30%, and there would also be waveguide losses and cavity losses if any resonant effects are used to get high enough electric fields. I would be surprised if there's much hope of that competing with conventional jet engines on efficiency.
> I'm pretty sure those don't exist and if they did they'd be very large and heavy
I'm clueless on these things, but what order of magnitude for "heavy" are we talking about here? If we take a 1Kg microwave and add 4 orders of magnitude we're at 10,000Kg and a 747 weighs in at ~200,000Kg, so by that measures it seems achievable. I'd also assume existing terrestrial ones aren't optimized for weight in any way.
> I would be surprised if there's much hope of that competing with conventional jet engines on efficiency.
Efficiency isn't the only measure, there if energy can be cheaper than fuel then less efficient can win out. There may also be applications for long running, low weight flight powered by solar like starlink.
> Scaling that up by 4 orders of magnitude would need a 10 MW RF source or amplifier. I'm pretty sure those don't exist and if they did they'd be very large and heavy.
Probably a dumb question, but it's it just a matter of more power and cooling for the magnetron? I'm thinking the size of the magnetron is determined by the wavelengths you're trying to produce. (I don't mean to diminish the challenge of applying this tech, a 10 MW power source would still be quite large for an airplane.)
Yes, I think so. If the frequency is high enough so that it's not the limiting factor the size would mainly depend on how much power you can dissipate in a given volume.
The highest power RF amplifiers I've ever seen were the klystrons used for the LEP and ESRF accelerating cavities. Those were large in part because of the relatively low frequency of 350 MHz but I believe there was also a large water-cooled absorber that dissipated whatever power was left in the MW+ electron beam given the conversion efficiency.
I'm sure it will be a long road to practicality, but that it ionizes all of the air and drives it out is fascinating. Of course it also then requires strong electric drivers since it's basically driving the engine with the magnetron.
I can kind of imagine improved transistors and other technology making scaling the magnetron easier, hopefully all that work on fusion containment has helped us understand plasma better here too!
That comparison wouldn't be as bad you might think.
The first functional Diesel engine for example had about 13kW of power and was only about 5x larger than a modern car engine.
This was in 1896. From 1923 onward, the engine type was small and powerful enough to be used in lorries (trucks).
The first petrol engines compare even better to modern engines (in terms of power-to-weight ratio).
When you say "continuous", would you consider some very high frequency solid state switching amplifier to be "continuous" enough for this application? I realize it's a different order of magnitude, but those GaN/Si transformers make me wonder if we aren't far off from some kind of megawatt scale solid state amplifier shakeup...
It's conceivable. I've seen solid state RF amplifier modules successfully combined into some quite high power units and that was over 20 years ago. I haven't followed the technology since so I'm not up to date on that at all. In any case I think a 10 MW amplifier and associated power supplies and cooling equipment would need to be large and heavy.
A single jet engine generates about 20 MW of power and I'd be very surprised if you could pack 10 MW of RF amplifier into that kind of size and weight envelope.
Makes sense. Maybe we'll see a fusion plasma thruster before the power/energy storage shrunk enough to make this viable as is (in the atmosphere and gravity of Earth at least).
Most high power microwave devices aren't build on semiconductors like Silicon (solid state), yet. They are normally valves of some sort, like magnetrons, klystrons or travelling wave tubes.
Some radars would be in the mega-watt range, but only pulsed with low duty cycles or 1%.
It sounds like the technology hasn't evolved much in the last 20-30 years then.
Around 1995 I was involved with a project where an engineer designed and built a multi-kW CW (I don't recall the exact value) pre-amp out of transistor based modules. There was a plan to apply the same approach to a later project at much higher power levels (100's of kW's) where the conventional approach would have been to use a klystron. I left that team (and the field) though so I don't know if the larger version was ever successfully implemented.
EDIT: I should note that this wasn't in the microwave regime, it was 500 MHz.
An even bigger problem would be efficiency, which this article doesn't even mention (haven't looked at the original paper). High power RF amplifiers aren't particularly efficient, I would guess around 30%, and there would also be waveguide losses and cavity losses if any resonant effects are used to get high enough electric fields. I would be surprised if there's much hope of that competing with conventional jet engines on efficiency.