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It's so near the noise threshold. Most of the energy goes into heating the thing; only a tiny fraction comes out as possible thrust. The heating prevents using enough power to get more possible thrust; the thing would burn out.

Very frustrating.

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If we had any idea how they work (if they work) we could work on optimised designs that generate more thrust and are thus easier to study. But we're not yet anywhere near the stage and instead have to study a design that if it works, is so inefficient that we can hardly tell whether it does anything. Very frustrating indeed

Yet it's happened before in engineering history. Edison discovered the "Edison effect" - he put an extra plate in an incandescent lamp, and discovered that some current, not much, would flow to it through vacuum. This, in 1875, was the beginning of the vacuum tube. But lacking the theory to understand it, it couldn't be improved. Early on, it was thought that there had to be some air, or something, in the tube for it to work. But then it was discovered that hard vacuum worked better. This confused electrical inventors of the era - how could electricity conduct through a vacuum? It wasn't until 1904 that tubes started to be figured out.

Tube filaments are quite different than lamp filaments - tubes need a material where electrons are easily emitted from the surface. Tungsten is good for lamps but terrible for tubes. A little thorium oxide or barium oxide helps a lot. But there was no theory to indicate in what direction to go, and it took 30 years before trial and error produced results.

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