@EmilioPisanty that's a really interesting note. I had no idea it was purely classical. Thanks!

@dmckee It's that bad? Oh well. I'm curious about it in practice, but if it's only possible with unreasonable simplification, that's worth knowing too.

thanks for the link! Perfect for my needs. If I have correctly understood the equation on page 12, I need 283.6µT on the surface of the bubble. Which means a 10m radius loop carrying 2e9 A. Given the recent superconductor critical current record of 1e6 A/cm^2, I can see that this can't work in our atmosphere.

Ah. The heat source could be on the ground, I didn't mean to imply the magnetic field source could be too. I was imagining a device physically the same as an M2P2 spaceship, only switched on in Earth's atmosphere, kept hot by beamed power instead of sunlight, and being propelled by buoyancy rather than solar wind. My written communication seriously needs to be improved.

Thanks for your answer! Based on what I have seen of the mini-magnetospheric plasma propulsion concept, there seems to be a two-way force between a magnetic field and a plasma within that field. I do not know how strong that coupling is. For temperature, I am assuming something in the order of 10,000 K. The overall pressure would, naturally, match the surrounding air — starting at 1.2kPa for 30km, and decreasing to 32 mPa at 100km. The volume would have to be in the order of 1.62 cubic kilometres at 100km for a 1 ton payload.

@Jim Thanks! That's the kind of answer I was looking for :)

@Jim Space is not a perfect vacuum; the air pressure at the Kármán line is 32 mPa, and the the container is a magnetic field. Some of the work being done for mini-magnetospheric plasma propulsion (the same setup, but in interplanetary space) suggests 15-30km radius plasma filled bubbles from a very small source.