It would require a titanic number of super-cooled magnets. And a non-trivial number of extremely expensive vacuum pumps. A chamber that big is going to leak and outgas at an absurd rate; it would be really tough to achieve a hard enough vacuum.

I used to work on vacuum chambers at one job I had. Our chamber was a hexagon about a meter across, used for semiconductor manufacturing, and it took something like 20 hours for it to pump down to a vacuum low enough -- and we didn't need anything like the kind of vacuum that fusion would require. The cryopump we used cost something like $50K. For a chamber a mile long to do fusion, you'd need hundreds of those, maybe thousands.

Our target was usually 10^-7 torr. But if we could do better, our customers were happy.

It was kind of neat watching a chamber pump down. A diaphragm pump would get it down to about 3 torr, and then we'd open the hatch on the cryopump, and the pressure would drop about 4 orders of magnitude in about 30 seconds.

(If you're not familiar with the unit, normal atmospheric pressure is about 750 torr, depending on the weather and your altitude.)

Not liquify, freeze. The pump head of the cryo is cooled to about 20 degrees kelvin using liquid helium. At the kind of pressures where you use a cryo, the gas particles bounce around the chamber like billiard balls, and when they strike the cryo they stick.

Over time ice builds up on the cryo, so about every two weeks you have to "regenerate" it, which means closing the slot valve, warming it up to room temperature, passing warm dry nitrogen over it to melt off the ice, cooling it back down to 20K, and then opening the slot valve again.

It's a sin to "dump a cryo", which means you accidentally open it to normal atmosphere. It instantly coats thickly with ice, and then you have to regenerate it. (It doesn't harm the cryo, but regenerating a cryo takes several hours.) Someone who dumps a cryo generally takes a bit of ribbing.

The vacuums we achieved in our chambers was far harder than anything available in space unless you went out to the orbit of like Neptune...

Pete, I don't believe that, because outgassing would prevent it. (I believe that they're making the claim, but I don't believe they'll succeed in reaching -14.)

Outgassing is a real problem. Our chambers were treated with a secret process which drastically reduced the amount of outgassing, but we still got some and that's why we had to struggle to even reach -7.

I don't think it's possible to eliminate outgassing entirely. (If someone ever figures out a way to do it, they'll get rich.)