> It's impossible to make a magnet in the shape of a ball that squishes on all sides equally...
Ah, my favorite theorem, The Hairy Ball Theorem!
Ah, my favorite theorem, The Hairy Ball Theorem!
To phrase it another way, could you explain why the magnetic field is tangent to the sphere and not normal to it?
The force magnetic fields apply is always perpendicular to the field: F ~ v × B (right hand rule, and all that). So if you want F to point inward, the one component of B that doesn't matter is its radial component---ie. the tangent component is all that counts. But it's not clear to me that you can't compensate for the restrictions of the hairy ball theorem by having a nonuniform velocity distribution. Of course, there's no "hairy torus" theorem (because it's not true!) and this immediately suggests the tokamak design.
It's also not possible to take a sphere and have the radial magnetic field point inward everywhere (or outward everywhere), because Maxwell's equations prohibit magnetic monopoles.
And of course, the last point about no magnetic monopoles should be sufficient to make a spherical field impossible.