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Not everyone is making a donut-shaped reactor. There's an entire field of fusion research on what's called inertial confinement fusion, where a pellet of fuseable material is held in place and hit with high-powered laser pulses. I think what you're talking about is magnetic confinement fusion. I had to go to the wiki to look up why donuts (known as toroids in geometry) are used. Essentially, they use magnetic fields to confine a moving plasma. An efficient way to do this is to build a solenoid, which is just a wire coiled up like a spring, with current flowing through it. A plasma inside the coil won't be able to escape out the sides, but might escape out the ends. A way to prevent this is to bend the coil so it's ends connect, forming a donut, which is why that's commonly used.In other words, the magnetic field produced by that donut shape is the right kind to keep a plasma trapped within the body of the donut.http://en.wikipedia.org/wiki/Magnetic_confinement_fusion#Compact_toroids
One of the major problems with a sphere is that a magnetic monopole is inherently unstable.
QuoteOne of the major problems with a sphere is that a magnetic monopole is inherently unstable.I thought magnetic monopoles were only hypothetical at present
I'm not quite sure about magnets and ions. One note suggested that positive ions migrate towards the magnetic North Pole, and thus would be repelled by the magnetic South pole.
Magnetic monopoles are way sci-fi-ish right now, but I hope someone invents/discovers it in near future.
In a fusion reactor the material undergoing fusion must be contained. There is nothing by way of a normal container that could work -- any material known vapourises at much lower temperatures. But a plasma of ions can be contained by magnetic fields.