The fictional and non-fictional science of lightsabers. Here’s more from the Science Museum:

Confining a plasma into a cylinder-like shape is not straightforward due to what is called the ‘hairy ball theorem’, says [Federico Felici of the EPFL, the Swiss Federal Institute of Technology in Lausanne]: imagine a ball covered in long hairs (which are equivalent to the magnetic field lines). If one were to try to comb those hairs down, there will always be at least one spot where the hairs will not lie flat. ‘One of the hairs will always stand up and there is no way to confine the plasma.’

To wrap the plasma in magnetic fields effectively, the answer is to use the same approach as in nuclear fusion and harness the toroidal shape of a tokamak. Felici believes the plasma in a lightsaber may look cylindrical to the naked eye but in fact consists of an elongated doughnut shape, with a narrow cavity at its core. “From a topological point of view, this would be the same as a tokamak and overcome the hairy ball problem,” he says.

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