Editing Stellarator (section)

=== Magnetic confinement ===
A simple confinement system can be made by placing a tube inside the open core of a [[solenoid]]. The tube can be evacuated and then filled with the requisite gas and heated until it becomes a plasma. The plasma naturally wants to expand outwards to the walls of the tube, as well as move along it, towards the ends. The solenoid creates magnetic field lines running down the center of the tube, and the plasma particles orbit these lines, preventing their motion towards the sides. Unfortunately, this arrangement would not confine the plasma along the ''length'' of the tube, and the plasma would be free to flow out the ends.{{sfn|Spitzer|1958}}

The obvious solution to this problem is to bend the tube around into a [[torus]] (a ring or donut) shape.{{sfn|Spitzer|1958}} Motion towards the sides remains constrained as before, and while the particles remain free to move along the lines, in this case, they will simply circulate around the long axis of the tube. But, as Fermi pointed out,{{efn|[[Andrei Sakharov]] also came to the same conclusion as Fermi as early as 1950, but his paper on the topic was not known in the west until 1958.{{sfn|Furth|1981|p=275}}}} when the solenoid is bent into a ring, the electrical windings would be closer together on the inside than the outside. This would lead to an uneven field across the tube, and the fuel will slowly drift out of the center. Since the electrons and ions would drift in opposite directions, this would lead to a charge separation and electrostatic forces that would eventually overwhelm the magnetic force. Some additional force needs to counteract this drift, providing long-term ''confinement''.{{sfn|Bromberg|1982|p=16}}{{sfn|Spitzer|1958}}