Editing Tokamak (section)

=== Basic problem ===
Positively charged [[ion]]s and negatively charged [[electron]]s in a fusion plasma are at very high temperatures, and have correspondingly large velocities. In order to maintain the fusion process, particles from the hot plasma must be confined in the central region, or the plasma will rapidly cool. Magnetic confinement fusion devices exploit the fact that charged particles in a magnetic field experience a [[Lorentz force]] and follow helical paths along the field lines.{{sfn|Wesson|1999|p=13}}

The simplest magnetic confinement system is a [[solenoid]]. A plasma in a solenoid will spiral about the lines of field running down its center, preventing motion towards the sides. However, this does not prevent motion towards the ends. The obvious solution is to bend the solenoid around into a circle, forming a torus. However, it was demonstrated that such an arrangement is not uniform; for purely geometric reasons, the field on the outside edge of the torus is lower than on the inside edge. This asymmetry causes the electrons and ions to [[Guiding center|drift across the field]], and eventually hit the walls of the torus.{{sfn|Bromberg|1982|p=16}}

The solution is to shape the lines so they do not simply run around the torus, but twist around like the stripes on a [[barber pole]] or [[candycane]]. In such a field any single particle will find itself at the outside edge where it will drift one way, then as it follows its magnetic line around the torus it will find itself on the inside edge, where it will drift the other way. This cancellation is not perfect, but calculations showed it was enough to allow the fuel to remain in the reactor for a useful time.{{sfn|Wesson|1999|p=13}}