3 The X-factor
To prevent the plasma from interacting with the walls, which would otherwise contaminate it, many tokamaks use sets of poloidal magnetic coils (not shown) to generate a so-called X-point configuration of the poloidal field. The X-point defines the edge or "separatrix" of the plasma (pink surface), which causes any particles that escape through it to be swept onto the divertor targets. Charged particles travelling in tight "gyro-orbits" around magnetic field lines drift due to non-uniformity of the magnetic field, in some cases tracing out banana-shaped orbits. Currents from particles on neighbouring banana orbits largely cancel (for example where the green and blue orbits shown touch each other). But because orbits nearer the core of the plasma contain more particles with higher velocities, this cancellation is not complete and a net current arises that is transferred into a helical "bootstrap" current via collisions with other particles.