Collisional diffusion of particles. The collision pushes the particles towards magnetic field lines other than the one around which they were orbiting. (Source: Goran tek-en, Wikipedia.org)

The gradual movement of charged particles in a magnetically confined plasma across magnetic field lines. The movement is caused by collisions between the particles, which shift them to a different magnetic field line from the one they first orbited. Through diffusion for example, particles can escape from the magnetic cage that holds them and smash into the walls of the vessel. Diffusion worsens the confinement time of the particles and is one of the important factors why thermonuclear fusion is so difficult to achieve. There are many types of diffusion, e.g. Bohm, classical, and neoclassical. Each one is related in a different way to the temperature of the plasma and the strength of the magnetic field.