File:Ion collisions in weak and strong magnetic fields compared.svg

English: Diffusion of a plasma within a magnetic field is a geometric process. A plasma consists of a gas-like mixture of ions and electrons. As these particles are electrically charged, they will orbit around the direction of the magnetic field due to the Lorentz force. Any original velocity they had, which is a function of the temperature of the plasma, is retained. The result of the circular motion and remaining linear motion causes the particles to trace out a helix through space. Diffusion occurs when two particle's orbits intersect, which is a function of the area of the circle and the speed of the particle measured along the magnetic lines. The resulting formula is thus dependant on the square of the magnetic field strength, B², which defines the area, and the temperature, T, which defines the speed along the line of net motion. On the left, we see particles within a weak field, which results in a larger orbital area and thus the possibility of collision. On the right are the same particles in a stronger field, reducing the chance of collision. Thus the diffusion rate in a weaker field is much faster than a stronger one, independent of the temperature of the plasma. For this reason, it was long expected that increasing the strength of the field in a fusion reactor would lead to practical plasma confinement. However, a host of previously unknown instabilities demonstrated that the classical diffusion calculation resulted in loss rates far below what was being seen in experimental devices.