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Particle transport

The movement of charged particles in plasma across magnetic field lines, which leads to a loss of particles and energy from the centre of the plasma and thus to reduced confinement. The primary causes of transport are particle collisions and turbulence. When particles collide, the magnetic field line around which they orbit changes, and so they gradually move out of the plasma. Different models describe the measure of their motion, or diffusion rate. Bohm’s theory assumes diffusion linear with temperature and inversely linear with the strength of the confining magnetic field, while classical diffusion is inversely proportional to the square of the magnetic field. At present, the neoclassical model best describes diffusive particle transport, which includes the geometric influence of the toroidal magnetic cage. In addition to collisions, there is also anomalous transport due to turbulence, which accounts for an order of magnitude or two higher particle loss than pure neoclassical transport would predict.