The behaviour of fast ions in tokamak experiments

Fast ions with energies significantly larger than the bulk ion temperature are used to heat most tokamak plasmas. Fast ion populations created by fusion reactions, by neutral beam injection and by radiofrequency (RF) heating are usually concentrated in the centre of the plasma. The velocity distribution of these fast ion populations is determined primarily by Coulomb scattering; during wave heating, perpendicular acceleration by the RF waves is also important. Transport of fast ions is typically much slower than thermal transport, except during MHD events. Intense fast ion populations drive collective instabilities. Implications for the behaviour of alpha particles in future devices are discussed.