Tokamak plasmas: a paradigm for coronal equilibrium and disequilibrium

Tokamaks operate over a wide parameter space, allowing access to plasma conditions relevant to astrophysical plasmas. For high electron density discharges, for example, the central electron density and temperature are ~ 3 × 10 14 cm -3 and ~ 1.5 keV, and the central plasma region is in coronal equilibrium. Towards the edge of the plasma, however, many ion species will be far out of coronal equilibrium. A novel feature of the edge region is the seemingly contradictory property that it is, simultaneously, both a strongly recombining and a strongly ionizing plasma. Recent tokamak observations of strongly recombining plasmas also show that the G parameter (the ratio of forbidden plus intercombination to resonance lines) is larger by a factor of 3 than the ratio of statistical weights of the triplet-singlet series. Such observations can be of direct consequence to the interpretation of non-equilibrium astrophysical plasmas.