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Transitions in tokamak edge confinement

Transitions in tokamak confinement are discussed using a simple illustrative model in which the diffusion coefficients predicted by cylindrical classical theory are used. The transport coefficients depend on the plasma parameters in such a way that transitions in the particle and energy confinement can occur near the edge of a tokamak where particle sources, and hence convection, are important. There may be a transition to a second steady state or to a regime where there is no steady solution, in which the density rises continuously. In practice such a density rise would be curtailed by high radiated power or by instabilities. Many experimental observations of L-H mode transitions in tokamaks resemble the non-steady solution rather than a bifurcation to a second steady state