THEORY OF THE SPATIOTEMPORAL DYNAMICS OF TRANSPORT BIFURCATIONS

The development and time evolution of a transport barrier in a magneti cally confined plasma with nonmonotonic, nonlinear dependence of the a nomalous flux on mean gradients is analyzed in the context of a one-fi eld model. Upon consideration of both the spatial inhomogeneity and th e gradient nonlinearity of the transport coefficient, it is shown that the transition develops as a bifurcation front with radially propagat ing discontinuity in the local gradient. The spatial location of the t ransport barrier as a function of input flux is calculated. The analys is indicates that for powers slightly above threshold, the barrier loc ation x(b)(t) similar to [D(n)t(P - P-c)/P-c](1/2), where P-c is the l ocal transition power threshold and D-n is the neoclassical diffusivit y. This result suggests a simple explanation for the high disruptivity observed in reversed shear plasmas. The basic conclusions of this the ory are insensitive to the details of the local transport model. (C) 1 997 American Institute of Physics.