ON THE DYNAMICS OF TURBULENT TRANSPORT NEAR MARGINAL STABILITY

A general methodology for describing the dynamics of transport near ma rginal stability is formulated. Marginal stability is a special case o f the more general phenomenon of self-organized criticality. Simple, o ne field models of the dynamics of tokamak plasma self-organized criti cality have been constructed, and include relevant features such as sh eared mean flow and transport bifurcations. In such models, slow mode (i.e., large-scale, low-frequency transport events) correlation times determine the behavior of transport dynamics near marginal stability. To illustrate this, impulse response scaling exponents (z) and turbule nt diffusivities (D) have been calculated for the minimal (Burgers') a nd sheared flow models. For the minimal model, z = 1 (indicating balli stic propagation) and D similar to(S-0(2))(1/3), where S-0(2) is the n oise strength. With an identically structured noise spectrum and flow with shearing rate exceeding the ambient decorrelation rate for the la rgest-scale transport events, diffusion is recovered with z = 2 and D similar to(S-0(2))(3/5). This indicates a qualitative change in the dy namics, as well as a reduction in losses. These results are consistent with recent findings from dimensionless scaling studies. Several toka mak transport experiments are suggested. (C) 1995 American Institute o f Physics.